In 1975 Captain Laurie Taylor was commissioned to write an appraisal of the B747 in BOAC service in a series of articles for Shell Aviation News. This edition of the appraisal is based on the first three of these articles.
LAURIE TAYLOR recently retired as a senior captain with British Airways Overseas Division after a career spanning thirty years, during which he operated all the airline's principal aircraft. In this series or articles he summarises his experience with the B747, the largest commercial type in regular service. Captain Taylor is now Executive Secretary of IFALPA. He is a past member of the Council of the ARB, and of the ICAO Airworthiness Committee


Part 1: Overview
Part 2: Operational Performance and Flight Planning
Part 3: Automatic Flight Control and Inertial Navigation Systems
Part 1: Overview


WHEN the Boeing Company obtained the first order for the B747 from Pan American and decided to go ahead with the production of the aircraft at a new and specially built factory at Everett, north of Seattle, many doubts were raised as to whether the aircraft could be successfully operated over the world's air routes.

'Experts' stated that the aircraft was too big, too complex, too expensive and had arrived too soon to be digested by the civil air transport operating industry. However, Pan American's often demonstrated determination to follow the example of the Civil War General whose dictum was 'Be fustest with  the mostest', pushed a number of airline boards into ordering the aircraft in order to stay competitive over the major air routes and particularly over the North Atlantic.

Similarly, airport authorities found themselves committed to new expenditures by the decisions reached by Boeing at Seattle and by Pan American at New York, and the whole civil air transport industry was lifted to a new plateau of investment and achievement. By May 1975, 252 aircraft had been delivered to 32 airlines and a further 41 aircraft were on order. Although several versions of the aircraft are in service, this appraisal will concern itself with the basic 747A, fitted with Pratt and Whitney engines. The earliest aircraft had the JT9D-3 fitted, but many operators have now substituted the -7 or 7A engines.

The B747 is currently operated over many routes including the North Atlantic, Pacific, Africa and Australasia. It flies to and from hot and high airfields as well as sea-level cold weather airports. Some short sectors are operated by a number of airlines, and many of the difficulties forecast were either circumvented by good planning or overcome by ingenuity and airmanship. The worst problems that remain are caused by factors other than the aircraft itself.

My first acquaintance with the aircraft came in 1968, when representatives from a number of pilot associations met in Seattle for a technical and air safety meeting sponsored by the USA's Air Line Pilots Association. We were invited to visit the new aircraft plant at Everett to take a look at the 747. Because I had already inspected the Lockheed C5A, I was determined not to be too impressed by this later (and smaller) product of U.S. aircraft technology, and I ended that first brief acquaintance more impressed by Boeing's achievement in clearing virgin forest and constructing a huge new factory of 58 acres of floor space in only two years than I was with the first production aircraft. (There was no prototype.) Then and now I believed that the aircraft would be greatly improved in appearance (and ground handling and flying qualities) when it is 'stretched' by about 30 feet.

It is only fair to add that my fellow pilots on the visit were as pleased by the prospect of eventually flying the aircraft as they would have been by the gift of an E-type Jaguar car on their eighteenth birthday. The only criticisms heard concerned mainly the dimensions and design of the flight deck area, and the absence of any really significant new technology in flight deck displays. Many of us had hoped that Jim Gannett, the Boeing test pilot whose work on flight deck displays for the projected Boeing SST set a new target for every other designer, would have been given his head on the 747 and encouraged to produce a flight deck more worthy of an aircraft that will almost certainly still be in front line service in the 1990s.

Later and closer acquaintance with the aircraft modified my early opinions somewhat, and now that I have operated the B747 for more than three years, my views may have a little more validity, although l cannot claim that they are necessarily shared by my employers, British Airways, by pilot associations within or without the International Federation Of Air Line Pilots Associations, and more particularly by pilots who have flown the aircraft over a longer period of time and for more hours than I.

Pilot training
A most welcome event in recent years had been the growth of consultation between aircraft manufacturers, aircraft operators and pilot organisations before ground school, flight simulator and flight training programs, and training aids including Operations and Flying Manuals for the use and training of line pilots are determined. So far as l am aware, the B747 training methods were the first to be devised under the new system, and the first that enabled a remarkable degree of standardisation of methods and documentation to be achieved among all the airlines operating the aircraft.

When BOAC (now British Airways Overseas Division) ordered the B747, one of the most fundamental decisions made - because of its potential effect on the flight crew training program - was to determine the eventual crew complement of the aircraft. It was decided by BOAC that the basic crew would eventually be two fully qualified and type rated pilots, plus a licensed specialist flight engineer who would be integrated into the crew by receiving training in additional flight operations duties. The flight engineer was to reduce pilot workload by taking over in-flight Company communications both by VHF and HF radio, copying broadcast weather information, generally 'monitoring' the flight path of the aircraft with particular emphasis on the vertical profile, and calling out all significant instrument and system warnings and malfunctions observed in flight.

Other airlines agreed other crew complements and procedures with their pilots. Without discussing here the pros and cons of all-pilot crews, specialist crew members, large crews, or small crews, it is perhaps fair to state that the decisions taken by the airlines concerned were as much affected by their previous individual practice, the industrial issues involved, costs, route structure, and attitude of the Regulating Authority as by any deeply held conviction that a particular crew complement was better than any other. I have heard passionate arguments over this particular topic, and I hold firm views myself, but the consensus is that although a particular crew complement might be preferred, neither the all-pilot three man crew nor the two pilot/one flight engineer crew is unsafe.

The Author learnt to fly in the RAF in 1941, subsequently specialising in the large multi-engine transport types with which he has been associated ever since. Here he is at the end of his final B747 flight for British Airways, a London-Miami operation, before taking up his IFALPA appointment

The flight engineer conversion course to the B747 in British Airways begins eight weeks ahead of the pilots, because of the airline's requirement for the flight engineer to become involved in trouble shooting and defect rectification - particularly at 'off line' airports. The flight engineer therefore studies the location and performance of components and systems in greater detail than the pilots, particularly those other than on the flight deck.

The pilots spend four weeks in ground school studying the aircraft and its operation. From the beginning they use the same manuals in the ground school that they will use in flight. An initially confusing mosaic of electrics, hydraulics, pneumatics, controls and indicators eventually forms a single picture, but the pace of the course was such that all of us found it necessary to study in the evenings and over the weekends.

The lectures are all given by specialists in the various systems. Each individual student's progress is checked by a frequent bombardment of multi-choice questions and answers, the students being spared the ignominy of recording their occasional failure to understand a lecture by the use of coded push buttons which light a display unit in front ofthe instructor, showing him which students have the right and wrong answers. The whole philosophy of the pilot course is based on a 'need to know'; it concentrates on controls and their effects in the various systems, the displays of system status as seen on the flight deck, and the effects of malfunctions in both systems and displays.

The emphasis is on normal modes of operation, then Alternate modes, and then Emergency modes. Pilots, being pilots, tend to concentrate on all the things that may go wrong which the designer has apparently failed to take into account, and only the pace of instruction prevents each course from carrying out a major re-design of the whole aircraft.

Towards the end of the course I found myself determining my answers to difficult questions by attempting to recall the information given in the lecture, and when that failed, to answer on the basis of what ought to happen - for example, to an autopilot in use during an automatic landing when that particular ILS localiser receiver failed. The provisions made by the Boeing design teams and my own beliefs based on more than thirty years of experience as to what should happen in a particular circumstance were usually identical, and although an academic purist might argue that a memory of complex technical points made during lectures is preferable to a common sense analysis of an event and the consequential actions, there is perhaps something to be said for pilots being more than naked apes drilled into unthinking responses to certain stimuli.

In view of criticisms I make later about some features of the B747, perhaps now is the time to state that my own personal thinking-man's response to unscheduled events during flight would not have been successful in previous aircraft types I have operated, whether of American or British origin. The design of those aircraft and their systems seemed not to have paid a great deal of attention to what the pilot needs - I might call the previously missing design factors logic and common sense!

Training sequence
The necessary breaking-down into particular systems of so complex an aircraft as the B747 provides some problems when putting it all together as an integrated vehicle. To the best of my recollection I did not visit an aircraft during the whole of the ground school course, but any adverse effects from this apparently surprising omission were overcome by the use of the latest training aids.

Scheduled check in the Overseas Division's Maintenance Base, Heathrow Airport - London

Systems panels that look and respond exactly like the aircraft systems are used to perfect operating and malfunction procedures. The fuel system, for example, can show typical loading, usage and even jettison procedures. When all the different systems have been learned, the trainee pilots and flight engineer can sit at the 'Cardboard Bomber' from which the relative positions of the systems displays are learned from a paper/cardboard representation of the various flight deck displays. At a later stage the trainees use the Cockpit Procedures Trainer, which can best be described as a static simulator, where the crew can perfect all their skills except those of navigation and piloting. Pre-flight checks and normal, alternate and emergency drills can be performed by a trainee crew with faults being injected by a ground engineering instructor.

The Cardboard Bomber and the Cockpit Procedures Trainer enable economies to be effected in the use of the full flight simulator which, although much cheaper than an aircraft, is still an expensive item of equipment.

British Airways has been able to give initial and continuation training to the crews for a fleet of sixteen 747 aircraft, and to train crews for other airlines, using only one C.A.E. flight simulator. This operates seven days per week over a sixteen-hour daily schedule. The simulator has the Visual Anamorphic Motion Picture (VAMP) attachment, which is of some value in training for operations in reduced weather minima, but which does not in my opinion come close to providing a faithful simulation of the visual cues used by the pilot in take-offs and landings. The shift from analogue to digital techniques in flight simulators has enabled their performance and flying qualities to resemble more closely those of the actual aircraft.

In the United Kingdom, pilot complaints about early flight simulators resulted in the chief test pilot of the Air Registration Board (now Civil Aviation Authority) becoming involved in their 'certification'. Unless he is satisfied with the fidelity of the simulator the airline cannot substitute flight simulators for aircraft, for training and pilot licensing purposes. Even now the crews of British Airways B707s have some flight checks rather than all-simulator checks because the simulators do not reproduce the aircraft's performance and flying qualities sufficiently closely. The B747 flight simulator, however, is now fully approved, and following the initial conversion course and one flight check about six months after initial qualification, flight crews do all their training and checks on the flight simulator.

The normal training schedule calls for the Cardboard Bomber and Cockpit Procedures Trainer to be used during the initial four-week ground school course. Later the trainees move to the flight simulator. Each flight simulator training detail is preceded by a two hour briefing session when the exercise to be 'flown' is explained by the use of the Cardboard Bomber, airways and airfield charts, performance and navigation manuals and flying and cruise control manuals. Take-off calculations are performed by the trainees for every simulator training exercise. The flight simulator details last for four hours shared by two pilots, and include normal pre-flight checks and engine starts. Eight of these Cardboard Bomber/Flight Simulator Type-conversion training details are scheduled, during which some corners of the flight envelope and some emergency situations are experienced that are not demonstrated in flight - including loss of hydraulic power to wheel brakes, abandoned take-off, offset and steep approaches, and so on.

In the 'Cardboard Bomber', a pre-simulator spatial representation of the B747 flight deck used for systems briefings

After the satisfactory completion of the simulator program, the flight training phase of the course begins. This concentrates heavily on circuit work, including engine failures on take-off, instrument approaches, automatic and manual three and four engined ILS approaches including an Instrument Rating renewal, taking a total flight time of eight to ten hours per pilot. Only six route sectors under supervision including one requiring INS navigation then separate the pilot from his operations certificate on the type.

A final base flight check not later than six months after qualification may be said to complete the course, but it is naturally not the end of training, although no more in-flight training is normally undergone. All check and refresher routines, including Instrument Rating renewals, are carried out on the flight simulator and each pilot is scheduled for simulator training four times per year. The routines are alternately Check and Refresher routines selected from a programme which over a year covers all likely and most unlikely circumstances, including loss of all engines, complete loss of pressurisation, under-floor fires and so forth.

There is also a Safety Equipment and Procedures Training and Check once per year when ditching and emergency evacuation films are shown, and all emergency equipment and drills are examined. The additional safeguard the airline takes to ensure a continuing and high standard of proficiency among its pilots is the route check system, whereby a supervisory pilot occupies the jump seat as extra crew member and, without participating in any way, checks the operating standards of the captain and his supervision of the crew. A written questionnaire on technical and operational complexities, which takes at least a whole day to answer, is completed by each pilot and has to be correct before the route check is regarded as being complete.

It will be seen that after a pilot is fully qualified on the aircraft type he spends at least six days and one supervised flight being trained and checked each year. It is not surprising that pilots feel that they are the most trained, checked and supervised of all professionals. They are not, however, resentful of this, recognising the very direct connection between their professional skills and flight safety.

During the flight training phase, the first landing a new pilot sees is an automatic one. Here the training captain flies a holding pattern manually before positioning the B747 for a demonstration


Part 2: Operational Performance and Flight Planning


A LOOK at the B747's range and payload figures show the aircraft to be a remarkable step forward over preceding airline types. The JT9D-3 powered aircraft were something of an operational (and engineering) headache for early operators, and the improvement in reliability and thrust provided by the Dash 7 engine has been put to good use by airlines blessed with its availability. Take-off and landing weights have been increased by approximately 10,000 kg to 332,500 kg and 265,351 kg respectively, and in the passenger configuration there are very few route sectors in which these structurally limited operating weights impose any significant penalty.

In the British Airways operating configuration, the fully equipped empty operating weight is about 166,200 kg - just 50% of the structurally limited maximum take-off weight. Full fuel tanks (141,400 kg at a typical kerosine SG) would leave 'only' about 25,000 kg available for payload, but this load - say 300 passengers and baggage - could be flown from London to Los Angeles over the route distance of 4808 n miles with ample fuel reserves.

Although I have been operating the B747 over route sectors such as London-Miami, London-Anchorage and London-Nairobi in all seasons, I never had full fuel tanks although of course many of my flights were at maximum structural take-off weight. The increased landing weight now permitted has not yet been accompanied by an increase in maximum zero fuel weight (still at the original 238,816 kg), and so only becomes useful if the captain needs to land with larger than normal fuel reserves. This can be of advantage in these troubled days. If an unscheduled landing has to be made, less fuel needs to be burned off or jettisoned.
 
Sector fuels
A London-Miami single sector operation using British Airways fuel reserves (greater than the ATA formula and that of many other operators) produces an available 43,800 kg of payload over the 4004 n miles against a 55 kt headwind component. The Sydney-Hong Kong sector during the summer period has the greatest fuel requirement of any British Airways B747 operation. The 85% wind component is only -10 kt but the alternate airport, Taipai, is 516 n miles from Hong Kong. In this case the sector fuel is 127,500 kg, which leaves 38,800 kg available for payload if the take-off conditions permit maximum take-off weight to be used. The British Airways policy on the carriage of fuel in the B747 requires that the sum of the following quantities be carried:

1. Fuel to destination (from take-off roll to touchdown)

2. Fuel from destination to alternate (from wave-off at destination to touchdown) - diversion fuel

3. An alternate reserve of 7500 kg (6000 kg to provide 30 minutes holding at 6000 ft, plus 1500 kg remaining in the tanks)

4. A contingency reserve, to allow for meteorological forecasting inaccuracies and ATC restrictions on the flight level and route. This reserve varies with the length of the planned sector as follows:
1-1000 n miles 2000 kg 13 minutes
1001-1500 2500 17
1501-2000 3000 20
2001-2500 3500 23
2501-3000 4000 27
3001-3500 4500 30
3501 plus 5000 33 

In certain specified circumstances an island reserve fuel may replace the diversion fuel. The quantity required is 19,500 kg, which provides 2 hours holding at 20,000 ft plus 1500 kg remaining after landing. This island reserve procedure is presently used at Bermuda and Darwin, because of the non-availability of alternate airports at those locations.

The B747's requirement for special ground facilities to load and unload means that some airports that are operationally suitable for the aircraft are unsatisfactory from a ground handling standpoint. They are therefore not used as destination or alternate airports at the planning stage. This factor tends to increase the amount of fuel carried.

Flight planning
Many of British Airways' stations are now linked to the company owned London Airport computer facility, and computed flight plans are often provided 'down line'. When the take-off conditions, or the length of the sector to be flown, show that the payload that can be carried is limited, consideration is given to flight planning to different destination and alternate airports, with a view to re-flight planning on the way; or to nominating an en route alternate which enables the contingency reserve to be reduced to that appropriate for a shorter route sector. (See Professional Challenge below.)

The over-riding requirement is of course to be able to reach a destination and an alternate airport from any point along the route after take-off, forecast weather conditions at the time of intended landing being taken into account.

Take-off. The penalties of a contaminated runway are greater . . . owing to the drag increment of the multiple landing gear

Take-off performance
In British Airways, a take-off performance chart is produced for every runway likely to be used for each aircraft type. Because the charts are individual to each runway, the airfield elevation, runway slope, runway length - including take-off run (TOR), take-off distance (TOD) and emergency distance (EMD) - obstacle clearance, and noise abatement limitations (if applicable) are all built in.

The pilot enters the chart with the surface temperature only to find the intersection with the headwind/ tailwind quantity. He then reads the uncorrected performance limit weight directly from it. This uncorrected performance limit weight can be, and at sea level airports often is, greater than the maximum permitted structurally limited take-off weight of 332,500 kg. Every other item affecting take-off performance is then applied as a weight correction, as follows:

Crosswind correction. Deduct 300 kg per knot of crosswind greater than 15 kt, to compensate for aerodynamic drag created by control surface deflections.

Runway precipitation. Water, slush, wet snow 3 mm to 13 mm (0.5 inch) - deduct 75,000 kg.

Dry snow 20 mm to 60 mm (2.5 inches) - deduct 75,000 kg. (A further decrease is made of 2500 kg per 100 ft by which the EMD is less than 10,000 ft down to the minimum permitted EMD of 9000 ft, plus a deduction of 3500 kg per knot of tailwind.)

Ice or compacted snow - deduct 10,000 kg plus 6000 kg per 100 ft by which the EMD is less than 10,500 ft. Also deduct a further 6000 kg per 1000 ft of pressure altitude AMSL.

The penalties of a contaminated runway are greater for the B747 than for any other aircraft type owing to the drag increment incurred by the multiple landing gear.

ABNORMAL PROCEDURES
Weight reductions are required when operating with certain items of equipment unserviceable, such as fan or turbine reversers, EPR indicator, wing fuel tank jettison system (regulated take-off weight must not exceed landing WAT limitations), or APU inlet doors. Should one air driven hydraulic pump (back-up for the engine driven hydraulic pumps) be unserviceable in the system(s) that retract the landing gear(s), the flight is treated from a performance standpoint as a gear-down operation.

INTER-MIX ENGINES
All British Airways B747 engines are JT9D-7, but provision is made for operating with one or two Dash 3 engines. If one Dash 3 engine is fitted, the take-off weight reduction varies with RTOW (regulated take-off weight) and temperature. A 332,000 kg, Dash 7 operation at ISA plus 12°C would be reduced by 6000 kg. If two Dash 3 engines are fitted, all engines and the aircraft are operated as a Dash 3 aircraft. The reduction in RTOW from a 330,000 kg Dash 7 operation at ISA plus 12°C becomes 12,800 kg.

WET RUNWAY
A wet runway take-off is considered by reducing V1 by 10 kt, and accepting that the 'screen' height will be reduced from 35 ft to l5 ft in the event of a continued take-off with the critical engine failing at V1.


The sum of the weight corrections is applied to the uncorrected performance limit weight to obtain a corrected performance limit weight. The lesser of the corrected performance limit weight and the maximum structurally limited take-off weight then becomes the maximum RTOW for the conditions obtaining. Each millibar reduction in pressure, each knot of headwind/ tailwind, has an immediate effect on the degree of profitability of any payload limited operation.

This British Airways system of working from simplified take-off performance charts for each individual runway, whereby an uncorrected performance limit weight is obtained and then adjusted for every deviation from standard conditions and a standard aircraft, works very well in practice and the generalised take-off performance chart is rarely used. All the performance manuals and other operational documents have been improved through simplification and size reduction, so that they are suitable for use at the flight deck crew members' operating stations. The changes to flight deck documentation were a major factor in obtaining pilot agreement to a three man crew for the British Airways B747.

Professional challenge
An example of the way in which the flight planning options described earlier are often exercised may be shown by an actual operation over the Nairobi-London sector last February. This northbound operation was scheduled to depart at about midnight local time, when take-off conditions are most favourable. Nevertheless the airfield elevation, air temperatures, sector length and high payloads offered, all combined to stretch the aircraft's performance to the limit. Political considerations have increased the air route distance between Nairobi and London, and securing the maximum payload while maintaining the highest operational and safety standards provides a challenge to pilots which is met with professionalism.

On the night in question the following circumstances prevailed:
Airfield elevation 5327 ft amsl
Runway length 13,500 ft
Runway slope  0.36% downhill (Runway 06)
Surface wind 360°/7 kt (Headwind component 4 kt)
QNH 1020.5 mb
Temperature 15°C
Aircraft empty operating weight 166,223 kg
Passengers 294
Payload to be carried (passengers,      baggage and cargo) 30,000 kg approx
Standby cargo As much as the aircraft could carry in the prevailing circumstances
 
This flight was scheduled to be non-stop to London. Yet the payload offered required that consideration be given to a different operation. Either a technical fuelling stop could be planned in order to maximise payload, at the risk of annoying passengers who expect the airline to operate to the published timetable; or the flight could be planned direct to London using Manchester as alternate, requiring some of the cargo to be offloaded (and none of the low rated standby cargo to be carried). Alternatively, a compromise could be sought.

Athens or Rome could normally have been considered as en route alternates or as technical fuelling stops, to reduce the amount of fuel required to be carried. That night, however, Athens Airport had poor weather and Rome (Fiumicino) had a strike, so there was no point in considering either. Nominating and flight planning Zurich as destination, with Frankfurt as alternate, looked a better choice and permitted a larger payload than London as destination with Manchester as alternate. This option was selected, so all the cargo already on board stayed on board, 4000 kg of standby cargo were loaded together with every possible kilo of fuel, in order to operate at the maximum regulated take-off weight with a very good possibility of re-flight planning in the air to operate direct to London.

Nairobi's elevation above sea level imposes a WAT limit on almost every operation. Thus on the night in question the uncorrected performance limit weight was only 307,300 kg. There were no reductions for crosswind, runway conditions or aircraft unserviceability, and the higher-than-standard QNH permitted an increase in weight of 1300 kg.

TAKE-OFF
The coincidence of a 'high and hot' airfield and a long runway allows a variation in take-off calculation and technique, to improve Second Segment climb performance when the operation is WAT limited. The performance requirement is to maintain a 3% gross gradient climb on three engines at V2, with landing gear retracted. An 'improved take-off climb' speed allows the scheduled V2 speed to be increased by up to 5% with corresponding increases in V1 and VR speeds. This provides a better engine failure climb performance, and hence a higher WAT curve weight, at the expense of greater runway length requirement.

The technique is built into the individual runway charts for airfields such as Nairobi, Teheran and Johannesburg. Normal VR and V2 speeds are taken from a table of take-off weights, then 'corrected’ to provide improved take-off climb, following which the V1 reduction is applied to the corrected VR.

The final figures came out as follows:
Take-off weight 308,000 kg
Flaps 10°
V1 150 kt
VR 171 kt
V2 174 kt
V2 + 10 184 kt
V2 + 40 206 kt (based on original V2)
V2 + 80 246 kt
Maximum EPR (power setting) 1.50

The increased V1 and VR speeds prolong the ground roll of the aircraft, and in zero winds push the tyres to near their performance limits. I believe this gives rise to a number of tyre casualties. However, one can tolerate tyre failures on an 18 wheel landing gear and the operation is in every way acceptable from a safety viewpoint.

INITIAL CLIMB. The take-off on the night in question was uneventful. On passing 1500 ft above the airfield level, power was reduced to climb settings and a low rate of climb accepted in order to clean up the aircraft. A climb rate of about 400 fpm allowed the aircraft to accelerate, and the take-off flap setting of 10° was reduced on the following schedule:
1.3 Vs Structural limits
Speed greater than 206 kt (and less than 240 kt) Flap 5°
Speed greater than 226 kt (and less than 265 kt) Flap 1°
Speed greater than 246 kt Flap 0°

On all B747 take-offs in British Airways, consideration is given to the consequences of three engine performance and to the failure of a second engine before the aircraft has been cleaned up. Should one engine fail at or after V1, the aircraft is climbed at V2 to 800-1000 ft above airfield elevation (higher if there is a terrain problem), and then flown level in order to accelerate to raise the flaps. A second engine failure before the aircraft is accelerated and cleaned up gives the pilot serious performance problems. At ISA and weights greater than 305,000 kg the maximum three engine cruise flight level for a Dash 7 powered aircraft is FL 230 (23,000 ft pressure altitude), and FL 190 for a Dash 3. The two engine cruise flight level becomes FL 40 (4000 ft) for a Dash 7 powered aircraft, while on a Dash 3 the weight must be reduced to less than 265,000 kg for this level to be maintained.

The elevation of Nairobi Airport is 5327 ft amsl. In view of the foregoing it should not surprise anyone to learn that most captains brief the co-pilot and flight engineer that they will begin dumping fuel as soon as a first engine failure has been dealt with when airborne.

A recent change to the Flying Manual now permits the use of take-off thrust (with an EGT limit of 915°C for Dash 7 engines) for 10 minutes, provided that (a) an engine has failed, (b) the obstacle clearance flight path requires it and (c) the ambient temperature does not exceed 36°C. Nairobi is a case where this concession is likely to be used.

CLIMB. The climbout north from Nairobi is flown using INS (cross-checked by VOR/DME and ADF) to put the aircraft's flight path over the lower ground between Mount Kenya and the Aberdares. On this occasion, at the 308,000 kg take-off weight the maximum cruising flight level was FL 310, and the climb time with Dash 7 engines was 30 minutes from sea level. This rate of climb is lower than that achieved by other aircraft types like the B707 and DC-8 and often produces difiiculties for air traffic controllers. It is appreciably worse for Dash 3 powered aircraft, and so unpopular were the Dash 3 engines among British Airways crews that (so it is rumoured) a two minute silence was observed for the retirement of the last one before the party began!

I believe that one of the earliest U.S. Presidential Election campaigns was run on the slogan: 'What this country needs is a good 5 cent cigar'. B747 pilots might paraphrase that to read 'What this aircraft needs is a good 50,000 lb thrust engine'!

CRUISE. The need to climb to a higher flight level as soon as possible, in order to conserve fuel, made the incredible accuracy of the Delco Carousel INS really prove its value on this occasion. Zero track deviation, and instantaneous ETAs, ground speeds, and spot winds, enabled frequent fuel checks to be made.

Almost 30,000 kg of fuel had to be burned before a climb to FL 350 was possible. By this time the aircraft's position required that HF communications be established with Khartoum and later with Cairo for ATC purposes, and there was a delay before a climb to FL 350 was approved at Abu Simbel. From Abu Simbel to Caraffa at the toe of Italy, the wind component was -50 kt and the fuel consumption was greater than 11,000 kg/hr. It appeared likely that the passengers were to be subjected to an unscheduled stop at Zurich, with a consequent delay to their arrival in London.

When Caraffa had been overflown, a further climb to FL 390 became possible when the weight reduced to 230,000 kg; and although it would have been preferable to burn off more fuel before climbing, FL 390 was immediately requested before any other aircraft made the same request and occupied the flight level. The slow climb took the aircraft through the tropopause and the situation was transformed. Fuel consumption was reduced to 10,000 kg/hr, with a further reduction to 9000 kg/hr before top of descent was reached. The wind component fell to -7 kt, and by the time Rome was passed it was clear than the flight could be re-flight planned to London with Manchester as alternate.

Minor power adjustment during the descent. 'The B747's only competitor in an aeronautical downhill slalom . . . would be the Trident'

DESCENT. The B747 may have a sluggish climb performance, but its capabilities in the descent leave nothing to be desired. With a zero wind component, a descent from FL 390 to the runway can be performed in a track distance of 100 n miles provided that ATC imposes no height or speed restrictions. The distance may be shortened if spoilers are used, though this is seldom necessary.

In British Airways the Mno/Vno condition is imposed, but operators of U.S. and other State registered aircraft are often heard using a Mne/Vne technique which permits the IAS to be increased by some 50 kt. The only other aircraft that could compete with the B747 in an aeronautical downhill slalom contest would be the British Airways (European Division) 'pocket rocket' Trident.

At 6 o'clock in the morning, London Control imposed no restrictions and an optimum descent and approach was flown, the fuel remaining in the tanks at engine shutdown being about 14,500 kg - enough to reach Manchester. So ended an operationally interesting flight . . . and a profitable one!

Landing
The landing performance of the B747 is worth looking at in a little detail.

Landings are normally made with full landing flap - 30° - but information is provided to permit a 25° flap landing if the aircraft is WAT limited in the one-engine inoperative missed approach case, at a hot and high airfield. This used to be the case at Johannesburg, where the airfield is 5557 ft amsl and the flight is scheduled to arrive in the afternoon. Fitting Dash 7 engines in place of the Dash 3 largely removed the necessity, but the new increased landing weight of 265,351 kg may possibly reintroduce it.

The present position is that maximum structurally limited landing weights can be accepted at Johannesburg with full flap (30°) at temperatures up to 15°C, with a reduction of 3000 kg per 1°C above that temperature. lf 25° flap is used, the maximum landing weight can be accepted at temperatures up to 24°C, with the same 3000 kg weight reduction per 1°C above that temperature.

As in the presentation of take-off data by British Airways, each individual runway likely to be used for landing is tabulated with the maximum weights for both 30° flap and 25° flap operation. A further column of data shows the maximum tailwind that can be accepted on each runway at maximum landing weight, or the still air runway-limited landing weight. It is not normally necessary to use the generalised landing performance charts.

Abnormal circumstances, such as operation with one or two Dash 3 engines installed, or use of nacelle anti-ice, or icing conditions during any part of the flight with temperature at landing forecast to be less than 8°C, are handled by entering the table normally but reducing the tabulated temperatures against maximum permitted landing weight by a given amount (16°C in the case of two Dash 3 engine installation).

The shortest runway for which data is tabulated is Runway 09/27 at Tampa, Florida, which is 7020 ft long. The Dash 7 powered B747 can operate into it at maximum landing weight with a 3 kt tailwind at 30° flap, or at 255,000 kg with a landing flap setting of 25°.

The landing roll is in fact shorter than that of the B707 at maximum landing weight, thanks mainly to a greatly improved anti-skid braking system (and automatic wheel brakes in later aircraft). If maximum automatic wheel braking is used, the landing roll can be as short as 3100 ft at a landing weight of 256,000kg. However, such a procedure could hardly be reconciled with the old BOAC slogan 'BOAC Takes Good Care Of You', for many passengers would dislike the fierce deceleration.

Separate summary tables of landing weights are provided for autoland, for manual landings on slippery runways, and for autoland on slippery runways. Each of these additional tables produces lower landing weights than the basic manual landing on a dry runway, so perhaps the old fashioned, early-model human pilot is not yet made obsolete by automatic flight control systems.

ILS is always used when available for B747 profile guidance on the approach, because normal VASIS do not provide adequate landing gear clearance over the threshold. (T-VASIS overcome this deficiency by providing positive angular information to the pilot). During a visual manual landing, British Airways technique is for the co-pilot to call radio altimeter heights at 100, 80, 50 and 30 ft, when the flare is initiated
Part 3: Automatic Flight Control and Inertial Navigation Systems



British Airways B747 flight deck

THE BASIC DESIGN of the B747's primary flying controls, as a fully powered system operating split control surfaces without manual reversion, makes the aircraft particularly well suited to the installation of a sophisticated automatic flight control system using autoland techniques to achieve very low weather minima.

The equipment offered by Boeing on the basic B747 aircraft is two autopilot/flight directors with two INS, two VOR/ILS receivers and two low range radio altimeters. I believe that only three of the 32 airlines operating the type have bought the more complete package with three instead of two of every AFCS/autoland item. British Airways was one of these, and was evidently the first to be authorised by the State regulating authority to operate the B747 in Category IIIA weather minima - less than 100ft cloud base and quarter-mile visibility.

During the type conversion course, the first landing demonstrated to the pilot is usually an autoland. Once he receives the type endorsement on his licence, he is fully qualified to perform autolands in Category I, and better, weather minima. After a short period of line operation each pilot returns to school to attend lectures on Category II and III equipment and procedures, and to see Blind Landing Experimental Unit films taken under real life conditions. He then receives flight simulator training with VAMP equipment to qualify for Cat III operation.

In the early stages of British Airways B747 flying numerous autolands were carried out in good weather conditions, many of which were later analysed from flight recorder read-outs. When the system had proved its reliability and accuracy of performance, a number of autolands were required to be performed on each runway to be so used in low weather minima, in order to demonstrate the performance of the ground equipment. After a long and expensive programme of pilot training and equipment proving, the authorised minima for specified runways with high quality ILS were gradually reduced for qualified crews. (ALL crew members must be qualified before the operation is permissible on a particular flight.)

The lowest weather minima authorised for British Airways B747 operation was on London Heathrow's Runway 28L, with a radio altimeter decision height (DH) of 20 ft and a Runway Visual Range (RVR) of 330 metres. The Cat II minima on the same runway are a DH of 125 ft and an RVR of 390 metres. It will be appreciated that with these low DHs, cloud base measurements and observations become irrelevant and the controlling operational criterion is the RVR.

Weather minima
British Airways' Aerodrome Operating Minima for B747 aircraft are described and tabulated in the Navigation Manual. One of the most important paragraphs in this manual states: "'Aerodrome Operating Minima" means the cloud ceiling and RVR for take-off, and the DH and RVR for landing below which operation is not permitted. OPERATION DOWN TO THESE LIMITS IS NOT COMPULSORY, but is at the discretion of the aircraft commander, who will take into account all relevant operational factors.'

It is because this paragraph means exactly what it states, because there have been years of consultation between pilots and managers and because of the pilot training programmes that have been completed, that British Airways is now beginning to derive some return in the form of improved operating regularity from its heavy investment in autoland development.

The altimeter setting policy for the airline's B747s - and for all future British Airways Overseas Division types - is to use QNH, thereby ending (I hope) the longest on-going argument in aviation. I have long believed that the QFE setting adherents have never been to Bogota, Mexico City or Johannesburg!

When British Airways tabulates take-off and landing minima for, say, Chicago O'Hare Airport, each value is itemised. There is a tabulation for all eleven (11) runways at O'Hare, listing each approved procedure and the appropriate weather minima. Fifty different lines of information on weather minima are given for this one airport. If anybody is wondering how an airfield can have eleven runways, let me assure them that there are fourteen, but British Airways does not authorise B747 landings on Runways 04R and 18/36.

The tabulations for Frankfurt and all other airports outside the USA would show the RVR/VIS column in metres. There are also pages of differing State regulations about what components of a landing system (i.e. REILS, TDZLs, HIRLs) need to be serviceable before an approach can be begun in specified weather conditions. In short, the whole problem is more complicated than it need be. ICAO has many more years of work to do, it seems, in the standardisation of equipment and procedures before the BA Navigation Manual can be reduced in size.

A selection of some of the more esoteric abbreviations used in this international but far-from-standardised business is as follows:
ALS Approach Light System
HAT Decision Height expressed as height above Touch-down Zone (TDZ)
DH RA Decision Height expressed as a Radio Altimeter reading
OCL Obstacle Clearance Limit
HIRL High Intensity Runway Lights
ILSC Minima applicable to an auto-coupled ILS approach (landing performed manually)
ILSC x As above, but ILS glide path unsuitable for use below Decision Height because it gives insufficient wheel clearance (less than 28 ft) over the threshold
ILSC 2 Cat II ILS
ILSC 3 Cat III ILS
ILSH Minima applicable to a hand-flown ILS
ILSNGP ILS with unserviceable glide path
MALS Medium intensity Approach Light System
RCLM Runway Centre Line Marking
RCLS Runway Centre Line Lights
REIL  Runway End Identification Lights
RVR Runway Visual Range. Visibility along the runway measured by a human observer, or transmitter with associated computer, and taking into account visual aids, background brightness etc.
RVV Runway Visibility Value. A less precise measurement than RVR, applying in the USA only. In certain circumstances expresses visibility in sixteenths of a mile instead of feet.

This list, though exhausting, is not exhaustive. I must admit that the fine difference between RVR and RVV tends to become forgotten when approaching a decision height of 125 ft at a rate of descent of 800 ft per minute.

Note the triplex AFCS/autoland installation comprising engage switches for the three autopilots (A); VHF Nav controller for Rx 1 (B); VHF Nav controller for Rx 2 & 3 (C); Radio Altimeters 1 & 2 (D and E) - Rad Alt 3 is out of sight to right of co-pilot's wheel; Flag Watcher for No. 3 Nav System (ILS, INS & Rad Alt) (F); Control/Display Units for INS 1, 2 & 3 (G, H, l); and the Flight Engineer's engine power computer readout (J)

Autoland
In April 1975, British Airways decided to discontinue their efforts to achieve Category III operation with the B747. The reasons given were the unsolved problem of lateral displacement from the runway centre line, and the high cost of retrofitting an automatic roll-out control system. Accordingly the third autopilot, flight director, ILS receivers and associated hardware have been removed or deactivated, though the capability to restore the system at a later date has been retained. The determining factor would be a re-appraisal of the costs/benefits should a larger number of airlines become interested in procuring the automatic roll-out guidance control system, thereby helping to reduce unit costs.

Nevertheless in the following description I have retained the Category III case, because it illustrates the way that British Airways believes this must develop and, as such, may be of unusual interest. It can be assumed that when Cat III again becomes a BA B747 operation, the techniques and procedures will be very similar to what now follows, plus the provision of add-on equipment to solve the lateral displacement and ground roll control problems. The autoland status of each aircraft is placarded on the Automatic Flight Control System (AFCS) glare shield panel as CAT II or CAT III. All the components of the aircraft's autoland system (and those of the ground facilities) must be serviceable for Category III operation.

Two successful autolands at Cat II status are required to upgrade the aircraft from Cat II to Cat III. This latter status is lost with any failure or unserviceability of a required sub-system, and the aircraft is automatically downgraded to Cat II as shown on the operations/engineering procedure sheet.


Operations engineering procedure sheet. This is a trouble-shooting flow chart for downgrading or upgrading the B747 autoland system between Cat ll and Cat Ill performance standards

The following items of aircraft equipment must be serviceable for Cat III operation:
Three autopilots Including Pitch Computers; Roll, Landing Control & Logic Units; Accessory Units; Actuators; Stabiliser Trim Units
Lateral Accelerometers 3
Normal Accelerometers 3
Altitude Rate Unit 1
Air Data Computers 2
Radio Altimeters 3
Standby Horizon 1
Attitude Director Instruments 2 (with Rising Runway indication)
ILS receivers 3
INS 3
Autopilot Wailer 1
Flight Mode Annunciators 2

The flight instrument panels for the two pilots are identical, except that the co-pilot has an additional radio altimeter. They comprise:

Attitude Director Indicator (ADI). This instrument provides the following information:
(a) Aircraft attitude, with pitch and roll attitude angles marked in degrees.
(b) Flight Director (vertical and horizontal command bars similar to the earliest Sperry Zero Reader).
(c) Speed Error. The left-hand vertical window shows speed error from the speed command bug setting on the Machmeter/ASI, which is remotely controlled from the AFCS glareshield mounted control panel.
(d) Raw Glide Slope, in the right-hand vertical window.
(e) Localiser. The runway marker at the bottom of the indicator moves left to right.
(f) Radio Height. The runway marker rises to meet the wheels of the ADI aeroplane symbol.
(g) Decision Height. Warning light at top right of the instrument case.
(h) Rate of Turn. Indicated at bottom of instrument.
(i) Slip. Black ball display at bottom of instrument.

British Airways' B747 Attitude Director Indicator (ADI)

Horizontal Situation Indicator. The HSI is conventional except that it displays either radio (VOR/ILS) or Inertial Navigation System (INS) information.

Remote Magnetic Indicator. The RMI is conventional, one being mounted on each pilot's panel and displaying on two needles ADF or VOR bearings according to the selection made. (Many pilots would have preferrcd two RMIs each, so that two ADF and two VOR indications could be displayed simultaneously.)

Autopilot/Flight Director Flight Mode Annunciator. Gives information on the mode and status of the three autopilots, viz. whether dual or triple autopilot operation; auto throttle status; and whether the desired captures of NAV (Localiser/VOR/INS), ALT/S (selected altitude), G/S (glide slope), Flare, and Go Around modes have been achieved.

ILS Deviation Warning Lights. These show 1/4 dot localiser or 1 dot glide slope deviation below a radio altitude of 500 ft. The lights extinguish if recapture is achieved before reaching 200 ft radio altitude. After 200 ft they remain illuminated, and a missed approach is required.

Central Instrument Warning System (CIWS). A flashing light is provided to obtain the pilots' attention, plus individually captioned amber lights for Heading (HDG), Attitude (ATT) and Monitoring (MON) failures. The flashing light (red) monitors the warning flags on the pilots' instruments and flashes when Attitude, Glide Slope (after Glide Slope capture), Navigation, Heading (on HSI) and Radio Altimeter warnings are displayed.

Flag Watcher. Because there are only rwo sets of pilots' instruments but three INS, ILS receivers, and Low Range Radio Altimeters (LRRAs), an additional set of flag watchers (red dolls' eyes) is provided on the central instrument panel for No. 3 system.

Annunciator Panel. A large multi-captioned annunciator panel is provided on the central instrument panel. It can handle 36 items, though not all are in use.

Pilots' Glare Shield Panel. On this panel controls are provided for:

(j) Radio or INS selection to the HSIs.
(k) VOR/DME channel selection (with test facility).
(l) Flight Director ON-OFF switch and heading and pitch control.
(m) Auto Throttle ON-OFF and speed select.
(n) A, B and C Autopilot engage control in manual or command modes.
(o) Autopilot/Flight Director input controls - COURSE, HEADING, VOR/LOC, ILS (LOC plus Glide Slope) and LAND.
(p) Mode Selector for Autopilot and Flight Director - TURBULENCE, OFF, VERTICAL SPEED, and IAS.
(q) 'Back Beam' selector (Flight Director only).
(r) Altitude Selector (for Autopilot, Flight Director and Altitude Warning System).


Pilots' Glare Shield Panel. Note engage switch for the third autopilot, since deactivated although restoration capability has been retained; also the Autothrottle speed selector (A/T SPEED), Altitude selector (ALT SEL) and A/P altitude command control to its right       

Executing an Autoland
All the above systems and other warning systems, the Autopilot Wailer, Radio Altimeter and AP/FD Flight Mode Annunciators, are checked before a decision is made to begin a Category III approach.

The stand-by horizon must be serviceable, and the electrical supply system is 'split' to prevent electrical faults from causing the total failure of an approach.

Single autopilot operation is used in the cruise and during the descent, either the IAS or Vertical Speed Command mode performing the descent to the selected Flight Level/Altitude. The Autopilot will capture the selected Flight Level/Altitude and the Auto throttle system will maintain the selected speed. The heading flown to intercept the localiser course will be that commanded by the HDG Select control on the glare shield panel.

At this stage the second and third autopilots will be engaged in the command mode, but the first autopilot to be engaged will continue to operate the aircraft. The Flight Mode Anunciator should show:
TRIPLE armed (White light) Three autopilots engaged
NAV armed (White light) Controlling autopilot awaiting radio Nav signal
ALT/S Green Altitude captured
GLIDE SLOPE armed (White light) Controlling autopilot awaiting glide slope signal

The Flight Director part of the AP/FD (Autopilot/Flight Director) panel will be showing identical lights to the Autopilot part.

On capturing the Localiser, the NAV annunciator will show 'Green' and the aircraft will maintain the Localiser. Before capturing the Glide Slope the landing gear is lowered. On capturing the Glide Slope, the G/S annunciator will show 'Green' and the aircraft will maintain the Glide Slope.

The throttles will close to maintain the selected speed down the Glide Slope. If there is a speed discrepancy greater than plus or minus 10 kt, the Auto throttle annunciator will show amber.

At an altitude of approximately 1500 ft, with Glide Slope and Localiser captured, all three autopilots will be activated. The TRIPLE light will show 'Green' and the Flare annunciator will show ARMED (white light).

The aircraft will then roll 5° left to achieve a 1/5 dot beam displacement and the blue Test Light will show for 5-7 seconds. When the test is complete, the aircraft will regain the Localiser course. From this moment the autopilot with the median command signal will fly the aircraft - control will be shifting A-B-C-A-B continuously!

Full flap is lowered at about this time, and the aircraft will stay tightly coupled to the Localiser and Glide Slope.

Flight Directors are switched OFF at 300 ft. At about 70 ft above the selected Decision Height (DH), the radio altimeters will provide an aural warning. This increases in volume until Decision Height is reached, when only the DH warning light will stay on. At 53 ft the Flare annunciators will show 'Green', and the flare will begin as the throttles close. After the landing is complete, both Autopilot and Auto throttle are disconnected by the normal control column disconnect button.

If a missed approach is to be flown, palm switches on Nos. 2 & 3 throttles put the aircraft in Go Around mode - GA annunciator lights show 'Green'. Gear and flaps are handled normally and the pilot selects the thrust. The aircraft follows a programmed climb profile at a climb rate of 300 ft per minute with 25° or 30° of flap, and 1000 ft per minute with a flap setting of 20° or less. The ground track angle is that flown during the late stages of the approach.

A manual Go Around may be flown if desired, in which case it is necessary to rotate the aircraft to 12° nose up in order to achieve a positive rate of climb.

Autolands and Go Arounds are handled very well by the autopilot(s). The major problem has been lateral displacement from the runway centre line. Compounding this problem is the wide landing gear track of the B747. If the aircraft is displaced from the Localiser course, the expanded Localiser scale in the ADI and the Excess Deviation Warning Lights will show at about 1/2 dot Localiser displacement.

If the displacement is less than 1/2 dot, the warning lights will not come on, and the pilot will have to assess his displacement from the centre line by means of the runway light pattern and the expanded Localiser scale. The Cat III lateral dispersion tolerances are:
1/2 track of B747 landing gear 21 ft
Standard ICAO Beam Tolerance 10 ft
Deviation Warning at touchdown 37 ft (approx. 1/2 dot)
Total 68 ft displacement of the outboard wheels from the runway centre line

It was on account of these possibly additive errors that Cat III authorisation was limited to London's 28L 300 ft wide runway. A lot of effort is being expended on improving the performance of the B747 Autoland system's lateral component especially in regard to the accelerometers - but improvements are slow in appearing. Unless such improvements are made, the aircraft will in effect have no more than Cat II capability on runways less than 300 ft wide.

One of the AP/FD Flight Mode Annunciators. This shows whether the desired autopilot mode has been achieved, and the status of the whole auloland system. Indication is green for mode achieved, red or amber for failures. The test button is being pressed here for the purposes of photography, to illuminate the various displays

FAULTS. Minor problems with the system have been nuisance channel disconnects. Generally, however, the system performs well. If three autopilots are engaged and one of them has an invalid sensor, its annunciator light will show amber. The pilot would disengage the faulty channel, whereupon his weather minima would become Cat ll. If two autopilots are engaged the warning will be red, and disengagement of the faulty channel would leave him at Cat I, with a manual landing from a coupled approach. (No Autoland on one autopilot.)

Autolands are only permitted with two or three autopilots engaged, and with four engines operating normally. The crosswind and tail wind limits are more restrictive for automatic than for manual landing, thus:
Autoland Manual
Crosswind 10 kt gusting 15 25 kt gusting 30
Tail wind 5 kt 10 kt

Autolands also require longer runways than manual landings.

Nevertheless there is a good deal to be said in favour of Autoland. The system never gets tired or has 'bad days', nor is it subject to stress. It saves the pilot from having to be an 'operative' under the most critical conditions, allowing him instead to become a 'manager'; moreover, in an emergency it can provide a means of landing when a landing might otherwise be difficult or impossible. (One B747 made an automatic landing at London with the pilots unable to see through damaged windscreens.) There is no doubt in my mind that a triplex level Autoland system provides a safe means of landing in weather worse than Category I, although Category III performance seems difficult to achieve.

In British Airways the procedure is for the co-pilot to stay 'head down' throughout an approach and to call out all display warnings and information, all the way to the flare 'Green' at 53 ft - not an easy assignment for the nervous! The Captain looks at his flight instruments and for external visual references, and handles any malfunctions of the equipment.

Landings performed by the B747's Autoland system are generally good, and always safe. The only system design deficiency is the placing of the Glide Slope antenna for No. 3 System on the nose gear doors, which means that the landing gear has to be selected down early with a consequent need for a lot of thrust - and an increase in noise on the approach.

The autopilots and flight directors use the same system components, computers, control laws and logic, and it is possible for say 'C' flight director to monitor the performance of 'A' or 'B' autopilots. The same 'Flare' command used in the autopilots is available to the flight directors, although it is British Airways policy to switch off the flight directors at 300 ft on the approach and not to select them until reaching 1500 ft after take-off or go around. All autopilot landings and auto go arounds are therefore monitored from raw data, and manual landings, manual go arounds, take-offs and initial climbs are similarly performed from raw data.

There has not been the same problem of motivating British Airways pilots to use the AFCS that some other airline operators have experienced. This, in my opinion, is because the pilots have been trained and encouraged to use the system in good weather conditions, and because the element of 'compulsion' attempted by some managements has not been applied here.

Inertial Navigation System (INS)
The British Airways B747 is equipped with a triple Inertial Navigation System. Of all the aircraft's features that pilots like, this is the favourite. In my experience it is the only aeronautical innovation within the past 30 years that has lived up to the claims made for it in terms of accuracy, usefulness, reliability and reduced crew workload.

After more than 500,000 hours of experience with INS, there is unanimous agreement in the airline about the value of the equipment. The accountants love it because it has finally disposed of the crew member specifically assigned to navigation duties, and because the fuel savings made possible pay for the equipment in about one year. Even on airways sectors the INS is used to navigate the aircraft over VOR and NDB defined airways, and the uncanny way in which it calculates the moment to turn according to ground speed, and change of track angle, saves minutes per hour of airways navigation. I do not understand why some airlines are purchasing large new jets without INS in these days of high fuel costs.

The pilots like the equipment because it is reliable and simple to use in all circumstances, and because it is pilot operated. For the first time we now know where we are, rather than where we have been; and the instant read-outs of distance, and time to go, make the lives of pilots and air traffic controllers so much easier. The wind velocity and drift angle read-out - which make nonsense of much meteorological theory - are invaluable in holding patterns and during approaches when conditions of wind shear exist.

An important additional benefit is the quality of the INS inputs to the flight attitude instruments. Rotation during take-off to a precise pitch attitude is easier on the INS equipped B747 than on any other aircraft I have flown, and this shows in flight recorder read-outs of aircraft/pilot performance during take-off, when precision flying is necessary to obtain the scheduled performance.

Ground engineers like the equipment because of its MTBF figures and because, as one of them put it, 'at last we are rid of those damned gyros and gyro-compasses'.

Application
The system depends upon the information fed to it by the flight crew. It is therefore vitally important that good operating procedures should be devised and adhered to. The basic need is to ensure that every item of information is double checked before, during and after insertion in the INS computer, and in British Airways the pilots and the flight engineer each have a role to play. The procedures ensure that no single action by a single crew member remains unchecked by the others, particularly if a route and the way points are changed after take-off. For about a year after the start of B747 operations British Airways carried a back-up Loran set, but this was never used and has now disappeared.

The most critical navigation sector flown by British Airways is the London-Anchorage route. This has the aircraft within the Polar area of compass unreliability for several hours. There are very few ground based navigation aids, and reversionary operating procedures have been devised to contend with the loss of any INS navigation information. The minimum equipment list (MEL), or acceptable deferred defect list (ADD), requires that two INS shall be serviceable when a B747 is despatched on the London-Anchorage, Anchorage-Tokyo, Auckland-Melbourne, North Atlantic, or any other route that would previously have required a licensed flight navigator. In practice most pilots would accept despatch with two INS on all these routes except London-Anchorage, for which it seems reasonable to demand that all three INS should be serviceable.

Once en route, the Civil Aviation Authority approved operating procedures require the aircraft to return to the airport of departure should two INS not be serviceable at the point where the region of magnetic compass unreliability is entered. Were a total failure to occur after this point the flight would be completed by using a gyro steering (and precession compensation) technique. Multiple INS failures are extremely rare, and when they have occurred have almost always been traced to an interruption of the electrical power supply (GPU or APU) on the ground during the alignment process. In such circumstances the INS will continue to operate on their own individual stand-by batteries but are deprived of cooling air. Accordingly the current procedure is to switch off all the INS if power cannot be restored in less than 30 seconds, and so avoid 'cooking' the equipment.

On airways sectors it is normal practice to leave the autopilot coupled to the INS, and to use the flight instruments to display VHF navigation information as a cross-check. Even after a longer-than-3500 n mile stage from Miami there is always one (and usually two or three) INS sufficiently accurate to navigate the aircraft along the centre line of the airway to London Airport. A typical system error is 0.5 n mile per hour of flight. Maintenance is called for if the error reaches 3 n miles per hour.

On Polar, Oceanic and Desert sectors where there are no VHF (or MF) ground navigation facilities, the INS is switched into the flight instruments including the flight director command bars. The heading references then all become True instead of Magnetic.

All things considered, the AC/Delco INS equipment fitted in British Airways B747s has proved a thoroughly sound investment, and pilots have thrown away their protractors, dividers, navigation tables and computers without a moment of regret.

One of the  B747's INS controllers. The keyboard permits the pilot to enter lat/long of up to 9 waypoints. Multi-purpose readouts above it will continuously display Nav information as selected by the rotary control switch. Below the Weather Radar controller may be seen the Flight Engineer's computer control, which gives maximum power settings for take-off, climb, cruise & go-around configurations. Ambient temperature is entered, the appropriate selection is pressed, and desired engine power read-out appears on the central instrument panel. This computer eliminates the need  to consult power charts

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