Comet 4 Development to CoA
History was made just over four years after the Comet Mk.1s were withdrawn from service. The first series Mk.4 (G-APDA) made its maiden flight on 27th April 1958 at Hatfield. It was airborne for 83 minutes and was crewed by John Cunningham (Chief Test Pilot), Peter Buggé (Chief Development Test Pilot), E. Brackston-Brown (Chief Flight Engineer), J.L. Johnson (test flight observer) with J. Marshall (instruments observer).
The Comet 4 was a completely new aeroplane: one with over 50,000 hours of test and operational flying and a programme of scientific proving and ground testing which was unprecedented in aviation history, behind it – 80%, of which had been completed, with the Mk.3.
Comet 4 simply had to be the safest aeroplane ever to fly.
The test-rig programme set up to investigate the fatigue failures had continued to work with a view to gaining more information about both materials and new anti-fatigue design concepts. The program was then re-directed to test new components for the Mk.4. Once the designers were satisfied that particular components met their ‘new’ specification they were incorporated into a ‘whole’ structure, e.g., into a complete wing. This was then rig-tested, eventually to destruction. Component testing covered materials, windows, canopies, the centre section and spars as well as the undercarriage. All seams and joints were carefully investigated for soundness or signs of impending failure.
All this was necessary because the aim of de Havilland, and the ARB, was to ensure a safe-life for the Comet above its useful operational life – assuming the latter was not less than 30,000 hours. To achieve this exacting requirement components were often tested to factors of 5 times (as in the case of the wing) or 6 times (for the pressure cabin) in excess of this ‘life’.
The effectiveness of this programme was demonstrated when, at a later stage, it was decided to produce the short-medium range Comet variant – the 4B. This aircraft would be subjected to greater stresses than long haul types because it would have to operate in more turbulent conditions prevalent at lower altitudes and make many more frequent takeoffs and landings. The basic structure proved to be extremely robust – so much so that the 4B only needed minor modifications.
de Havilland had learned that, in practice, it was not always possible to design components to be fail-safe but, wherever it was possible to do so the company adopted that approach. Sometime though a compromise was necessary and a ‘safe-life’ for any component or structure would have to be demonstrated.
The number of changes incorporated into the Mk.4 were enormous. They reflected not only the experience gained with Comet 1 but also, after a gap of some eight years, the many new concepts and practices that evolved since.
An example, and this was due to experience gained directly from the Comet 1, it was not sufficient just to cant outwards the engines jet pipes, for the Comet 4 there had to be extensive modifications to the rear fuselage to enable it to withstand better the effects of jet-blast. The tailplane and elevators too were given thicker skins and the ribs doubled-up to help withstand the ‘noise’ vibration induced by the turbo-jets. A new ‘aged’ aluminium alloy (24ST) was used on the lower tension surfaces of the wings and, on the same theme, Zinc-alloy sinks were eliminated from the fuselage. Much stronger steel lug-forgings were used for the attachment of the wings and tailplane etc. In fact the Comet 4 differed in almost ever respect from the Comet 1.
Many lessons were learned too with the operational Comet 1 with respect to aircraft systems. As a result many changes were incorporated into the Mk.4. For example, electrical distribution was by separate busbars backed up with separate emergency busbars. More powerful alternators were used at 350 amps and all the electrical equipment had been explosion tested! Problematic multi-pin plugs had been eliminated where possible and American type connectors, inverters and terminals were now used.
The Comet still featured fully powered flying controls but modified with adoption of ‘Q feel’ The system gave an artificial feel to the controls proportional to airspeed. The system was applied to the elevators and also used to restrict rudder movement above 180 knots (this was done because too much rudder application at speed could induce excessive yaw and thus induce possible over correction). The system was not applied to the ailerons. Detail changes e.g. to reducing friction within the control system resulted in the criticised break-out force of the Comet 1 being halved to around 10lb.
In the Comet 1 the crew had to manually compensate for trim displacement when using the autopilot. The Mk.4 had the Smiths S.E.P.2 autopilot which automatically adjusted the trim. Also new was an automatic approach (I.L.S.) coupler included in the autopilot system. A stick shaker had been added to give warning when a stall was imminent – this was far superior to the previous stall warning indicator i.e. the whole airframe shaking! This stick shaker, and alterations to the elevator gearing (making displacement of the stick greater during takeoff and landing – i.e. more coarse – than during normal flight), had been incorporated into the Comet 2 also.
A Mach sensitive device (which had been devised for the French and Canadian Mk.1As) was incorporated into the elevator circuits which applied an amount of up-elevator when a pre-set Mach. number was reached – in this case 0.77M. Other safety features included duplicated powered input and output control of the elevators to guard against system failure. To this end too power to the ailerons was duplicated – in fact all the major control systems and undercarriage had duplicated hydraulic supplies Also revised were the recommended flap settings and there was the addition of a yaw-damper.
AiResearch experience had enabled Normalair to produce a very sophisticated pressurization and air-conditioning system for the comet 4 – and it was to a great extent was automatic in operation. Once the altitude at which pressurization was to begin was determined and set, such things as temperature, air mass-flow, air recirculation and rate of climb were automatically dealt with. There were special cold-air units incorporated into the system and these were made by de Havilland Propellers under licence. De-misting was taken care of by having gold-film heating elements embedded in the cockpit glass.
The Avon engines were protected by a new anti-icing system as was the airframe. These revised systems had been extensively tested on the Comet 2X. In the cockpit the black background to the instruments had been replaced by the fashionable grey – which was said to make the instruments and controls stand out better. The engineers panel had been revised with the various banks more logically arranged. For the 4B and 4C the pilot’s windscreen panels were made some 4 ins. deeper and greatly improved visibility.
The fuel system too was extensively revised and, as in the Mk.1, each engine had an independent fuel feed. New safety pressure-relief valves now protected the pressure refuelling system. Fire safety had been a particular concern of the American Civil Aviation Authority because of the Comet’s buried engines. R.R. built a special test ‘fire tunnel’ at Huchnall to demonstrate the extinguisher systems. Rolls-Royce developed noise suppressors for the jet pipes and were developing thrust reversers – at this stage only tested on the Comet 3 (Note: a prototype thrust reverser had been demonstrated in tests on the 2X).
All these modifications made the Mk.4 far superior to its predecessors. For the passenger too a new ‘bright’ revised cabin interior added to the sense of luxury and was more than a match for its rivals. BOAC were planning to carry 16 passengers, four abreast, in seats with a pitch of 56 ins. in their deluxe class cabin and 43 passengers, five abreast, at 40 ins pitch in tourist class. The spacing was generous by any standards.
Statistics for the Mk.4
Range was now 2720 nautical miles in still-air at maximum AUW, initially 156,000lb with reserves (note: maximum range was calculated as 4030 n. miles in ideal conditions – dry, still air – with no reserves) Optimum cruising speed was 505 m.p.h. (438 Knots) at 28,000ft when the rate of fuel consumption would be 9,650lb/hr. Cruising altitude in excess of this was often used, 36,000ft being typical.
So the Mk.4 had North Atlantic capability … however a stop-over was necessary in most cases operating westbound. Capacity payload was 20,286lb and gross weight, less fuel and payload 75,424lb. Max. take-off weight was 158,000lb (it was gradually increased in stages to 162,000lb) and landing weight 120,000lb.
Cabin volume was 2,815 cubic feet with a freight/ baggage volume of 570 cu.ft. The usable floor area was 439 sq.ft in a fuselage with internal dimensions 71ft 8in long, maximum width 9ft 7in. and height 6ft 6½in.
The Mk.4 would accommodate – as a typical mix – 24 first-class passengers and 43 tourist class. Capacity depended on pitch adopted and number of seats abreast: In Deluxe class with a four abreast/ 56in pitch seating could be provided for 40 passengers; in first-class 40in pitch for 56 passengers; or five abreast/40 in pitch for 71 tourists; and finally economy-class with 34in pitch for a cramped 81 passengers (see Dan Air).
Power came from four RR Avon 524s (RA.29/1) which were rated at 10,500 lb.st each. Fuel capacity was 8,898 Imp. gallons (the pod tanks each held 440 Imp.Gall.)
From the outset it was planned to set up a second production line and this was done at the Hawarden Aerodrome, Chester – which was part of the de Havilland Company. In fact fourteen of the nineteen Mk.4s for BOAC were to be built there. In all 16 Mk.4s, 2 Mk.4Bs, 9 Mk.4Cs and 5 C.4s were built at Chester.
The Air Registration Board had spent more than 10 flying hours in the Comet Mk.3 on certification work. Limited certification had been granted for the Mk.4 to enable training and route proving flights to begin. The ARB still required, however, another 250 hours of test flying before full certification would be granted and 100 hours of that had to be under operational route-proving conditions.
In practice basic evaluation by de Havilland took few flying hours – such was the accuracy of the Mk.3 data. Some new systems (not found on the Mk.3) were adopted for the flying controls and these, and their new emergency back-up systems, were relatively easily assessed. There were no problems.
Route proving therefore took up most of the necessary 250 hours for the ARB and BOAC were well placed and ready to put the Mk.4 to use. BOAC had at a late stage decided to have emergency passenger oxygen supplies fitted before their scheduled services were due to start. This was already a requirement in the U.S. The Comet system was designed by Walker Kidde Ltd. and in use the masks automatically spring from the hat-rack in the event of decompression.
Demonstration flights and records
de Havilland meanwhile undertook a series of their own long distance certification trials/demonstration flights. With G-APDA, for example, John Cunningham covered a distance of 7,925 miles on route from Hong-Kong to London in a flying time of 16 hrs 16 min. – average speed of 487 mph.
During August 1958 G-APDA returned form tropical trials in Africa at Khartoum, Wadi Halfa, Nairobi, and Entebbe.
Towards the end of August ‘DA made history crossed the North Atlantic New York to Hatfield in 6 hrs and 27 min. knocking 1 hr. 17min off the previous record! Crew: John Cunningham, Peter Buggé and Peter Wilson joined by BOAC pilots Capt. N.A. Mervyn-Smith, Ward and C.T. Farndell.
September saw ‘DA fly out to Hong Kong for route proving and for a special occasion too – the inauguration of Hong Kong’s new runway at Kai Tuk.
On the return flight further records were set – a distance of 7925 miles in an elapsed time of 18 hrs. 22 min with two refuelling stops at Bombay taking 1 hr. and at Cairo 1 hr. 6 min.
The final de Havilland demonstration / test flight in G-APDA added 48 hours to the ARB total covering 23,000 miles in ten days via Ottawa, Gander, Toronto and to de Havilland’s Canadian factory at Downsview. Vancouver, Mexico City, Lima, Buenos Aires, Rio de Janeiro, Caracas and then on to New York and from there a non-stop flight to Hatfield.
On this tour some 600 people flew in the Comet.
Records were set on every sector it flew. Later when asked about the Comet’s serviceability record John Cunningham said that on the third 23,000 mile tour there had been no snags of any kind. All that had been required was fuel and three pints of oil! On the previous flights too reliability had been excellent. There had been only one minor snag – and that was the failure of an electric actuator. The aircraft had also demonstrated its ability to rely on its own batteries for starting in the absence of ground power supplies. Very impressive.
In September 1959 de Havilland announced an increase in the payload weight of the Comet Mk.4 and Mk.4C from a certified 158,000lb to 162,000lb. This increase (without increasing the tare weight) had been made possible by adopting the 4B stub wing for the Mk.4 and 4C. Carriers could either extend the range with capacity payload or increase payload over longer stages.
Certificate of Airworthiness
On September 29th 1959 the Minister of Transport recommended that the ARB issue a C of A for the Mk.4.
The following day – on schedule – de Havilland delivered G-APDB and the second production G-APDC to London Airport for the hand over. Present at the hand over ceremony was the Minister – Mr. Aubrey Burke – who brought with him the Certificate. This he duly handed to Sir Geoffrey de Havilland who, in turn, handed it to Sir Gerard d’Erlanger, Chairman of BOAC.
BOAC transatlantic services began on Saturday October the 4th. just before Pan American introduced transatlantic scheduled services. Comets were only destined to ply the north Atlantic for two years.