While the Comet 4 was under development de Havilland started to consider a number of variants.
The first such announced was the Comet 4A. In the ‘Enterprise’ (the internal magazine of de Havilland Companies) August 1956, details of this Short to Medium-haul Express Airliner were given. Not only that but the Enterprise was, “pleased to be able to include the announcement which gives us all reason to be justly proud”. This was news of an order worth £19 from Capital Airlines Inc., of Washington D.C. which included Mk.4A.
The 4A – it was described as offering airline operators a high-speed jet aircraft capable of operating over short and medium stage routes at competitive costs.
Based on the longer range Comet 4 it differed from the 4 in that the fuselage was 40 inches longer – thus up to 70 passengers could be carried four abreast in first class or up to 92 five abreast in tourist class.
Wing span was reduced from 115ft (4) to 108ft. This change, along with some structural strengthening of the rear fuselage and tail, enabled the Comet 4A to operate at higher cruising speeds benefiting commercial unit cost.
Expected performance characteristics for the 4A were derived from data gained with the Comet 3. Special operating techniques were tested to give improved economy. Because the speed of sound varies with altitude 4A was to cruise at a lower level therefore it’s true airspeed was increased relative to the limiting Mach number (Mno).
Although the lower cruising altitude resulted in greater fuel consumption this was more than offset by the increase in cruising speed. This technique further required that the rate of climb and descent be increased thus maximizing the time spent at cruising a altitude.
Adopting these procedures the 4A was to cruise at between 520 and 545 mph at 23,500ft – differences being due to variation in the ambient air temperature.
The 4A would offer good operating economy on stage lengths of 500 miles or more and this technique held good for stage lengths up to some 2000 miles. Above this distance higher altitudes were recommended – as for the Comet 4. It was said that, if need be, the 4A could be operated on the long-range technique but the penalty of doing this was that the longest practical stage length was effectively decreased. Logically the 4 was more suited to main trunk routes. So 4As advantage was in its greater degree of operational flexibility over short and medium distance routes.
At its maximum payload of 22,690 lbs (seating 92) maximum stage lengths in still air (with full fuel reserves) was quoted as 1880 miles. This figure increased to 2040 miles when carrying 19070 lb. e.g. with the 70 seat, all first class, configuration. But any combination between the maximum 70 first class and 92 tourist class configuration were possible.
The 4As all-up weight was planned to be the same as for the Comet 4 i.e. 152,000lb. Take-off and landing performance was said to be very close to the 4 which was described by most pilots as being superb. It was intended to fit corrugated jet pipe nozzles to further improve external noise suppression – these you will recall were first demonstrated on the Comet 3 (Greatrex type).
The 4A was to be powered by four Rolls-Royce RA 29s. When calculating their performance data for the 4A, de Havilland made use of RR test-bed data. However RR were expecting specific fuel consumption to be improved early into the engines production life. Even so the de Havilland 4A data assumed a cruising rpm. well below the RA29s capabilities. Lower operating rpm. resulted in less ware and enabled longer overhaul intervals to specified. At this time RR expected overhaul life reached 1500 hours within three years of the start of commercial operation.
The ‘Enterprise’ also commented that the RA.29 was not new to de Havilland. There was much in flight experience with this unit totalling some 10,000 hours of high-utilization in the Comet 3 and the Comet 2X.
The Comet 3 had been operating with RA.26 engines rated at 10,000 lb.st. thrust. The RA.26s were replaced with RA.29s (rated at 10.500 lb.st.) and combined with the shorter wingspan of the 4A it was to be used for certification flying for the 4 and 4A. Thus a fully representative prototype would be flying eighteen months before the 4A was completed.
Capital Airlines planned to operate Comets on their New York – Chicago route from the 1st January 1959. As a result of the Capital contract another 800 staff would be needed within de Havilland and it would also mean £25,000,000 worth of business for Rolls-Royce.
Unfortunately due to financial problems Capital had to cancel the order and the 4A was never produced. Capital had got into short term difficulties but by the time matters were resolved United Airlines had virtually absorbed Capital lock, stock and barrel.
United it will be recalled were formed from Boeing’s own commercial airline – so they were hardly likely to by Comets anyway.
THE COMET 4B.
The projected 4A indicated to BEA that a variant on the 4 may suit their routes too. The fuselage was lengthened to 118 ft and the wing span reduced to 107’ 10″. Wing area was thus reduced to 2059 sq.ft. Height remained the same as the Mk.4.
Range obviously varied with load and stage-length but from 2500 to 3350 miles could be expected. Capacity payload was 24,137lb and gross weight less fuel and payload 78,363lb. Max. takeoff weight was 158,000lb (increased in stages to 162,000lb) and landing weight 120,000lb.
Cabin volume was 3,160 cubic feet with a freight/ baggage volume of 640 cu.ft. The usable floor area was 529 sq.ft in a fuselage with internal dimensions 78’ 2″ long, maximum width 9’ 7″ and height 6’ 6½”. This allowed seating from 84 to a very high density 119 (achieved by Dan-Air London)
Power came from four RR Avon 525Bs (RA.29/1) which were rated at 10,500lb.st each. Fuel capacity was reduced to 7890 Imp. gallons and the cruising altitude reduced to 23,500 ft although, as will be seen later, on the longer routes 30,000ft plus was used.
The 4B was the fastest Comet capable of up to 532 m.p.h. but optimum cruising speed was 520 m.p.h. at 23,500ft and 135,000lb giving a rate of fuel consumption of 11,500lb/hr.
Although originally only purchased by BEA as a stop-gap measure – (pending the arrival of the DH. 121 they were losing traffic to the Caravelle) it proved to be ideally suited to BEA’s short-haul European routes. As the certificated weight increased so did its range and eventually it was used on long charters. To most the 4B variant was the most attractive of all the Comet models.
An interesting development resulted from the following incident – Capt. Peter McKeown and crew were on a BEA proving flight from Copenhagen when, in mid-Feb. 1960, the 4B took-off from a slush covered runway. Moments later two engines began to run roughly – these were hastily shut-down. Fate being what it is soon after that a third engine ran rough. Capt. McKeown managed to restart one of the shut-down engines and managed to make an emergency landing on two engines.
The problem was traced to slush being thrown up from the nose wheel – grit contained within the slush damaged the engines. Three of the Avons were changed.
Following this incident de Havilland tested nose-wheel tyres with a different tread-pattern and also the effectiveness of slush guards. If tests proved successful it was planned to incorporate these modification on all Mk.4B Comets. However, conditions at Copenhagen had been unusual as a heavy snow fall was followed by a rapid thaw. On this occasion the slush was deeper than would usually be experienced – such conditions had to be catered for though.
Developing the 4B
Much of the development work for the 4B fell to the 3B and provided ‘hands on’ experience for trainee crews.
Data produced on the 3B was carefully check on the first production 4Bs. The 4B, in short to medium-haul use, would naturally experience far more stress than the Mk.4 but it was more than up to the task.
Modification were found to be necessary to the wing because once the pod tanks had been removed the shortened wing became susceptible to pre-stalling buffet. Not only was this disconcerting but the sometimes severe vibration induced could be felt in the cockpit.
Such behaviour was unacceptable to de Havilland and the ARB. The solution was to fix spoilers to the leading edge and move the boundary fence some 4ft inboard. This resulted in a length of conduit which ran chordwise over the top of the wing – the purpose of which was to cover wiring which had previously been concealed in the fence in it’s original position. An effective if not particularly elegant solution!
4B manners then became impeccable – the stall was classic and although, in certain conditions, a wing would drop it was easily correctable with a touch of aileron.
One of the more spectacular demonstrations laid on by de Havilland were ‘tail-scrapping’ take-offs at Hatfield. It was said that apart from the unnecessary stresses such manoeuvres impose on the airframe the greatest problem likely to be encountered by the pilot was with his arm muscles! The 4B was a very safe aeroplane.
Registered for BEA G-APZM was leased to Olympic Airways from July 1960 to April 1966
In November 1960 a BEA 4B taxiing for takeoff from Zürich suffered a fracture of the nose-wheel axle and the loss of both nose-wheels.
There had been a similar incident at London Airport a week earlier and that had resulted in a temporary withdrawal of 4Bs for inspection.
After the first incident de Havilland called for crack inspection by dye penetration, chromium deposit and shot-peening. After the second incident a number of axles were replaced and de Havilland recommended that an axle sleeve be fitted to strengthen it.
It was very significant though that no such problems had been found on the Mk.4 or 4C and that particular 4Bs had accumulated more takeoffs and landings than had any other.
Flying the 4B
Assuming that the aircraft was at the maximum take-off weight of 158,000lb (71,650Kg) full throttle would spin the Avons to 8050 rpm.
Vr (rotate) would come up at approx. 133 Knots and V2 (take-off) at 143.
By 1,500 yds. the aircraft was airborne and, if noise abatement measures were needed, a climb rate of 150 Knots I.A.S. was adopted. Flaps were retracted at 1,500 ft (170-180Knt.I.A.S.). Power was pulled back at approx. 2000ft and cruise-climb used from 290 Knots and held to approx. 6000ft.
Speed was then allowed to increase to 330 Knots (or Mach 0.76) for the rest of the climb to cruising altitude. At cruising altitude power was adjusted taking account of the outside air temperature. Height-lock or speed-lock was then selected on the autopilot as desired.
On approach to the destination, at say 200 st. miles, a descent was begun. As with the Mk.4 various techniques could be used. In the event of decompression a rapid descent would be necessary. With the use of air-brakes 10,000ft/min. would be possible – but this was a maximum rate. 7,000 to 8,000ft/min. being more acceptable – generally the 4B would be operating at lower altitudes than the 4 in any case.
On a normal descent the outers were throttled back to idle speed and the inners adjusted to around 6,500 rpm. At approximately a thousand feet 20° flaps were selected and the throttles adjusted to 6,500 rpm. (if into a strong wind another 200 rpm. could be required) or to give a speed of 170-180 Knts I.A.S. Gear-down was selected and 40° flap set. Speed would now drop to 140-150 Knts on the same power setting.
With the end of the runway lined up, and satisfied that the Comet would not undershoot, 60° flaps were selected. Speed would drop to approx. 130 Knots for the approach to the threshold. Full 80° flap knocked a further 7 Knots off the speed and once across the threshold, with the stick held back and the nose raised, a smooth landing was made. Once all the wheels were down reverse thrust was selected and held – amid great din – until 60 Knots reached when either the Comet was allowed to roll to a standstill or the Maxaret brakes were used.
THE COMET 4C.
The 4C was the next logical step in the evolution of the Comet. Essentially the configuration was the same as the Comet 4 but combined with the lengthened fuselage of the 4B. This hybrid had a span of 114’ 10″, length 118’, height 29’ 6″ and a wing area of 2,121 sq.ft..
Range obviously varied with load and stage-length but from 3560 to 4,350 st. miles could be expected. Capacity payload was 23,145lb and gross weight less fuel and payload 79,355lb. Max. take-off weight was 162,000lb and landing weight 120,000lb.
Cabin volume was 3,160 cubic feet with a freight/ baggage volume of 640 cu.ft. The usable floor area was 529 sq.ft in a fuselage with internal dimensions 78ft 2in long, maximum width 9ft 7in. and height 6ft 6½in. This allowed seating to be provided for 72 to 101 passengers – the 4C was generally fitted out at a lower density than the 4B.
Power came from four Avon 524s which were rated at 10,500lb.st each. Fuel capacity was 8990 Imp. gallons (the pod tanks each held 440 Imp.Gall.) and the 4C used cruising altitudes of up to 39,000ft.
The 4Cs optimum cruising speed was 500 mph (435 Knots) at 33,000ft and 135,000lb giving a rate of fuel consumption of 8,800lb/hr.
The 4C sold itself particularly to airlines based at high altitudes especially after the Mk.4 had demonstrated its exceptional capability when operated from Mexico City airport – some 7340 ft above sea level. Mexicana were the first airline to operate the 4C in 1960. U.A.A., M.E.A. and R.A.F. Transport Command also operated them.
An interesting development came in September 1961 when against competition from almost every jet airliner manufacturer in the world de Havilland sold a very special 4C. It was to be used as a mobile Royal Palace for King Ibn Saud of Saudi Arabia. Naturally it was to be specially furnished – the cost of the aircraft was put at about £1.25 m.
Comet 4C – 9K-ACE Kuwait Airways
In August 1961 de Havilland announced that the cruising speed of the 4C could be increased by 20 Knots because of a recommended increase in the cruising rpm. of the Avons. The difference of 0.03 Mach. allowed the corresponding increase in the permissible indicated airspeed.
Production to end?
In 1960 there were rumours in the press that de Havilland was to cease production of the Comet because of lack of orders. de Havilland issued a denial and stressed that prospects for sales had improved greatly due to, “the growing realization that a moderate sized jet was urgently needed to avoid the cream of the traffic going to those carriers which already possess jets.” Sadly though the number of firm orders for the Comet did not match Company expectations – the Company’s delivery rate had been exceptional and almost all the aircraft were handed over in advance of the contract date – sometimes to the embarrassment of the purchasers who’s crews were not yet fully trained.
The Comet was not abandoned but de Havilland were now heavily involved in the DH 121 ‘second generation’ of short – medium haul passenger jet transports.
However in January 1963 Hawker Siddeley announced that the Ministry of Aviation had placed an order for a Comet Mk.4C for delivery in the autumn of that year. It was to be used for experimental research into the problems of long-range navigation and instrumentation and would be based at A & A.E.E. Boscombe Down.
It was to be equipped to carry a large amount of experimental equipment. In addition to the flight crews and research technicians, it would carry it’s own maintenance crew so that it would be able to operate independently on extended sorties around the world.
This was in addition to the five 4Cs and 10 Comet 2s already operating with Sqd. 216 RAF Transport Command. This aeroplane was delivered to A & A.E.E. on the 2nd December 1963 as XS235.
COMET SERIES PRODUCTION.
12 Comet 1s between July 1949 and Nov. 1951.
10 Comet 1As between Aug. 1952 and May 1953.
11 Comet 2s between Jan. 1953 and Jul. 1957.
1 Comet 2E between Jan. 1953 and Jul. 1957.
3 Comet C.2s between Jan. 1953 and Jul. 1957.
1 Comet 3 by Jan. 1954.
28 Comet 4s between May. 1957 and May. 1967.
18 Comet 4Bs between May. 1957 and May. 1967.
26 Comet 4Cs between May. 1957 and May. 1967.
5 Comet C.4s between May. 1957 and May. 1967.
115 IN TOTAL