The Comet had grown: the prototype was bigger and, inevitably, heavier than 1946 targets. Span was now 115ft (35m), length 93ft 1inch (28.37m). and weight – initially increased by some 5000lb (2273Kg) – was to increase by a further 2000lb (909Kg) before introduction into service. To offset this, and improve the seat costs/mile ratio, capacity was increased to 36 – arranged as before.
Other details for the Mk.1 were:
Wing area of 2015 sq.ft.
Maximum range (with a maximum payload and allowing for standard airline reserves of fuel) of 2,100 st.miles (3,360Km) with a payload of 8,800 lb. (4000Kg).
Stage length was reduced to 1,830 st.miles (2,928Km) with a payload of 11,600lb (5273Kg).
Cruising speed of 490 m.p.h. at an altitude of 35,000 ft. (10668m).
All-up weight (AUW) of 105,000 lb. (47,727Kg) performance was marginally down on that projected.
Power was supplied by four Ghost 50 Mk.1(D.Gt.3) engines each now rated at 5,050 lb.st. at 10,250 rpm. Later the Mk.4 version of the Ghost 50 provided an equivalent 5,050 lb.st. but at a slightly lower 10,000 rpm. Integral wing tanks held a total of 6050 Imp. Gallons! (Note: for some variants an AUW of 107,000lb (48,636Kg) is given).
Ten Mk1 were delivered to the airline between the April (G-ALYP) and September 1952 – G-ALYZ being the final aircraft of the order.
One month after delivery G-ALYZ was written off when it failed to get airborne from Rome. Accident Investigators blamed this, and a later similar accident to a Canadian Pacific Mk.1A, on pilot error. The pilots were said to have over-rotated during the takeoff. This resulted in the nose of the aircraft being too high, thus the wing ‘angle of attack’ was too steep to get sufficient lift – it ground stalled.
1954 proved to be a momentous year in the history of the Comet. It had profound effects not only on de Havilland but also on BOAC who were to suffer the trauma of their most serious accident in eleven years of operation. Yoke Peter crashed in mysterious circumstances on 10th January 1954 near Elba, Italy. As described in greater detail in the next chapter there were no clues as to the cause but, as a precaution, all Comets were temporarily grounded. Some fifty modifications were made to the aeroplane based on what could only be described as conjecture.
Comet operations resumed with modified aircraft amid a blaze of publicity and, in general, most of the reinstated flights were heavily booked. There was, it seemed, no loss of public confidence in the aeroplane. Then fate struck again, two weeks later Yoke Yoke was lost near Stromboli, off the Italian coast.
Confidence was shattered. The Minister of Aviation Transport had no choice but to order the grounding of all Comet operations. Special crews were dispatched to ferry Comets home form wherever they were when the order to cease operations was received. No fare-paying passengers were carried aboard these aircraft.
A major investigation was set up and a Court of Inquiry sat to consider all the evidence. Apart from specific examinations of those aircraft having accumulated as many hours as the two lost Comets, three lines of investigation were pursued: one of the returned Comets was stripped down and used for extensive structural testing, another underwent special test flights with RAE and de Havilland crews. This aircraft was loaded with strain-gauges and huge amounts of data was obtained for further analysis. Lastly, despite the problems associated with the depth of water at the Yoke Peter crash site, it was hoped that enough wreckage could be recovered to enable the investigation team to pin-point the cause of the disaster.
As a result of these accidents and findings of the subsequent Inquiry the Comet 1 was abandoned for civilian use.
Canadian Pacific Airlines CF-CUM ‘Empress of Vancouver’ 1952
Soon after the Comet 1 was introduction into service the need for greater operational range and higher passenger payloads became readily apparent. Indeed the prospect of future foreign sales would hinge on rapid development of the type. The first result of de Havilland deliberations was the Mk.1A – an evolution of the Comet 1. Designed to carry up to 44 passengers over slightly longer distances (1770 miles) the increased all-up weight (now 115,000lb (52273Kg) was compensated for by the introduction of a slightly more powerful version of the Ghost. The 50 Mk.2 (D.Gt.3) was rated at 5,125 lb.st. at 10,350 rpm. and made use of methanol injection. Fuel capacity was increased by the installation of extra centre bag tanks giving an additional capacity of 856 Imp. gallons.
Externally the Mk.1 and 1A were little different. Dimensions of the 1A were as for the Comet 1 with respect to its length, height, span and wing area. Now 36-44 passengers could be accommodated seated four abreast. Range was estimated at 1770 miles cruising at 490 m.p.h. at 40,000 ft. Total fuel capacity was now 6909 Imp. gallons.
CPA were the first overseas airline to order the Comet – having ordered two Mk.1A’s. Sadly, as explained in the previous chapter, these Comets did not go into service with CPA. A ‘ground stall’ incident (very similar to that which resulted in the writing-off of BOAC’s Yoke Zulu) befell the Mk.1A on it’s delivery flight while attempting to takeoff from Karachi on 3rd March 1953 – it failed to get airborne. All onboard were killed. Investigators attributed the cause to pilot error.
In both these incidents the induction of ground stall was attributed to poor operational technique – i.e. over-rotation – and not to aircraft design. de Havilland however decided that modifications were necessary to prevent even the inadvertent application of ‘over-rotation’ resulting in ground stall. Mock-ups of a modified ‘drooping’ leading edge – ordered by Chief Designer R E Bishop – gave acceptable results during test flights and only had a marginal detrimental effect on the aircraft’s speed.
Interestingly prototype G-5-1 had been fitted with leading-edge slats to prevent stalling but, in numerous tail-down take-offs, they were found to offer little benefit. On the negative side they increased significantly mechanical complexity of the wing and contributed to an additional weight penalty (already problematic) so it was decided to dispense with them. In testing it had been proved that at normal weights and operational temperatures the Comet would take off, tail bumper touching the ground, with no problem.
In fact both ‘ground stall’ incidents had one factor in common – both occurred when the aircraft was ‘heavy’ and operating at higher than average temperatures. Normally scraping the tail in no way affected (critically) takeoff performance.
Peter Bois (de Havilland Test Pilot 1950 – 1956 – Comet Development) explains, “In both these accidents the high weight and temperature were critical. The crew rotated to a point where the wing stalled – thus the drag was so high that the aircraft could not accelerate beyond the stall speed, before running out of concrete”.
So why did the crew attempt to take off in such circumstances? de Havilland, it transpired, had demonstrated to Canadian Pacific crews tail-down takeoffs during both day and night training flights.
“The most probable explanation of this accident was that it was caused by extreme fatigue. The pilots spent the whole day before departure making final arrangements, which they did not complete until about 2 a.m. the next morning. They had, perhaps, three of four hours of sleep before a, circa 8 a.m., departure for Beirut. The flight time was about 5½ hours. The Beirut ground stop lasted some two hours and the flight to Karachi another 5½ hours. A further lengthy ground stop meant that the crew had been active for some 40 hours, with inadequate sleep, before the accident occurred.”
The most tragic fact was that the crew almost got away with it.
“Score marks on the runway indicated that the tail bumper was touching for a distance of about eight hundred metres. Shortly before the end of the runway, the marks ended. Tracks in the sand, past the runway end, showed the nose wheel in ground contact. Some distance on, the nose wheel tracks ceased, followed by those of both main landing gear. Clearly, the aircraft became airborne for a short time before striking a low stone wall bordering a deep gully, into which it plunged and exploded.”
Now operating procedures differ. Take-off speeds are laid down for all normal conditions and are calculated for each particular take-off. Now ground stalls rare because of the introduction of the concept of ‘Rotation Speed’. The pilot does not attempt to lift the nose – rotate – until a certain speed – Vr. This along with the ‘decision speed’ – V1 and V2 (initial climb speed) – allow a smooth and safe transition between having all the wheels on the ground, maintaining a speed above the stall, to being airborne at V2.
The Royal Canadian Air Force purchased the Mk.1A for military transport use. These, after many successful operational flights, were reluctantly taken out of service as a result of the grounding of the Mk.1. They were initially put into storage at de Havilland’s Canadian factory at Downsview, Toronto in April 1954. Following the Court of Inquiry most Comet 2 pressure hulls were re-built with heavier gauge skins and in addition their jet-pipes were ‘swept out’ to reduce buffet. These changes were incorporated into the two RCAF 1As at Broughton, Chester, the first of which arrived on 24th May 1956 after a night-stop at Goose Bay. The modified 1As were designated 1XBs. Once the modification were completed it was intended that the 1XB’s be ferried back to Canada by John Cunningham and crew. The cost of the work was put at £142,000.