4A6(1) Structural Design
Group 6
The de Havilland Comet
Fatigue Failure
Background
In September 1946, Sir Geoffrey de Havilland began design work, under the
direction of Ronald Bishop, for the Comet. It was the world’s first turbojet powered
airliner, and with four de Havilland Ghost 50 turbofan engines, of 22kN thrust, it would
fly at double the then cruising altitude of 12.2 km. The Comet was the first aircraft to use
hydraulically actuated controls, to use high pressure refuelling and to have a highly
pressurised, and air-conditioned, cabin. De Havilland also pioneered glued skin panels
(‘Redux’ panels) to keep the weight of the aircraft down. The Comet could carry 36
passengers, with a cruising speed of 725 km/h and a range of 4000km.
The de Havilland Comet 1 (G-ALYP)
On the 27th July 1949, the DH 106 Comet flew for the first time. This was the first
prototype flight but still 16 aircraft had already been signed for, 14 to British Overseas
Airways Corporation and 2 to the Ministry of Supply, and a production line set up. This
was a huge managerial risk to take as any alterations to the design from now on would be
costly to implement.
After 3 years of testing, the first passenger flight took off from London for
Johannesburg in May 1952. The flight time was cut from 40 hours to 23, and due to the
pressurised cabin the plane was able to fly above storms, hence providing a much quieter,
smoother flight. In its first year of operation the Comet carried 28,000 passengers and
travelled over 104 million miles.
4A6(1) Structural Design
Group 6
The Problems Begin
However the success was soon to be outlived, on the 26th October 1952 the first
accident occurred while departing from Ciampino airport in Rome. Luckily there were no
fatalities and the incident was blamed on poor pilot handling of the aircraft, however it
seems de Havilland knew there could be a problem with the aircraft during take-off but
were relying on the pilot’s skill to overcome this.
Following this, on the 3rd March 1953 a new Comet 1A (CF-CUN) was being
delivered and failed to gain altitude on take-off from Karachi, Pakistan. The plane
collided with a bridge and caught fire, killing 11 crew members and others on board. It
had been loaded to almost the maximum permissible weight but a flaw in the design was
also found, it was discovered that during a steep ascent a large area of the wing lost its
lift. A design review followed this and the leading edge of the wing was re-profiled, to
provide greater lift, and a wig fence was added, to control spanwise flow.
By May 1953, de Havilland had firm orders for 50 Comets and was negotiating
100 more.
The first fatal accident involving passengers was on 2nd May 1953. A Comet 1 (GALYV) broke up, mid-air, 50km from Calcutta. The crash was attributed to excessive
stresses in the airframe due to a tropical storm, but subsequent accidents indicated a weak
structure was the more likely cause.
On the 8th January 1954 one of the most serious accidents occurred, Comet GALYP, bound for London, exploded in good weather ascending from Rome to 8200m
and crashed into the sea near Elba. All 35 passengers died.
The fleet was grounded and an investigation began and the Royal Navy started to
recover the aircrafts remains. The results of the investigation led to a design review, in
which “modifications [were] embodied to cover every possibility that imagination [had]
suggested as a likely cause of the disaster”; shields were installed between the engines
and the fuel tanks, smoke and fire detectors were installed and the fuel lines were
reinforced.
On the 23rd March 1954 the fleet re-entered service. In less than a month, on 8th
April 1954 Comet G-ALYY ran into difficulties 30 mins into flight while ascending
towards 10.6 km. The wreckage was found in the sea near Naples and the fleet was
grounded once more. A full investigation by the Royal Aircraft Establishment (RAE),
under the direction of Sir Arnold Hall at Farnborough, began and a court enquiry was
established. The result of which was that the plane suffered a catastrophic explosion of
the fuselage and the Comets were withdrawn from service.
On 12th April 1954 the Ministry of Transport and Civil Aviation removed the
certificate of airworthiness.
The square windows of the Comet 1 were redesigned as oval for the Comet 2, and
the skin sheeting was thickened slightly. The remaining Comet 1s and 1As were either
scrapped or the windows modified. But the Comet did not resume commercial airline
service until 1958, when the Comet 4 was introduced (with better engines, greater fuel
capacity, and a lengthened cabin for additional passengers) and became the first airliner
to enter transatlantic service.
4A6(1) Structural Design
Group 6
However, the 4 year hiatus gave other aircraft manufacturers the opportunity to
profit from de Havilland’s hard-learned lessons The rival Boeing 707 and Douglas DC-8
had claimed the bulk of the market and only about 90 Comets ever reached the
commercial market. Had the Comet not been plagued by a fatal design flaw, Britain
might well still dominate commercial aviation today.
The Reasons
Investigators at the RAE examining the Comet G-ALYP carefully studied the
cabin pressurisation by loading a full-size Comet into a water tank, and repeatedly
pressurising and depressurising it, to simulate cabin loading at 35,000 ft, for a period of
9000 hours. When the tank was drained, a crack was located in the fuselage. This was a
fatigue crack due to stress concentrations at the rear ADF window cut-out of 315MPa
around the window and 70MPa at bolt positions. The stresses around pressure cabin
apertures were higher than anticipated, especially around sharp cornered windows. When
these cracks reached a critical length they would rapidly increase in size causing a rapid
depressurisation of the cabin resulting in an explosion that would destroy the aircraft.
The problem was exacerbated by the punch rivet construction technique used. The
windows had been engineered to be glued and riveted, but had been punch riveted only.
The imperfect nature of the hole created by punch riveting may have caused the start of
fatigue cracks around the window.
G-ALYU Fuselage failure – inside
G-ALYU Fuselage failure - outside
The technology introduced was cutting edge and with all new technology there
are risks involved with entering unfamiliar territory. There were new load cases and
failure modes to be analysed and new theories had to be developed to deal with all of this.
There was also a possible mis-match between service loads and fatigue test
procedure, a possible contribution from out-of-plane bending loads (bi-axial stresses).
Finally, a poor configuration due to wing roof engine placement, affecting the updating
potential, posing a possible fire hazard and affecting the structural integrity in case of
engine disintegration.
4A6(1) Structural Design
Group 6
The Results
The lessons learnt from the Comet disasters, led to the revision of the safe loading
strength requirements of airliner pressure cabins; even though the Comets had been
designed in excess of current requirements, 2.5P as opposed to 1.33P (where P is the
cabin proof pressure), they had still failed.
The design of the different elements in the cabin was revised to eliminate the
sharp corners which concentrated the stresses; hence the reason plane windows have
rounded corners. Full- scale testing of aircraft structures was implemented and attention
was drawn to detection of and the critical size for fatigue cracks in aircraft structures,
with the concept of ‘one-bay’ crack tolerance in fuselage properly formulated.
Finally, thanks to the pioneering experiments at Farnborough a better
understanding of fatigue testing was established, by accurately matching service and test
loads.
References:
“Structures or why things don’t fall down”, J.E. Gordon
“Fatigue Failure of the de Havilland Comet”, P.A. Whitney
www.tech.plym.ac.uk/sme/Interactive_Resources/tutorials/FailureCases/sf2.html
http://aerospaceweb.org/aircraft/jetliner/comet/
http://en.wikipedia.org/wiki/De_Havilland_Comet#Production_and_service_summary