De Havilland Comet-Disaster Project Group 6
Background
The de Hallivand Comet of Britain was a revolution in the civil aviation industry It was the first
commercial airliner to use turbojet engines, to have glued skin panels (Redux), to have a
highly pressurised cabin (8.25 psi) and to use high pressure re-fuelling. However the De
Havilland name is relatively unknown compared with market leading Boeing, who’s 707 only
made its first test flight more than 2 years after Comet’s first commercial flight!
So what happened?
The Comet's story began in 1929, when a Royal Air Force officer Frank Whittle proposed
powering an airplane with a gas turbine engine. Rather than applying power to a propeller,
this engine would force exhaust gases out the rear of the engine, generating enough force to
drive the aircraft forward. Eleven years of research and experimentation produced Britain's
first jet fighter, the Gloster Meteor. By 1952, the de Havilland Comet, the world's first
commercial jetliner went into service. Rather than lumbering through storms, the Comet flew
above the weather, eight miles up in the stratosphere. Its air-conditioned, fully pressurized
cabin provided passengers with a quiet, smooth ride previously unheard of in commercial
aviation.
Design
The Comet was an all-metal low-wing cantilever monoplane powered by four jet engines. The
first Comet would carry 36 passengers at a cruising speed of 725 kph. The clean, low-drag
design featured many unique design elements; a swept leading edge, integral wing fuel tanks.
Two pairs of Halford’s Ghost 50 Mk1 turbojet engines were buried in the wings close to the
fuselage. This configuration avoided the drag of podded engines and allowed a smaller fin and
rudder. The hazards of asymmetric thrust were reduced. The engines were mounted higher in
the wings. This reduced the risk of ingestion damage, a major problem for turbine engines.
These benefits were compromised due to increased structural weight including armour for the
engine cells in case of an engine explosion and a more complicated wing structure. The
Comet's thin metal skin was composed of advanced new alloys and was chemically bonded
using the adhesive Redux and riveted. This saved weight and reduced the risk of fatigue
cracks spreading from the rivets.
Failures
On 8 April 1954, the Comet G-ALYY ("Yoke Yoke"), on route from Rome to Cairo, crashed into
the waters near Naples. 60 structural items that were thought to be suspect were reinforced.
However it was only after the second major crash (Comet G-ALYY) were thorough tests
carried out. The Comet (Yoke Uncle) was tested in Farmborough for fatigue failure. The cabin
was supported in water to avoid extraneous weight effects and used to pressurize the cabin.
Calculations indicated that the energy release under cabin rupture using air pressurisation
would be similar to a 2.3kN bomb exploding in the cabin Under this form of pressurisation the
Comet failed after 3057 cycles, (1221 actual and 1836 simulated) well short of its design
cycles of 18,000.
What went wrong?
This discrepancy was attributed to the manner in which the Comet was tested. Instead of the
full-scale fuselage structure, only a part of the cylindrical fuselage fixed at the rigid steel wall
was initially provided for the internal pressurization fatigue test. When designed, the pressure
tests were carried out as static tests with 114kPa (twice operating pressure) being applied.
This did not take into the account the true loading that would be applied to the Comet during
flights. In Farmborough the fuselage was hydraulically pressurised in cycles as well as the
wings being flexed with jacks to simulate flight loads.
Under this true loading, failure would occur at a much earlier stage, i.e. around 3000 cycles.
This underestimation of the true loads involved meant that there was a gross error in the
design life of the Comet.
Technical Failures
The final RAE report determined that the crashes were a result of:
 Fatigue cracks associated with the stress concentrations at the corner of the windows
(due to their sharp edges) with stresses of 315MPa at the edge of the window and a
bolthole around the window of 70MPa stress at the bolt position. The windows were
almost rectangular, with small radius at each corner. Consequently, the stress
concentration factor was high, and fatigue cracks initiated readily.
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The possible contribution from out-of-plane bending loads (bi-axial stresses).
Improperly understood failure mode assessment procedures necessitated by new
technology.
Poor configuration due to wing root engine placement affecting updating potential, fire
hazard, and structural integrity in the event of engine disintegration.
Full-scale water tank testing of a comet (left). The fuselage of the comet (right). Note the square windows.
Managerial and Ethical failures
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A decision was made to set up the production line before the prototype had flown.
Before it flew, agreements to supply 14 aircraft to the British Overseas Airways
Corporation and two to the Ministry of Supply had been signed. Usually prototypes
went through 7 years of testing before widespread implementation could begin. The
comet was rushed through in a mere 3 years.
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It took several accidents before the decision was made to ground the fleet. Many lives
could have been saved if De Havilland had fully investigated the problems.
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Though never conclusively proved it has been suggested that De Havilland knew there
could be problems with the planes take-off performance and was relying on the skill of
the pilot to over-ride this.
Lessons learned
The Comet’s pressure cabin had been designed to a safety factor comfortably in excess of
that required by British Civil Airworthiness Requirements (2.5 x P as opposed to the
requirement of 1.33 x P) and the accident caused a revision in the estimates of the safe
loading strength requirements of airliner pressure cabins. In addition, it was discovered that
the stresses around pressure cabin apertures were far higher than had been appreciated,
especially around sharp-cornered cutouts, such as windows. As a result, future jet airliners
would feature windows with rounded corners, the curve eliminating a stress concentration.
From these accidents we have learned how to better prepare against fatigue failure and we
have a better understanding of the interaction of stress concentrations with fatigue
Technological Outcomes:
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Full-scale and dynamic testing of aircraft structures utilised in future aircraft.
Better understanding of fatigue testing achieved, i.e. match service and test loads (no
previous over-pressurisation cycles first).
Attention drawn to detect ability/critical size issues for fatigue cracks in aircraft
structures.
Concept of ‘one-bay’ crack tolerance in fuselage probably formulated. (Water tank to
encase a concept of ‘one-bay’ crack tolerance in fuselage.
First large scale reconstruction, use of underwater cameras and medical forensics as
part of an accident investigation.
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For the development of new aircraft, two full-scale structures must be provided for
testing: one is for the static load test, and the other is for the fatigue test including the
pressurization test
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In conducting the full-scale fatigue tests, in addition to the effect of the magnitude and
number of fatigue loads, the effect of the load sequence must be taken into
consideration.
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Stress concentration factors of cut outs and notches must be correctly evaluated, and
they must be minimized in the design of the structure as much as possible.
Conclusion
At the time of the accident investigation, the effect of pressure overloads on fatigue life had
not been correctly recognized. To make progress engineers must be prepared for the possible
risks involved and endeavour to foresee and prevent them. They will invariably be unable to
foresee and prevent them all. However, there were significant errors in the management of
the project, which resulted in the needless loss of more lives
References
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P A Withey (1997) Fatigue failure of the De Havilland Comet 1 Engineering Failure Analysis Vol. 2 No. 4
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