Comet Aircraft – The Worlds First Commercial Jet Airliner
Fatigue Failure
Background& Description
In the 30’s and 40’s, technological advances in military aviation marked the introduction
of jet-powered aircraft. In 1952, the de Havilland Comet became the first commercial jet
airliner. Sir Geoffrey de Havilland’s design work had begun in September 1946, and the
prototype first flew on 27th July 1949.
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.
The aircraft could carry 36 passengers at a cruising speed of 725 kmph with a range of
4,000 km on entering the service. For commercial viability, the weight of the aircraft and
fuel had to be kept to a minimum. Rivets were used in certain areas as well as gluing the
aircraft skin and stringers, a method known as “Redux” pioneered by de Havilland to
reduce the weight of the structure whilst maintaining strength. Four Ghost turbofan
engines of 22 kN thrust powered the aircraft. In order to run the engines as efficiently as
possible, the aircraft was expected to fly at double the cruising altitude of the then
commercial airline fleet (12.2 km). This high altitude meant that the passengers would
require artificial oxygen to be supplied, and the cabin to be pressurised to that of a cabin
flying at 2.5 km. This caused a pressure differential across the aircrafts’ skin of 56kPa at
cruising altitude. Other new features included high pressure refuelling, the hydraulic
actuation of the control surfaces, and an air-conditioned cabin.
Figure 1 - Comet G-ALYP
Things go wrong
Soon after the introduction of the Comet 1 it ran into difficulties and a number of crashes
tarnished its reputation. There were several crashes during take-off and on 3rd March
1953 the crew of the Comet CF-CUN was killed in one such incident. A year after its
introduction, the Comet G-ALYV broke up in mid-air 50km from Calcutta.
Two other accidents occurred under similar conditions within 3 months of each other.
The first was on 8th January 1954 involving the Comet G-ALYP or “Yoke Peter”. As the
Comet was ascending from Ciampino airport in Rome to 8,200 m in good weather
conditions it exploded and crashed into the sea near Elba. All 35 people on board died.
The fleet was grounded. The cause of the accident was difficult to detect as most of the
wreckage remained on the seabed. The fleet re-entered the service on 23rd March 1953.
The second major accident occurred on 8th April 1954. The Comet G-ALYY or “Yoke
Yoke” ran into difficulties only 30mins into its flight. Again this was as the aircraft was
reaching the top of its climb (10.6 km). The wreckage was found in the sea near Naples.
The Comets were again withdrawn from service. On 12th April 1954 the Ministry of
Transport and Civil Aviation removed the certificate of Airworthiness from the Comet.
Assumptions and mistakes
The take-off accidents were initially assumed to be due to the unfamiliarity of the pilots
with the new aircraft. When a Comet taking off from Rome overshot the runway an
investigation ensued. The conclusion was that it was the pilot, Foote, who was to blame.
Foote was forced to sign a confession stating that it was his fault. The truth was 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.
Another accident was blamed on the weather. The Comet G-ALYV crash was attributed
to the excessive stresses in the airframe because of the tropical storm.
Only after the second major crash (Comet G-ALYY) were tests carried out at
Farnborough. The immediate evidence from the wreckage was that the aircraft had
suffered a catastrophic explosion of the fuselage.
Real Reasons for failure
After investigation, there was several problems deemed to be the real causes of failure,
they were as follows:
Causes:
 After extensive testing of the Comet 1 (‘Yoke Uncle’), which had been donated
for testing by BOAC, investigators at the RAE determined the cause was due to a
fatigue crack associated with the stress concentrations of the rear ADF window
cut out, with stresses of 315MPa at the edge of the window and a bolt hole around
the window of 70MPa stress at the bolt position.
 New technology introducing new load cases (high altitude flight for turbojet
engines requiring cabin pressurisation).
 Mis-match between service loads and fatigue test procedure
 Possible contribution from out-of-plane bending loads (bi-axial stresses).
Design Failures:
 Improperly understood failure mode assessment procedures necessitated by
implementation of new technology.
 Poor configuration due to wing root engine placement (very few other aircraft
have had engines in this position), affecting updating potential, fire hazard, and
structural integrity in the event of engine disintegration.
Figure 2 - View from inside of failure. Note the square window
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 an ultimate load of 2 x P, P being the cabin ‘Proof’ pressure)
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
considerably 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. This was a noticeable
distinguishing feature of all later marks of the Comet.
Technological outcomes
 Full-scale 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 detectibility/critical size issues for fatigue cracks in aircraft
structures.
 Concept of ‘one-bay’ crack tolerance in fuselage probably formulated.
References
http://www.tech.plym.ac.uk/sme/Interactive_Resources/tutorials/FailureCases/sf2.html
http://shippai.jst.go.jp/en/Detail?fn=0&id=CB1071012&
http://www.aerospaceweb.org/aircraft/jetliner/comet/
http://www.neam.co.uk/comet.html
"Fatigue Failure of the De Havilland Comet 1"P.A.Whitney, "Engineering Failure
Analysis" Vol 4, No. 2, pp. 147-154, 1997