Group 6
4A6 (1): Disaster Report
“Comet Aircraft: Fatigue Failure”
Michael Beton
Charles Delaporte
Colin Murphy
Colin Smyth
Background & Introduction
The de Havilland comet was a revolutionary benchmark in the British aviation
industry. It completely re-assessed how airliners should be built and operate. The de
Havilland comet was the first of its kind in many ways, it introduced the use of
turbojet engines, the use of totally hydraulically actuated controls, glued skin panels
(redux), a highly pressurised cabin(8.25 psi) and the use high pressure re-fuelling. It
boasted being the most advanced and prestigious commercial airliner that provided
passengers with a quiet, smooth ride previously unheard of in commercial aviation
and could travel further distances in less time. Only making its maiden voyage in
1952, its strange that the name de Havilland is relatively unknown compared to
market leading Boeing, who’s 707 only made its first test flight 2 years after Comets
first commercial flight. This was a result of a fundamental engineering flaw that was
overlooked and consequented in several public disasters.
Development & Design
The initial idea of the Comet began in 1929 when Royal Air Force officer Frank
Whittle proposed a new idea for powering an airplane. He suggested a gas turbine
engine should be used instead of applying power to a propeller. The gas turbine
would suck in air from the front and force exhaust gasses out the rear thus
generating enough energy to drive the airliner forward.
It wasn’t until after WWII in 1946 that the design of the first commercial airliner
began. In July 1949 the DH 106 Comet was the first test flight and it lasted 31
minutes. Following that, the aircraft underwent almost 3 years of tests and
experimentation, until the first commercial flights began in January 1952. The first
passenger flight was in May from London to Johannesburg; the flight time was cut
from 40hours to 23 and was deemed a huge success.
The aircraft itself was an all-metal low-wing cantilever monoplane powered by four jet
engines. It could carry 36 passengers at a cruising speed of 725 kmph with a range
of 4,000 km. The Comet consisted of a clean, low-drag design that featured many
unique design elements. The engines were mounted higher in the wings. This
reduced the risk of ingestion damage, a major problem for turbine engines. 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 also reduced. 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. For commercial viability the weight of the aircraft had to be kept to a
minimum. This required that the Comet's thin metal skin was composed of advanced
new alloys and was chemically bonded using the adhesive Redux(pioneered by Sir
Geoffrey Havilland) and riveted only in certain areas. This saved weight and reduced
the risk of fatigue cracks spreading from the rivets.
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. During its first year
of service the comet transported 28000 passengers a total of 104 million miles. By
1953, de Havilland had confirmed orders for 50 more comets and was negotiating for
100 more from the world’s airliners.
Accident in Comet G-ALYP ("Yoke Peter")
On 10th January 1954, BOAC Flight 781, “Yoke Peter” crashed into the
Mediterranean off the island of Elba, killing all 35 people that were on board. Initially,
the reasons for this failure were unknown, and various assumptions were made to as
the aircraft actually failed.
In order to understand the actual method of failure, engineers at Royal Aircraft
Establishment in Farnborough, led by Arnald Hall, began an investigation into the
failure of the comet. They built another comet (“Yoke Uncle”), as a replica of Comet
Yoke Peter.
At Farnborough, the engineers built a water tank (112 ft. long, 60 ft. high, 20
ft. wide) (below) so that the fuselage of Comet Yoke Peter could be placed into the
tank, with the wings extending outside of the tank. With the comet placed inside the
water tank, the tank was filled with water, until the pressure around the comet was
8.25 lbs/square inch, similar to the pressure at its flying altitude of 40,000 ft. The
water pressure was held at this pressure over a period of time to simulate the
amount of pressure that Comet Yoke Peter would have had to withstand before its
failure. Initially, the Comet was able to withstand these pressures, but as time went
on, the metal of the comet began to become overstressed.
Eventually, cracks started to appear at the corners of the escape hatch window.
These cracks were as a result of the design of the windows, a square shape design
(as shown below).
Stress levels at the corners of the windows were measured to be much higher than
was expected. This was because the windows were designed to be glued and
riveted, but instead the windows were only punch riveted. Once this crack was
formed, the air pressure within the cabin was so great that it caused the aircraft to
explode, firstly the cabin roof blowing off, and then the wings breaking, and then
finally then main body and cabin tore apart while the fuel ignited the remains of the
main body as it descended.
Conclusion
The causes of catastrophic breakup of Comet aircraft in high altitude were first
misunderstood.
When Comet 1 G-ALYV disintegrated in flight after leaving Calcutta (May 1953),
violent storms were thought to be involved and some wreckage was recovered.
When Comet 1 G-ALYP disintegrated over Elba (January 1954), and fire was first
assumed the most likely cause.
As explained before, the Royal Aircraft Establishment (RAE) recovered and partially
reconstructed the wreckage of Comet 1 G-ALYP (1954). The RAE found
'unmistakable fingerprint of fatigue' at the low-drag fibreglass forward "window"
around the Automatic Direction Finder. A full-scale simulation test on a Comet 1 GALYU plane confirmed that a fatigue failure leading an explosive decompression had
caused the loss of the Comet aircraft.
The Comet aircraft were not properly designed for fatigue failure mode. There was a
mis-match between service loads and fatigue test procedure. This is due to the use
of new technology introducing new load cases: Comet aircraft was the first high
altitude flight; at 40,000 feet, the pressure differential across the fuselage is 56 kPa –
twice the value previously used. The cabin must be pressurised.
The failure mode assessment was not properly understood and procedures were not
adapted for this new technology.
The square windows of the Comet 1 were redesigned as oval for the Comet 2, which
first flew in 1953 and the skin sheeting was thickened slightly. The remaining Comet
1 were either scrapped or modified with oval window rip-stop doublers. All production
Comet 2 was modified to alleviate the fatigue problems.
Comet 3 should lengthen Comet 2 with greater capacity and range was constructed.
But after the fatigue accident, only two Comet 3 were constructed. Modifications and
improvements were made to the design of the aircraft. The Comet 4 entered service
in October 1958 on the trans-Atlantic route with 80 passengers.
The loss of 6 years to the Comet problems took a part in losing the competitive
advantage. The Boeing 707 flew the same route that the Comet 4 with more
passengers and a safer, more flexible engine design.
To conclude, from this 'error' we get a better understanding of fatigue testing
achieved. Full-scale testing of aircraft structures were generalized and utilized in
future aircraft. Finally, we get some detectability/critical size issues for fatigue cracks
in aircraft structures.
References
http://surf.to/comet
http://www.tech.plym.ac.uk/sme/interactive_resources/tutorials/FailureCases/sf2.html
http://www.time.com/time/magazine/article/0,9171,823674,00.html