4A6(1) Structural Design – Case Study Presentation Exercise
Comet Aircraft – The Worlds First Jet Airliner
Fatigue Failure
Background
In it’s day, the de Hallivand Comet of Britain was revolutionary in the civil aviation industry (being the
first commercial airliner to use turbojet engines, to use totally hydraulically actuated controls, to have
glued skin panels (Redux), to have a highly pressurised cabin (8.25 psi) and to use high pressure refuelling.) Yet nowadays the 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! All due to a relatively
simple engineering flaw.
In 1929, RAF officer Sir Frank Whitttle proposed powering an airplane with a gas turbine engine, which
would suck in air at the front and force exhaust gases out the rear of the engine, thus propelling it through
the air. (As apposed to the standard piston engines used at the time). After WWII, De Havilland was the
only manufacturer that had experienced building both engines and aircraft and so in 1946, they began the
design of the first commercial jet liner.
The DH 106 Comet first flew in July 1949. The aircraft underwent almost 3 years of test and fixes and so
the first commercial flights didn’t begin until January 1952. The first passenger flight was in May from
London to Johannesburg, the flight time was cut from 40hours to 23, traveling at up to 500mph. It’s fully
pressurized cabin allowed it to fly above storms as apposed to through them and the quiet, smooth ride
made the Comet an overnight sensation. During its first year of operation, the Comet carried 28,000
passengers a total of 104 million miles. By May 1953, de Havilland had firm orders for 50 Comets from
the world's airlines and was negotiating for 100 more.
Human and Managerial Factors
As with space travel, the Americans (Boeing) and the British (De Havilland) were in competition to
develop the first successful commercial aircraft. De Havilland began designing their jet aircraft in 1946.
Due to increasing political, time and market pressures, (eight Comets had already been sold before they
were even built), the project had an almost impossible deadline. It turned out that project was to require
three years from its final inception until its maiden flight, whereas previous aircraft tended to require seven
years. The production line was set up before the prototype had flown. This managerial decision was a risk
as there would be great financial losses if they had to change the design.
As the project neared completion, The Ministry of Supply and BOAC (British Overseas Airways
Corporation) had concerns over fatigue and the project was threatened with closure. It was decided that
full-scale testing would be carried out and the aircraft still went into service as planned (However, this fullscale testing was not actually carried out until the Comet fleet had been grounded after a number of
accidents, a few years later).
The Problems
The problems started in late 1952, when the De Havilland Comets had been in service for almost one year.
The first major accident occurred on October 26th 1952 while the plane was preparing to depart from
Ciampino airport in Rome. There were no fatalities and the accident was attributed to the pilot Captain
Foote. However, an official report stated “An error of judgment by the captain in not appreciating the
excessive nose-up attitude of the aircraft during the take off.”
4A6(1) Structural Design – Case Study Presentation Exercise
A second crash on March 3rd 1953 was also initially deemed to be the result of pilot error. This plane
failed to gain altitude on takeoff, collided with a bridge and caught fire. Although the plane had been
loaded to almost the permissible maximum weight limit for a Comet, examinations carried out after the
accident also discovered a flaw in the planes design: if the plane was pulled up too fast a large area of the
wing lost its lift. Consequently a design modification was carried out to improve the wing leading edge
allowing for greater lift at the Comets low speeds.
The weather was blamed for the third accident; At 10,000ft the plane encountered a tropical storm and
disintegrated spreading the wreckage over an area of 20km^2. However subsequent accidents indicated
that perhaps the Comet had a weak structure which would be a more likely cause.
The next crash was probably the most critical as it suggested that there was a serious problem or flaw in
the Comet design. A Comet flying to London on January 10th 1954 lost radio contact and disappeared.
Fishermen reported to have witnessed the plane exploding and diving into the Mediterranean near the
island of Elba amid a cloud of black smoke. Comet flights were suspended and recovery of the wreckage
for investigation was carried out. Based on the limited data recovered from the investigation engineers
revised some aspects of the plane. For example, shields were installed between the engines and fuel tanks.
Smoke and fire detectors were installed and the fuel lines reinforced. These measures were hoped to have
remedied the problem. Unfortunately that was not to be the case, when two months later a Comet flying
from Rome disappeared in similar circumstances. The wreckage of the Comet was discovered in the sea.
A full investigation was then carried out by the Royal Aircraft Establishment (RAE) and a court of enquiry
was established.
The Real Problem
The real problem causing these planes to blow apart in actual flight was metal fatigue.
An investigation team was led by Sir Arnold Hall, Director of the Royal Aircraft Establishment at
Farnborough. Models of full size aircraft and replicas were subjected to the most elaborate tests ever
carried out on an airliner.
The investigation included examination of cabin pressurization. A large tank was built so that it could hold
one of the grounded comets. The wings were allowed to protrude from water tight slots in the sides of the
tank. Both the tank and cabin were flooded with water. The water tank was used to simulate cabin loading
at 35,000ft (with water pressure raised to eight and a quarter pounds per square inch). Hydraulic jacks
were used to move the wings both up and down so as to simulate the flexing that occurs in aircraft wings
during flight. The testing continued 24hours/day until a sudden pressure drop occurred after the equivalent
of 9000 flight hours. The water was drained and a crack in the fuselage was observed. The rivets at the
corners of the escape hatch windows suffered high stress concentrations, leading to metal fatigue crack (ie.
failure due to repeated exposure to loads below design load). The investigators discovered that this small
weakness would deteriorate rapidly under pressure, causing the fuselage to break up leading to an
explosion (due to sudden cabin depressurization). Here underwater testing prevented such an explosion.
(Ironically, the American Aviation Authority hadn’t granted them permission to fly in the states due to
concerns about the shape of said windows.)
Eventually, the Navy recovered 70% of the craft lost and confirmed that fatigue crack growth was the
cause of the accident. It confirmed the test results – that 70% of the aircrafts ultimate stress under pressure
was concentrated on the corners of the aircraft windows. It was these square windows that were the major
flaw of the Comet’s design.
It took 4 years for the Comet to be re-engineered sufficiently, but at this stage their reputation preceded
them. Also, American Boeing had already gone into service providing bigger planes with farther range and
so Boeing stole Comet’s market share and went on to become the most dominant name in the industry
4A6(1) Structural Design – Case Study Presentation Exercise
A Comet being tested in the water tank
Notice the shape of the Comet’s windows.
Lessons Learned from the Comet Air Disasters

No matter how much we test a new idea it is impossible to cover all unforeseen possibilities. The
comet was the first jet airliner and as such was an engineering challenge. To make progress
engineers must be prepared for the possible risks involved and endeavor to foresee and prevent
them.
Quoting the court of inquiry into the comet air disasters, Concerning Lord Brazabon of Tara
Chairman of the Air Registration Board’s Statement“The Elba accident was completely unexplained to us. The Comet was a machine that was
being talked about all over the world, being as it was the most remarkable machine in the
world and if we grounded the type of every aircraft that had unexplained accidents you would
scarcely have a machine in the air. The cause of the accident was the adventurous pioneering
spirit of our race ...here was a great imaginative project ...we were conscious of the dangers
that were lurking in the unknown…Everything in the realm of human knowledge and wisdom
was put into this machine. It is metallurgy not aeronautics that is in the dock.”

From these accidents we have learned how to better prepare against fatigue failure. The comet
used a factor of two to protect against fatigue but this proved insufficient as more complex
phenomena like the interaction of stress concentrations with fatigue must be taken into
consideration. The quote above illustrates that along with aeronautical knowledge greater
investigation was needed in failure mechanisms of materials used in relation to specific situations.
Clearly fatigue is a serious factor in aircraft design and must be provided for by minimizing stress
concentrations and strengthening fatigue resistance in the future

As the first disaster of this kind a number of new and innovative methods of aircraft testing and
accident investigation were developed, including:
Use of a water tank to encase and test a whole aircraft
Use of underwater cameras in the collection of wreckage.
First large scale reconstruction as part of an accident investigation.
Use of medical forensics to solve an air accident.
Testing should simulate real life conditions as closely as possible. It is clear that the initial static
testing used on the comet was insufficient. This has led to the current standard practice of dynamic
testing of aircrafts.
References:
http://www.dlyoung.freeserve.co.uk/DH106/comet_1inq.htm
www.geocities.com/CapeCanaveral/Lab/8803/fcometcr.htm
www.wordiq.com/definition/De_Havilland_Comet
www.geocities.com/Heartland/Ridge/1747/timessumm.htm
http://www.rafmuseum.org.uk
http://www.theknownuniverse.com/comet.htm