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 re-fuelling.) 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 full-scale 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.”
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
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: