Failure Analaysis

   Fracture Mechanics

   Failure as a Design      Criterion

 Structural Failures
 ::  Unforeseen Loads & Consequences
- Tacoma Narrows Bridge
- Comet Airliner
 Human System Interaction  Failures
 ::  Flawed Decision Making
 :: Flawed Safety culture
 Failure of Design Management
 ::  Visionary Management Style
 ::  Inaccurate Assessment of Market Needs


Jet transportation age began in on May 5 1952 when the De Havilland Comet 1 began scheduled flights from London to Johannesburg. In April 1953, a Tokyo to London service was inaugurated – flying time for the 10 200 mile distance dropped from 85 hours to 36 hours. The Comet had a cruising speed of 490 mph at 35 000 feet and a range of 1 750 miles with a payload of 44 passengers.

Power came from 4 De Havilland Ghost turbojet engines of 5 000 lbf thrust. Engines were mounted in the wing root – this minimises yaw accompanying loss of engine on take-off, but poses a hazard in the event of engine fire/disintegration and does not allow for easy uprating of engines (cf. hanging engine pods under wing) – poor design for development. Fuel consumption of turbojets is lower at high altitude.

The cabin was pressurised to maintain a pressure equivalent to 8 000 feet at an aircraft altitude of 40 000 feet, which was required for efficient operation of the engines. This gave a pressure differential of 8.25 psi (56 kPa) across the fuselage – twice the value previously used. De Havilland conducted ‘many tests’ to ensure structural integrity of the cabin. Other innovations included high pressure refuelling, hydraulic actuation of control surfaces and cabin air conditioning. It seemed that the future was bright for the British aircraft industry, with orders from France, Canada and the UK.

However, a series of 3 accidents occurred where Comet aircraft disintegrated in flight:
  • G-ALYV after leaving Calcutta – May 1953. Violent storms were thought to be involved and some wreckage was recovered. No firm conclusions drawn as to cause.
  • G-ALYP over Elba – January 1954 after 1 286 cabin pressurisation cycles. Little wreckage was recovered and no major problems found in fleet inspection. Fire was assumed the most likely cause and modifications made to improve fire prevention and control. Aircraft returned to service.
  • G-ALYY flying as SA 201 after leaving Rome – April 1954.
and all Comet 1 aircraft were subsequently withdrawn from service.

A more intensive effort was made to recover the wreckage of G-ALYP using underwater television cameras for the first time. About 70% of the aircraft was recovered and reconstructed at Farnborough. The engines were recovered more-or-less intact, showing that engine disintegration was not the cause of the accident, and neither was any evidence of fire found.

Comet G-ALYU, which had experienced 3 539 flying hours and 1 221 cabin pressurisation cycles, was subjected to full-scale flight simulation testing at Farnborough. The fuselage was hydraulically pressurised in cycles, while the wings were flexed with jacks to simulate the flight loads. Water was used for this pressurisation because calculations had indicated that the energy release under cabin rupture with air as the pressurisation medium was equivalent to the explosion of a 500 lbf bomb in the cabin. The cabin was also supported in water to avoid extraneous weight effects. After the equivalent of a total of 3 057 (1836 simulated cycles) flight cycles a 2 mm crack near the escape hatch grew to failure (Hatch Sketch). This was repaired and after 5 46 flight cycles a 4.5 m section of the cabin wall ruptured due to fatigue cracking. It was concluded that explosive cabin failure had caused the loss of the 3 Comet aircraft. Developing a detectable crack 6 mm long consumed some 95% of the cyclic life.

The Royal Navy was charged with getting the relevant fuselage piece of G-ALYP from the sea (using simulation trials, based on the way the aircraft was now thought to break up in flight, to establish the likely position of this part of the aircraft on the seabed. This was recovered within a few hours of searching and showed, in the language of the coroner, the ‘unmistakable fingerprint of fatigue’. The fatigue crack was associated with the stress concentrations of the rather square rear ADF window cutout (stress of 315 MPa at edge of window), and with a bolt hole around the window (although the stress at the bolt position was only 70 MPa).

The Chief Designer at De Havilland had wanted to glue the windows in position, but the tooling for the square shape was too difficult to make. A lower stress concentration shape would have been easier to manufacture.

The manufacturer had performed fatigue tests of the forward cabin area at about 10 psi (with cracking occurring at 18 000 cycles), but these were carried out after static tests of to up to 16.5 psi (twice operating pressure) had previously been applied. Cracks were also known to be present after manufacture, and the remedy was to drill 1.6 mm holes at the crack tip to ‘arrest’ them (such an arrested crack was present near the rear ADF window, which had not propagated until the final failure).

Modifications were made to the design of the aircraft and the Comet 4 re-entered service in October 1958 on the trans-Atlantic route with 80 passengers. A few weeks later the Boeing 707 flew the same route with 120 passengers and a safer, more flexible design engine design. The loss of 6 years to the Comet problems may have been instrumental in losing the lead in future jet transportation to the US. Parity in sales of passenger aircraft was established only in 1999 between Airbus and Boeing.

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 detectability/critical size issues for fatigue cracks in aircraft structures.
  • Concept of ‘one-bay’ crack tolerance in fuselage probably formulated.
Causes:
  1. New technology introducing new load cases (high altitude flight for turbojet engines requiring cabin pressurisation).
  2. Mis-match between service loads and fatigue test procedure.
  3. 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 uprating potential, fire hazard, and structural integrity in the event of engine disintegration.
    • References:
      1. P A Withey (1997) Fatigue failure of the De Havilland Comet 1 Engineering Failure Analysis Vol. 2 No. 4 pp147-154.
      2. J F Lancaster (1996) Engineering catastrophes: causes and effects of major accidents Woodhead Publishing.
      3. Johannesburg Sunday Star Review, March 22 1992 p.11 Super-fast jet takes a dive.
      4. http://www.hq.nasa.gov/office/pao/History/SP-468/ch13-2.htm
      5. http://surf.to/comet (Note that some of the information here with respect to crashes is inaccurate, e.g. fatigue is referred to as ‘crystallinity’).

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Structural Failures | Human System Interaction Failures | Failure of Design Management

Failure Analysis  -  Fracture Mechanics  -  Failure As A Design Criterion