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Damage tolerance, or safety by inspection, was developed as a design philosophy in the 1970s as an improvement on the fail-safe principle for structural deterioration.
The damage tolerance approach is based on the principle that while cracks due to fatigue and corrosion will develop in the aircraft structure, the process can be understood and controlled. A key element is the development of a comprehensive programme of inspections to detect cracks before they can affect flight safety. That is, damage tolerant structures are designed to sustain cracks without catastrophic failure until the damage is detected in scheduled inspections and the damaged part is repaired or replaced.
Unfortunately, history shows that it is an imperfect solution in practice.
Accidents and Incidents
The following events included Damage Tolerance as a contributory factor:
- A139, vicinity Sky Shuttle Heliport Hong Kong China, 2010 (On 3 July 2010, an AW139 helicopter was climbing through 350 feet over Victoria Harbour Hong Kong just after takeoff when the tail rotor detached. A transition to autorotation was accomplished and a controlled ditching followed. All occupants were rescued but some sustained minor injuries. The failure was attributed entirely to manufacturing defects but no corrective manufacturer or regulatory action was taken until two similar accidents had occurred in Qatar (non-fatal) and Brazil (fatal) the following year and two interim Safety Recommendations were issued from this Investigation after which a comprehensive review of the manufacturing process led to numerous changes.)
- A310, en-route, Florida Keys USA, 2005 (On 6 March 2005, an Airbus A310-300 being operated by Canadian airline Air Transat on a passenger charter flight from Varadero Cuba to Quebec City was in the cruise in daylight VMC at FL350 seventeen minutes after departure and overhead the Florida Keys when the flight crew heard a loud bang and felt some vibration. The aircraft entered a Dutch roll which was eventually controlled in manual flight after a height excursion. During descent for a possible en route diversion, the intensity of the Dutch Roll lessened and then stopped and the crew decided to return to Varadero. It was found during landing there that rudder control inputs were not effective and after taxi in and shutdown at the designated parking position, it was discovered that the aircraft rudder was missing. One of the cabin crew sustained a minor back injury during the event but no others from the 271 occupants were injured.)
- A320, Los Angeles USA, 2005 (On 21 September 2005, an Airbus A320 operated by Jet Blue Airways made a successful emergency landing at Los Angeles Airport, California, with the nose wheels cocked 90 degrees to the fore-aft position after an earlier fault on gear retraction.)
- A320, vicinity LaGuardia New York USA, 2009 (On 15 January 2009, a United Airlines Airbus A320-200 approaching 3000 feet agl in day VMC following take-off from New York La Guardia experienced an almost complete loss of thrust in both engines after encountering a flock of Canada Geese . In the absence of viable alternatives, the aircraft was successfully ditched in the Hudson River about. Of the 150 occupants, one flight attendant and four passengers were seriously injured and the aircraft was substantially damaged. The subsequent investigation led to the issue of 35 Safety Recommendations mainly relating to ditching, bird strike and low level dual engine failure.)
- A321, en-route, Gimpo South Korea, 2006 (On 9 June 2006, an Airbus 321-100, operated by Asiana Airlines, encountered a thunderstorm accompanied by Hail around 20 miles southeast of Anyang VOR at an altitude of 11,500 ft, while descending for an approach to Gimpo Airport. The radome was detached and the cockpit windshield was cracked due to impact with Hail.)
- A343, Helsinki Finland, 2009 (On 22 June 2009, an Airbus A340-300 being operated by Finnair suffered a single tyre failure during take off on a scheduled passenger flight to Helsinki and malfunction assessed as consequential by the flight crew occurred to the hydraulic system. The flight proceeded to destination and carried out a daylight landing there in normal visibility without any further aircraft damage. Because of a further deterioration in the status of the aircraft hydraulic systems during the landing roll, the aircraft was stopped on the runway and then towed into the gate. No persons were injured in this incident.)