Aircraft Fatigue

Malaysia Airlines Flight 370

b777

The leading news story the last few days has been Malaysia Airlines Flight 370 which mysteriously disappeared on March 8, 2014, somewhere over the South China Sea or perhaps the Strait of Malacca. The aircraft was a Boeing 777-200ER.

The cause or causes of the disappearance and presumed crash are simply unknown at this time.

But fatigue failure is one possible cause. Note that the Federal Aviation Administration (FAA) had previously issued the following warning regarding potential fatigue cracking in Boeing 777 aircraft.

Airworthiness Directives; The Boeing Company Airplanes
Document Number: 2013-23456
Type: Proposed Rule
Date: 2013-09-26

Agency: Federal Aviation Administration, Department of Transportation
We propose to adopt a new airworthiness directive (AD) for certain The Boeing Company Model 777 airplanes. This proposed AD was prompted by a report of cracking in the fuselage skin underneath the satellite communication (SATCOM) antenna adapter. This proposed AD would require repetitive inspections of the visible fuselage skin and doubler if installed, for cracking, corrosion, and any indication of contact of a certain fastener to a bonding jumper, and repair if necessary. We are proposing this AD to detect and correct cracking and corrosion in the fuselage skin, which could lead to rapid decompression and loss of structural integrity of the airplane.

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Pressurization Cycles

Aircraft fuselages undergo repetitive cycles of differential pressure with each flight. The difference between the cabin and the external ambient pressure is about 6 or 7 psi at an altitude of 36,000 feet.

Note that cabin pressure at high altitudes is maintained at about 75% of sea level pressure, which corresponds to the air pressure at 8000 ft. This is done by pumping air into the cabin. Note that there is some variation in these numbers depending on the aircraft model.

Pressurization cycles along with vibration, corrosion, and thermal cycling can cause fatigue cracks to form and propagate.

The following images show case histories of fatigue failures in aircraft.

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de Havilland DH 106 Comet

boac1

boac2
The de Havilland DH 106 Comet was the first production commercial jetliner, beginning service in 1952.

Several catastrophic failures occurred over the next two years.

Investigators eventually determined via testing that aircraft’s square windows had a “stress concentration factor” which generated levels of stress two or three times greater than across the rest of the fuselage. The window corners where thus prone to fatigue crack initiation, propagation, and fracture, particularly at the rivet holes.

As a result, the Comet was extensively redesigned with oval windows, structural reinforcement and other changes.

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Aloha Airlines Flight 243

aloha

Aloha Airlines Flight 243 between Hilo and Honolulu in Hawaii suffered extensive damage after an explosive decompression in flight, on April 28, 1988. The aircraft was a Boeing 737-297. It was able to land safely at Kahului Airport on Maui. There was one fatality — a flight attendant was swept overboard.

Fatigue cracks occurred due to disbanding of cold bonded lap joints and hot bonded tear joints in the fuselage panels. This caused the rivets to be over-stressed. A large number of small cracks in the fuselage may have joined to form a large crack. Corrosion was also a related factor.

* * *

Southwest Airlines Flight 812

SW_Yuma

Southwest Airlines Flight 812 suffered rapid depressurization at 34,400 ft near Yuma, Arizona, leading to an emergency landing at Yuma International Airport, on April 1, 2011.

Inspection of the 5 feet long tear revealed evidence of pre-existing fatigue along a lap joint.

The National Transportation Safety Board has concluded that “the probable cause of this accident was the improper installation of the fuselage crown skin panel at the S-4L lap joint during the manufacturing process, which resulted in multiple site damage fatigue cracking and eventual failure of the lower skin panel.”

* * *

Qantas Flight 32

aus_rr

Qantas Flight 32 suffered an uncontained engine failure on 4 November 2010 and made an emergency landing at Singapore Changi Airport. The aircraft was an Airbus A380 with Rolls-Royce Trent 900 engines.

The Australian Transport Safety Bureau concluded that “fatigue cracking” in a stub pipe within the No. 2 engine resulted in oil leakage followed by an oil fire in the engine. The fire led to the release of the Intermediate Pressure Turbine (IPT) disc.

Shrapnel from the exploding engine punctured part of the wing and damaged the fuel system causing leaks and a fuel tank fire, disabled one hydraulic system and the anti-lock brakes and caused No.1 and No.4 engines to go into a ‘degraded’ mode, damaged landing flaps and the controls for the outer left No.1 engine.

* * *

Landing Gear

landing_gear_fatigue

Landing gears are designed to absorb the loads arising from taxiing, take-off, and landing. Hard landing shock is a particular concern. Vibration is another concern. Fatigue cracks can form in the struts and trunnion arms as a results of these loads. Again, corrosion can be a related factor.

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See also:

FAA FR Doc No: 2013-23456

Aircraft Acoustics & Hard Landings

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– Tom Irvine

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