Effect of Vibration: Crawling of Cracks, 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.
1. Case Study: Southwest Airlines Flight 812
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.”
2. Landing Gears and Vibrations:
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.
2. Exhaust Cracks: