Analysis Of Air Accident: Southwest Airlines Yuma, Arizona

Abstract

On 04/01/2011 a Southwest Airlines aircraft experienced a cabin depressurization at 34, 000ft due to a 60x8 inch hole rupture in the aluminum skin of the Boeing 737 plane. A successful emergency landing ensured no fatalities though two people were injured. The rupture was caused by metal fatigue in lap joints on the fuselage due to propagation of cracks which slowly connected over time.

Analysis

The fuselage material was Aluminium 7075-T6 (Al-Zn-Mg-Cu) which is a solution heat treated and age-hardened alloy used for highly stressed parts with high toughness, ductility and corrosion resistant due to Al2O3 protective oxide layer. It has high strength-to-density ratio.

The fuselage was manufactured in two separate parts and assembled later. It consisted of an upper part and a lower part which were connected at a lap joint, bonded together to a doubler with polysulfide sealant and fastened together with three rivet rows 1-inch apart. Stringers were attached to the middle rivet row. Fatigue cracks initiated at the outer surface of the lower skin at the lap joint connecting the upper skin to the lower skin. The fracture on the fuselage affected 58 rivet holes which were misaligned; the lower holes appeared oval/double drilled and paint from the recent plane revamping had leaked from the fuselage into the rivet joints. Some rivets were underdiven, oversize and their heads appeared burred.

Discussion

Highly compressed air circulating in the cabin is taken from the compressor section of the engine before it is mixed with fuel or exhaust gases. On-board pressurization systems pump fresh, clean air and automatically control the outflow valve located near the tail of the aircraft. This action creates a differential pressure between the air pressure inside and outside the aircraft. The air pressure inside an aircraft increases with altitude hence the stress weakened the plane’s skin over time.

Crack formation from the rivet holes gradually grew during the use of the aircraft, joining together over time forming a large crack. Longest cracks were observed on rivet 85 and propagation decreased with distance further from its tip. Fatigue occurred because the material was subjected to repeat loading and unloading above a certain threshold, hence microscopic cracks begun to form at the stress concentrators at the rivet holes. Boeing engineers expected the lap joints to endure 60, 000 flight cycles of take-off and landing before they would need inspections, but the Southwest 737 that ruptured had completed only 39, 871 cycles after almost 5 years.

Conclusion

Material properties was ruled out as a cause of skin fatigue. The fracture toughness Kc of aluminium 7705 is high, as do most metals, due to its ability to resist fracture despite propagation of cracks over time i. e. Skin fastening design was poor since they used thousands of rivets to fasten overlapping joints and bonded together by adhesive.

Boeing used the same fuselage on a new aircraft design larger than its predecessor hence the instantaneous crack formation after manufacture. To avoid such accidents, frequent inspections and maintenance should be conducted on the aircrafts procedurally and detection of cracks using x-ray photography or high-frequency sound waves.