Area Of Cracking And Rupture Not Seen During Previous Inspections

In the probable cause report released last week concerning the rapid decompression of an American Airlines B757 departing from Miami on October 26, 2010, the NTSB says 'incorrect manufacturing' was the source of an 18x7 inch hole which opened up in the upper fuselage of the airplane. Fortunately, the 757 landed safely back at Miami, and no one was injured in the incident.

File Photo

NTSB Identification: DCA11FA004
Scheduled 14 CFR Part 121: Air Carrier operation of AMERICAN AIRLINES INC
Accident occurred Tuesday, October 26, 2010 in Miami, FL
Probable Cause Approval Date: 09/19/2011
Aircraft: BOEING 757-223, registration: N626AA
Injuries: 160 Uninjured.

Flight data recorder data revealed that, after takeoff from Miami International Airport (MIA), Miami, Florida, the airplane climbed for about 16 minutes and was passing through an altitude of approximately 32,000 feet when the rapid decompression occurred. The flight crew declared an emergency (the first officer was the pilot flying) and returned to MIA. Post accident examination of the airplane revealed fatigue cracking of the upper fuselage skin above the forward left passenger (L1) door. The fatigue cracking penetrated the fuselage skin, leading to a rupture and an approximate 18-inch-by-7-inch hole that depressurized the airplane.

NTSB examinations of a section of the fuselage crown skin (from body station 374 to body station 439) where the rupture occurred revealed fatigue cracking along the lower longitudinal step of the chemically milled pocket just above the stringer S-4L (left) lap joint. The fatigue cracking initiated on the interior surface of the skin at multiple locations and propagated through the skin thickness. The skin thickness at the base of the chemically milled step measured 0.035 to 0.037 inch—which is less than the 0.039-inch minimum thickness specified by Boeing. Calculations from an NTSB study of the fatigue striation density and propagation in the fatigue region indicate that it would take an average of 3,709 total cycles for a crack to grow through skin with 0.035-inch thickness and an average interval of 917 cycles for a crack to grow from a minimally detectable size and penetrate a 0.035-inch skin thickness.

The area of cracking and rupture on the accident airplane was not subject to any specific inspections, service bulletins (SB), or airworthiness directives (AD) at the time of the accident. Following the accident, on November 22, 2010, Boeing issued SB 757-53-0097, which called for repetitive external inspections (every 30, 200 cycles, or 300 hours depending on the inspection method) to detect cracks in the fuselage skin along the chemically milled step at stringers S-4L (left) and S 4R (right) between body station 297 and body station 439. On January 10, 2011, the Federal Aviation Administration issued AD 2011-01-15 (effective January 25, 2011) mandating the inspections recommended in SB 757-53-0097.

During its investigation, the NTSB learned of a United Airlines 757 and a second American Airlines 757 that exhibited cracking in the fuselage skin similar to the accident airplane; both had nonconforming thickness at the base of the chemically milled step at the stringer location specified in the SB. Records of manufacture for the skin panels on the accident airplane and the other airplanes with fuselage skin cracking were not retained, and were not required to be retained; therefore, a cause for the manufacturing nonconformance could not be identified.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
Fatigue failure of the fuselage crown skin due to incorrect manufacturing of the crown skin panel that resulted in a skin thickness less than the manufacturer-specified thickness.

FMI: www.ntsb.gov