History of Flight
On October 26, 2010, about 2130 eastern daylight time, American Airlines flight 1640, a Boeing 757-223 (registration N626AA), experienced a rapid decompression while climbing through 32,000 feet after departure from Miami International Airport (MIA), Miami, Florida. The flight crew executed an emergency descent and returned to MIA, landing without further incident. No injuries were reported among the 6 crewmembers and 154 passengers.
Damage to Aircraft
A ground inspection revealed a section of fuselage crown skin measuring approximately 18 inches longitudinally by 7 inches circumferentially had ruptured just above and aft of the forward left passenger (L1) door. The forward 13-inch-by-7-inch portion remained attached and flapped upward; the aft 5-inch-by-7-inch portion separated and was not recovered.
Crew Information
The captain held an ATP certificate and had 20,566 total flight hours, including 13,226 as pilot-in-command and 2,501 as PIC in the 757. The first officer held an ATP certificate and had 6,621 total flight hours, with 3,686 in the 757. Both held valid medical certificates without limitations.
Aircraft Information
The airplane, serial number 24584, was delivered new to American Airlines on August 14, 1990. At the time of the event, it had accumulated approximately 63,010 hours and 22,450 cycles. The accident skin panel was manufactured around early 1990, but no specific manufacturing records were available.
The crown skin panel on 757 airplanes is subjected to a single-step chemical milling process to form waffle-like pockets. Other 757 skin panels use a multi-step process that reduces thin-out tendencies. The 767 model uses a similar single-step process but in a different area.
Tests and Research
A 60-inch-by-18-inch section of the fuselage crown skin was examined at the NTSB Materials Laboratory. Fatigue cracking was found along the lower longitudinal step of the chemically milled pocket above the stringer S-4L lap joint. The cracking initiated on the interior surface at multiple locations and propagated through the skin. The skin thickness at the base of the milled step measured 0.034 to 0.035 inch—less than the 0.037-inch minimum specified by Boeing.
Striation analysis indicated that an average of 3,709 total cycles would be needed for a crack to grow through 0.035-inch skin, with an average interval of 917 cycles from a minimally detectable size to penetration.
Additional Information
At the time of the event, the rupture area was not subject to specific inspections, service bulletins, or airworthiness directives. Following the event, Boeing issued Service Bulletin 757-53-0097 on November 22, 2010, calling for repetitive external inspections for cracks in the crown skin along the chemically milled step at stringers S-4L and S-4R. The FAA issued AD 2011-01-15 on January 10, 2011, mandating those inspections. Boeing later provided an Alternate Method of Compliance (AMOC) extending inspection intervals to 620 flight cycles for SPEC inspections.
After the event, NTSB staff learned of two other 757s (a United Airlines and an American Airlines) with similar cracking and nonconforming thickness. About six weeks before the event, a 10.75-inch through crack was found on a United Airlines 757 after a whistling noise. About six weeks after the event, American Airlines found three through cracks on another airplane during inspections.
Effective April 16, 2011, records for manufactured articles must be retained for at least 5 years, and for critical components for at least 10 years. Due to the time elapsed since manufacture, these requirements would not have affected record availability for the investigation.