LAX95FA303

The aircraft experienced a rapid decompression at 33,000 feet msl. The aircraft made a rapid descent to 14,000 feet where it was able to maintain a 7,000-foot cabin pressure. The aircraft returned to Los Angeles and made an uneventful landing. Postaccident examination revealed that around fuselage station 1800, 20 stringer end fittings were severed, allowing the aft pressure bulkhead to separate from the fuselage crown. Metallurgical analysis revealed that the origional manufacturer had predrilled the stringer end fittings. During assembly not all holes aligned. The unused holes were plugged. The fastener edge distance was below minimums. Fitting eccentricity was also noted as a problem. Combined, they increased stress levels at crack initiation sites. Operator inspections have reported 97 aircraft without cracks and 29 with various cracks of the stringer end fittings.

History of flight

On August 23, 1995, at 2013 hours Pacific daylight time, a Lockheed L-1011-385-1, N781DL, operated by Delta Airlines as Flight 157, sustained substantial damage followed by a rapid decompression over the Pacific Ocean while in cruise flight at 33,000 feet mean sea level (msl). The flight crew reported hearing a loud bang immediately prior to the decompression event. The pilot initiated an emergency descent to 14,000 feet msl, and returned to Los Angeles, California, for an uneventful landing at 2217 hours.

The crew reported that they were unable to regain control of the pressurization; however, the cabin was eventually stabilized at 7,000 feet when the aircraft leveled off at 14,000 feet msl. Other than reports of ear discomfort, there were no injuries to the 226 passengers and crew of 10. Three passengers were taken to a local hospital for a brief check then released. Flight number 157, a regularly scheduled domestic flight, departed Los Angeles International Airport at 1810 en route to Honolulu, Hawaii. The flight was approximately 450 miles west of Los Angeles at the time of the depressurization.

Aircraft information

Postaccident examination of the aircraft revealed that around fuselage station 1800, 20 stringer end fittings were severed, allowing the aft pressure bulkhead to separate from the fuselage crown. The separation was over a length of about 12 feet of the pressure bulkhead circumference. Also, at fuselage station 1809, the "Z" frame was found bent and torn. No external fuselage damage was found. The crack extended from stringer 10 through 0 (top center) to stringer 55.

According to information provided by Lockheed, seven of the stringer end fittings had failed on the L-1011 fatigue test aircraft. This led to the issuance of service bulletin 093-53-105, which recommended the inspection/replacement of the stringer end fittings on serial numbers 193A-1002 through 1012. The accident aircraft is the third production aircraft, serial number 1003. A review of the Lockheed records revealed that no operators have complied with SB 093-53-105 as of this date.

Although the redesigned fittings were installed on serial number 1013 and up, Lockheed has received several reports of the redesigned fittings cracking on aircraft with serial numbers later than 1012.

The area of the failure is not covered on an inspection work card during routine inspections; however, the exterior of the aircraft is inspected. On July 23, 1995, a "C" check inspection had been performed on the accident aircraft. At that time, the aircraft had accumulated 51,951 hours and 25,691 cycles. At the time of the accident, the aircraft had 52,210 hours and 25,813 cycles.

As a result of this accident, on August 29, 1995, the FAA issued a telegraphic airworthiness directive (T95-18-52 amended with AD95-18-52 on September 28, 1995, and superseded by AD95-26-11) was issued to all operators of Lockheed model L-1011-385 series airplanes certificated in any category. The AD mandated the compliance/inspection called out in the text within a certain time frame. On August 24,1995, Lockheed Aeronautical Systems Support Company notified all foreign operators of the rapid decompression during flight and referenced S/B 093-53-105.

According to Lockheed, there are about 250 L-1011's in service. As of February 24, 1996, as a result of Lockheed's notification to operators, there have been 29 reports of aircraft having various cracks to the end fittings and 97 reports of no cracks. The cycles of these aircraft ranged from a high of 33,375 to a low of 21,657.

Metallurgical analysis

According to the report, during the manufacturing process, stringer end fittings were predrilled. During the assembly process not all of the predrilled holes aligned. The unused holes were then plugged/filled by the original manufacturer. Plugged holes oriented in the direction of maximum tension stress near the aft most fastener hole resulted in increased stress levels at crack initiation sites.

Short fastener edge distance on some of the stringer end fittings contributed to increased stress levels at crack initiation sites. (Ref.: FAA AC 43.13-1A, paragraph 99, figure 2.18.)

Fitting eccentricity also contributed to high stress levels at crack initiation sites. For more detailed information, refer to the attached Structure Technology Group Engineering Report (February 5, 1996).

Additional information

Due to the cost of repairs, Delta Airlines subsequently retired the aircraft. The aircraft was purchased by Lockheed Aircraft for aging aircraft testing. On October 11, 1995, the aircraft was ferried to Dobbins AFB, Marietta, Georgia. Prior to releasing the aircraft to Lockheed, Delta Airlines personnel removed the failed aft pressure dome components for metallurgical analysis.