National Transportation Safety Board
Office of Public Affairs
(Washington, DC) -- The National Transportation Safety Board has determined that the uncontained engine failure on a Delta Air Lines MD-88 in Pensacola, Florida on July 6, 1996, was the result of a fracture of the left engine's fan hub. The Board said that the fracture resulted from the failure of Delta to detect a fatigue crack in the fan hub.
On July 6, 1996, at 1424 central daylight time, a McDonnell Douglas MD-88, N927DA, operated by Delta Air Lines Inc., as flight 1288, experienced an engine failure during the initial part of its takeoff roll on runway 17 at Pensacola Regional Airport in Pensacola, Florida. Engine debris from the front compressor front hub (fan hub) of the No. 1 (left) engine penetrated the left aft fuselage. Two of the 137 passengers were killed and two others were seriously injured. The takeoff was rejected, and the airplane was stopped on the runway.
The Board found that damage occurred in the accident fan hub during the drilling process performed by Volvo for Pratt & Whitney, creating an altered microstructure and ladder cracking in the titanium. Fatigue cracks initiated from the ladder cracking in the tierod hole and began propagating almost immediately after the hub was put into service in 1990. Although the altered microstructure in the accident hub tierod hole was detectable by blue etch anodize inspection methods, Volvo did not identify it as rejectable because the appearance of the tierod hole did not match any of the existing inspection templates showing rejectable conditions.
The crack was large enough to have been detectable during the accident hub's last fluorescent penetrant inspection at Delta. The Board said that the lack of sufficient redundancy in the in-service inspection program contributed to the accident.
The Safety Board discussed the limitations of the blue etch anodize process, manufacturing defects, standards for the fluorescent penetrant inspection process, the performance of non-destructive testing, the use of alarm systems for emergency situations, and instructions regarding emergency exits. The Safety Board made 11 recommendations to the Federal Aviation Administration including:
• have all manufacturers of titanium rotating engine components reevaluate their manufacturing specifications and procedures;
• find ways to improve the likelihood that abnormal microstructure will be detected;
• establish and require adherence to a uniform set of standards for the fluorescent penetrant inspection process;
• require that all heavy rotating titanium engine components receive appropriate non-destructive testing inspections (multiple inspections, if needed) based on probability of detection and, as an interim measure, require that critical rotating titanium engine components that have been in service for at least 2 years receive a fluorescent penetrant inspection, eddy current, and ultrasonic inspection of the high stress areas at the engine's next shop visit or within 2 years, whichever occurs first;
• require that all newly-manufactured passenger-carrying airplanes operated under 14 CFR Part 121 be equipped with independently powered evacuation alarm systems;
• require that all newly-manufactured airplanes be equipped with cockpit indicators showing open exits, including overwing exit hatches, and that these cockpit indicators be connected to emergency power circuits.
The NTSB's complete report, may be purchased from the National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161. The telephone number is (703) 487-4650.
The National Transportation Safety Board (NTSB) is an independent federal agency charged with determining the probable cause
of transportation accidents, promoting transportation safety, and assisting victims of transportation accidents and their families.