Safety Case Studies

Safety Case Studies examines aviation accidents that involve maintenance as either a probable cause or factor. The information in Safety Case Studies is obtained from National Transportation Safety Board reports, regulatory authorities and accident investigation bureaus, and research by the Aviation Maintenance staff. The information in Accident Case Studies is presented here to help the aviation maintenance industry learn from accidents and incidents and prevent future mistakes.

HPT BLADE SEALS NOT INSTALLED

F-16D, Shaw Air Force Base, South Carolina, April 18, 2005. Onboard: one pilot, one passenger. No fatalities. Both pilot and passenger ejected safely following engine failure in an F-16D assigned to the 20th Fighter Wing at Shaw AFB. According to the Air Force Accident Investigation Board's report, "There is clear and convincing evidence that the cause of the mishap was the ME's [mishap engine's] catastrophic failure and sudden lack of thrust due to High Pressure Turbine (HPT) blade failure. There is clear and convincing evidence that the HPT blade failure was caused by the required blade seals not being installed in the HPT rotor assembly during scheduled maintenance." During the post-crash examination of the F-16D's General Electric F110-GE-129 engine, investigators found that no blade seals were present in the HPT rotor assembly. "Additionally," the Air Force report noted, "the HPT blades lacked witness marks, which are routinely made by the blade seals--further indicating no seals were ever present. The specific maintenance actions that led to the failure to install the required blade seals included not heeding a caution in a technical order, which warned that "missing HPT rotor blades seals will cause HPT rotor blade failure." The maintenance crew also did not accomplish steps in the technical order that included not only installing the seals but also inspecting the seals to make sure they are installed properly and testing their fit with a piece of shim stock. "Therefore, two additional opportunities to avoid the mishap were missed," stated the Air Force report.

CRANKSHAFT GEAR BOLT FAILURES

Piper PA32-301T Turbo Saratoga, Byram Township, New Jersey, September 8, 2002. Onboard: one pilot, three passengers. Fatalities: pilot, one passenger. The National Transportation Safety Board issued a probable cause for this accident on October 27, 2005. The accident occurred following loss of power. During the forced landing in trees, the pilot and one passenger suffered fatal injuries. The probable cause of the accident, according to the NTSB, was: "the improper manufacturing of the crankshaft gear bolt, which resulted in the failure of the bolt due to hydrogen embrittlement, and the subsequent loss of engine power. Also causal was the engine manufacturer's failure to remove the affected bolts from fixed wing engines during the 4-year period that transpired between the initial failures and removal of the bolts from stock from installation, until the accident flight. A factor in the accident was inadequate oversight (lack of action) by the FAA during the 4-year period in which the bolts remained in serviceable engines." The accident investigation centered on the Saratoga's Lycoming TIO-540-AH1A piston engine and found that the zinc-plated crankshaft gear attachment bolt was fractured, which means that the crankshaft no longer turns important parts of the engine. "Fracture surface analysis," according to the NTSB report, "revealed intergranular separation due to hydrogen-assisted cracking." Following two helicopter engine crankshaft gear bolt failures in 1998, due to hydrogen embrittlement, Lycoming removed zinc-plated bolts from its inventory and issued a bolt recall, as did the helicopter manufacturer. An FAA airworthiness directive issued in February 1999 called for replacing crankshaft gear bolts in affected helicopter engines. Additional failures of crankshaft gear bolts occurred, one in July 1999 in a Piper Saratoga (normally aspirated) at 327 hours; two on Royal Jordanian Falcon trainer airplanes between March 2000 and January 2001, at 292 and 179 hours respectively; in June 2002 on a Piper Saratoga with 448 hours; then the fatal accident in September 2002. In October 2002, the FAA issued an emergency AD to replace the bolts in all Lycoming engines in which the bolts were installed.

MULTI OPERATORS MESSAGE ISSUED

Boeing 777-200, Flight Level 380, Perth, Australia to Kuala Lumpur, Malaysia, August 1, 2005. No injuries. As the airplane climbed through Flight Level 380, the flight crew observed a low-airspeed warning on the 777's engine indication and crew alerting system (EICAS). The slid/skid indication on the primary flight display (PFD) deflected full right at the same time and the speed tape showed that the jet was approaching its overspeed limit and stall speed limit simultaneously, according to an Australian Transport Safety Bureau report. "The aircraft pitched up," the report stated, "and climbed to approximately FL410 and the indicated airspeed decreased from 270 kts to 158 kts. The stall warning and stick shaker devices also activated." After disconnecting the autopilot, the pilot-in-command was able to lower the airplane's nose. When the PIC turned the left autopilot on, the airplane banked left and the nose pitched down. He turned the autopilot off. The same problem occurred with the right autopilot on, but the airplane handled normally with both autopilots off. Although this investigation continues, Boeing issued a Multi Operators Message on August 9, according to the ATSB report, "recommending to all B777 operators that the aircraft should not be flown with an unserviceable SAARU [standby air data and attitude reference unit]."