What happened
While descending to maintain 27,000 feet in instrument meteorological conditions, the crew of a Boeing 757 was instructed by Air Traffic Control to maintain a high rate of descent and an indicated airspeed of at least 300 knots. During this descent, the TCAS indicator displayed a traffic alert for aircraft traveling in the opposite direction at 26,000 feet. As the flight leveled off at 27,000 feet, the opposing aircraft passed nearly directly underneath.
Immediately following this encounter, the Ground Proximity Warning System (GPWS) issued a "terrain, pull-up" warning. Following company flight operations manual procedures, the crew executed a prescribed escape maneuver involving aggressive thrust and a rapid nose pitch up to 20 degrees. However, flight data recorder analysis revealed the crew only achieved an 8-degree nose-up attitude. During this maneuver, g-loads fluctuated between +2.5 and +0.5 over a two-second period, resulting in injuries to 4 flight attendants, 2 of whom were seriously injured with fractured leg bones. At the time of the event, the passengers were seated with seatbelts fastened, while the flight attendants were standing in the aft galley.
The investigation
The investigation revealed that the GPWS unit installed on the aircraft was three upgrades behind the current configuration. Although Boeing had issued various service bulletins and letters since 1988 to address false warnings caused by overflying other aircraft, the airline had not implemented these improvements due to budget and priority considerations.
Maintenance records showed that in the 16 months preceding the accident, the GPWS and Radio Altitude (RA) systems had been reported as erratic or inoperable 45 times, with 18 discrepancies recorded in the 60 days prior to the event. On three previous occasions, the GPWS had issued terrain warnings at high altitudes during profiles similar to the accident flight. Corrective actions by maintenance typically involved only the removal and replacement of units rather than advanced diagnostic troubleshooting.
Further examination of the RA units by the manufacturer suggested that central processors could become desynchronized during power transfers, leading to erratic behavior. Additionally, a previous issue had been identified where incompatible antennas were installed due to errors in the airline's parts stocking system.