What happened
On March 3, 2002, a Lockheed L1011-385-3, registration CS-TMR, was performing a daytime takeoff from Milan Malpensa Airport, bound for Varadero, Cuba. The aircraft was carrying 252 occupants. Approximately 5 knots before reaching rotation speed (Vr), the number three engine experienced a surge, resulting in a sudden drop in Engine Pressure Ratio (EPR) from 1.645 to 1.186.
Because the decision speed (V1) had already been exceeded, the flight crew continued the takeoff roll and successfully executed the engine surge/stall emergency checklist, restoring normal engine operation. The crew decided to return to Malpensa, necessitating a fuel dump of 50 tons to reach the maximum landing weight. The aircraft landed safely at 16:53 UTC without further incident.
The investigation
The investigation examined the history of the number three engine and the maintenance actions taken following the event. It was discovered that an identical surge had occurred in the same engine during the takeoff phase on January 30, 2002. In both instances, ground run-up tests failed to reproduce the anomaly, leading to the aircraft being returned to service.
Following the March 3 event, maintenance personnel replaced the Bleed Control Valve and the VIGV Airflow Control Unit. While subsequent ground tests were normal, a follow-up test flight resulted in another engine surge. A subsequent borescope inspection of the High Pressure Compressor (HPC) revealed several bent and fractured blades on the 2nd and 5th stages of the compressor. These findings were consistent with the impact of a shock wave traveling from the HPC to the engine inlet.
Findings
- The engine was operating with over 8,400 flight hours since its last shop visit, significantly exceeding the typical 6,000 flight hour mean time between unscheduled removal (MTBUR).
- The VIGV Airflow Control Unit was found to be in a marginal state, showing signs of contamination and improper regulation of the variable inlet guide vane opening angles.
- The primary cause of the surge was a combination of degraded compressor blades (bent and fractured) and the malfunctioning airflow control unit, which reduced the compressor load and increased susceptibility to surges during high-power takeoff settings.