What happened
On 6 January 1999, a McDonnell-Douglas DC10-30ER, registration C-GCPH, arrived at London Heathrow Airport from Calgary. After landing on Runway 27L, the aircraft taxied towards Terminal 4. During the taxiing process, the commander requested additional thrust to expedite crossing Runway 27L via Block 85. Shortly after this increase in power, the crew noticed a vibration through the airframe.
While the crew initially considered the possibility of a tyre deflation, the vibration intensified as the aircraft approached its assigned stand. The flight engineer identified an abnormal reading on the No 1 engine N1 (fan) RPM, which was significantly lower than expected. Consequently, the commander shut down the No 1 engine to complete the arrival procedures safely. Following the shutdown, maintenance personnel discovered engine debris scattered across the taxiway in Block 91.
The investigation
An inspection of the No 1 engine revealed substantial damage to the 4th stage of the low pressure turbine (LPT) and metallic debris within the jetpipe. The investigation involved a detailed strip examination of the engine at an overhaul facility in Hanover. Investigators examined the low pressure turbine module, checking for evidence of bearing or oil seal failures, which were not found.
Detailed analysis of the LPT 1 rotor showed that 75 out of 128 blade roots were missing from their slots in the disc. The remaining blades had suffered severe damage to their aerofoils. The investigation focused on the mechanism intended to retain these blades axially within the disc. Examination of the stage 1 nozzle air seal revealed that the discourager leg had suffered cracking and material loss, and there was evidence of contact abrasion between the seal and the blade retainer upstands.
Findings
- The primary cause of the failure was the loss of the forward upstands on the stage 1 LP turbine blade retainers.
- This loss was caused by rotational rubbing and erosive wear between the blade retainer upstands and the LPT1 nozzle seal assembly.
- The wear weakened the retainers, allowing the turbine blades to move axially backwards and disengage from their dovetail slots.
- Once liberated, the blades traveled through the turbine, causing secondary damage to the 2nd, 3rd, and 4th stages of the LP turbine and the turbine casing.
- The investigation concluded that the LP turbine rotor was positioned further forward than the design allowed, leading to the contact between the seal and the retainers.