Jump to Content

The crew of the Fairchild Metro III aircraft, registered VH-DMI, heard a loud bang shortly after application of full power during the take-off roll. The crew immediately retarded both engine power levers and noticed that the left engine exhaust gas temperature was increasing so they shut down the left engine. When a passenger advised that `smoke and fire' were coming from the left engine, the crew discharged the fire bottle into that engine. The crew then shut down the right engine and ordered an evacuation out the right side of the aircraft.

An external examination by the operator's engineers found damage to the left engine turbine blades and shrapnel damage to the exhaust nozzle. The operator removed the engine and sent it to an approved workshop for examination and repair. The Australian Transport Safety Bureau (ATSB) did not attend the examination, but requested that all damaged components be forwarded to the ATSB for detailed examination.

After the engine was disassembled, the ATSB was advised that the engine failure was precipitated by a failure of the turbine first stage disc rotating air seal. The rotating air seal's outer rim was missing and the downstream turbine components received damage consistent with fragments of the rotating air seal passing though the turbine. The failed rotating air seal, the first, second and third stage turbine wheels and nozzle guide vane assemblies were forwarded to the ATSB for further examination.

Engine and component history

The Allied Signal TPE331-12UHR-701G turboprop engine, serial number P-70210, had accumulated 9,139.8 hours and 15,585 cycles since new and 3,066.9 hours since overhaul. In May 1997, it had Allied Signal service bulletin TPE331-72-2002 incorporated, which detailed replacement of the inner baffle with a new inner baffle part number 3108039-2. Service bulletin TPE331-72-2030, that detailed replacement of the compressor interstage seal assembly support, was incorporated in July 1999, at 6,073 hours, during engine overhaul, After overhaul, the engine was installed into DMI where it remained until the failure.

The rotating air seal, part number 3103839-3, serial number 2-23315-945, appeared to have been installed in the engine since new as its time and cycles since new were identical to those applicable to the engine. During the engine overhaul in 1999, it was inspected in accordance with the requirements current at that time and found serviceable.

The ATSB was advised that cracking of the rotating air seals had occurred in the past, but that it was rare to see a cracked rotating air seal on engines that have the requirements of the engine manufacturer service bulletins TPE331-72-2002 and TPE331-72-2030 incorporated.

Rotating air seal examination

The examination of the rotating air seal and other components from the failed engine is detailed in the ATSB's technical analysis report number 40/01. The examination revealed that the entire outer rim of the rotating air seal had separated from the flanged section. About seventy percent of the rim circumference was recovered and most material from the outer ten millimetres of the plate flange was lost.

One location, where the loss of material was substantially greater, exhibited a short length of fracture showing evidence of fatigue crack propagation. Heat tinting over the area of fatigue indicted that it was present prior to the event failure. The seal had no evidence of material or manufacturing anomalies.

Examination of the turbine components

The turbine disks and nozzle guide vane assemblies showed evidence of random impact damage to the blade leading edges. The damage was consistent with the separated pieces of the failed rotating air seal passing through the turbine.

A copy of the ATSB's technical analysis report, number 40/01, is available on the ATSB web site at or from the ATSB on request.

Engine manufacturer's action

The engine manufacturer reported a number of documented in-flight shutdowns due to separation of the rotating air seal plate rim. Their investigation into the events concluded that cracking in the rim area was due to elevated rim operating temperatures, primarily due to hot gas leakage from deteriorated first stage stator assembly hardware. To alleviate the problem, the manufacturer introduced service bulletins TPE331-72-2002 and TPE331-72-2030 and revised the engine maintenance manual to improve inspection of the relevant components at hot section inspection with the intention of preventing hardware prone to gas path leakage from returning to service.

 

The engine manufacturer advised that investigation into previous failures of the rotating air seal concluded that the cracking in the rim area was due to elevated rim operating temperatures, primarily due to hot gas leakage from deteriorated first stage stator assembly hardware.

To alleviate the problem, the manufacturer introduced two service bulletins, TPE331-72-2002 and TPE331-72-2030 and revised the engine maintenance manual to improve inspection of the relevant components at hot section inspection with intention to prevent hardware prone to gas path leakage from returning to service. The requirements of the service bulletins were incorporated in this engine in 1997 and 1999 respectively. The workshop that overhauled the engine indicated that incidents of rotating air seal cracks were rare on engines that had been modified in accordance with the service bulletins, but they had no service data on the failed seals.

The subject rotating air seal had accumulated over 6,000 hours before the engine had the requirements of the service bulletins incorporated. At that time, the seal was inspected in accordance with the current requirements. However, the possibility that the rotating seal failure was related to damage incurred during the seal's prior time in service could not be excluded.

The investigation determined that fatigue crack propagation led to separation of the rotating air seal outer rim from the flanged section. The fatigue cracking was consistent with the air seal being operated with elevated rim temperatures prior to the incorporation of the engine manufacturer's service bulletins.

 

Fairchild Industries Inc., SA227-DC, VH-DMI

1. FACTUAL INFORMATION
1.1 Introduction

Extensive radial cracking of the flangeOn 12 August 2001, the take-off of a Fairchild Metro 23 aircraft (VH-DMI) was discontinued as a result of the failure of the left engine. To investigate the event, the Australian Transport Safety Bureau carried out an examination of major components from the first, second and third turbine stages of the affected engine.

  • Centre rotating air seal plate (p/no. 3103839-3, s/no. 2-23315-945)
  • First-stage turbine wheel (p/no. 3103897-10, s/no. 3-03229-1075)
  • Second-stage nozzle guide vane assembly
  • Second-stage turbine wheel (p/no. 3103927-5, s/no. 981217201126)
  • Third-stage nozzle guide vane assembly
  • Third-stage turbine wheel (p/no. 3103838-2, s/no. 0-01345-1837)

An earlier disassembly of the engine had found these components damaged to varying degrees.

To further assist the examination, two additional rotating air seal plates were supplied. Extensive radial cracking of the flange section had rendered both of these items unserviceable.

1.2 Component history

The failed engine was an Allied Signal (Garrett) TPE331-12UHR-701G, serial number P-70210. At the time of failure, the engine had operated for a total of 9,140 hours and through 15,585 flight cycles. The following life information was also provided for the rotating components.

Item TSN (hours) CSN Life Limit (cycles)
Air Seal Plate
9,140
15,585
20,000
S1 Turbine Wheel
9,140
15,585
20,000
S1 Turbine Blades
3,067
5,728
Unknown
S2 Turbine Wheel
6,073
9,857
15,000
S3 Turbine Wheel
4,614
8,545
12,000
 
General details
Date: 12 August 2001 Investigation status: Completed 
Time: 1030 hours EST Investigation type: Occurrence Investigation 
Location   (show map):Orange, Aero.  
State: New South Wales Occurrence class: Technical 
Release date: 10 September 2002 Occurrence category: Technical Analysis 
Report status: Final Highest injury level: None 
 
Aircraft details
Aircraft manufacturer: Fairchild Industries Inc 
Aircraft model: SA227 
Aircraft registration: VH-DMI 
Serial number: DC-839B 
Type of operation: Air Transport Low Capacity 
Damage to aircraft: Minor 
Departure point:Orange, NSW
Departure time:1026 hours EST
Destination:Sydney, NSW
 
 
 
Share this page Provide feedback on this investigation
Last update 13 May 2014