Pilot Debrief

BEECH B100 near Bakersfield, CA — 2014-07-17

Final reportWPR14IA297
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Date
2014-07-17
Location
Bakersfield, CA, USA
Airport
KBFL
Aircraft
BEECH B100
Registration
N6756P
Category
Airplane
Highest injury
None
Fatalities
0
Phase of flight
Cruise

Probable cause

The failure of the 2nd-stage turbine wheel due to an improper repair of the 2nd-stage stator assembly, which the manufacturer does not consider a repairable item. Contributing to the incident was the designated engineering representative's approval of the repair process.

Contributing factors

Contributing to the incident was the designated engineering representative's approval of the repair process.

NTSB narrative

The pilot reported that, while in cruise flight, the left engine of the twin turboprop airplane lost power. The pilot secured the left engine and performed an uneventful single-engine landing at an airport. Examination revealed damage to the left nacelle and external damage to the left engine, consistent with an uncontained engine failure. Further examination of the left engine revealed an overload failure of the 2nd-stage turbine wheel. The investigation determined that a repair to a brazed joint on the 2nd-stage main nozzle casting support baffle and outer flange had failed. This baffle directs cooling air to the aft face of the 1st-stage turbine wheel and knife-edge seal ring. Once the baffle failed and cooling air was lost, the temperature of the knife-edge seal ring cavity increased, which precipitated its fracture. Once the seal ring fractured, it sprang open slightly from normal internal stresses, became loose in the cavity, and then migrated axially aft until it contacted the rotating 2nd-stage turbine wheel web and started to machine the web material, weakening it until the overload failure occurred. According to the engine manufacturer, without the cooling air, metal temperatures of the knife-edge seal ring at the forward and aft ends were estimated to be 1446°F and 1263°F, respectively. Intergranular fractures were initiated after exposure to these high temperatures. The nominal cavity temperature should be 1060°F. A review of the repair history of the 2nd-stage stator assembly revealed that the forward braze joint had been repaired about 5 years before the accident using a repair process specification that was not applicable for the assembly part being repaired. Further, the engine manufacturer does not consider the part a repairable item and advises that it be removed from service. However, the Federal Aviation Administration designated engineering representative had approved the repair process and did not include technical substantiation.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Repair of the 2nd stage stator assembly was unapproved and should not have been performed.

NTSB coding

  • Loss of engine power (total) · Enroute-cruise
  • Engine Failure Enroute

Evidence available

  • Photos
  • 8 docket documents
View NTSB final reportView NTSB docket

Docket documents8

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