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Technical Analysis Report

Summary

Technical Analysis Report: Examination of Components from a Failed Turbomeca Arriel 1S1 Turboshaft Engine
Sikorsky S76 Helicopter, VH-EXX

1. FACTUAL INFORMATION
1.1 Introduction

A Sikorsky S76C helicopter (VH-EXX) sustained a failure of the number-two engine during cruise flight. The failed engine was a Turbomeca Arriel 1S1 turboshaft engine, serial number 15038 and had accumulated 7,935 hours and 6,784 cycles since new.

Reports from the flight crew indicated that the engine failure was associated with a loss of gas-generator turbine speed and an escalation of turbine outlet temperatures. Fire warnings for the engine were also received, prompting the pilot commanded shutdown of the engine and discharging of the fire suppression system.

1.2 Engine examination

Disassembly of the engine (figure 1) was carried out at the Bankstown (NSW) facility of Turbomeca Pty Ltd, in the presence of representatives from the engine manufacturer, the helicopter operator and the Australian Transport Safety Bureau. The examination revealed the following significant damage to the operating components of the engine:

  • Outer wall of the centrifugal diffuser cracked and separated into seven pieces over half the circumference (figure 2).
  • First-stage gas-generator turbine blades oxidised and burnt over the outermost third of their length (figure 3).
  • Second-stage nozzle guide vanes extensively overheated and partially melted on the convex (trailing) face and on the trailing edges (figure 4).
  • Second-stage gas-generator turbine blade number 16 fractured beneath the platform. Remaining blades burnt and mechanically damaged on tip edges (figure 5).
  • Second-stage NGV housing indented and punctured, circumferential cracking extending from this area (figure 6).
  • Power turbine NGV missing two vanes; the remainder showing mechanical damage (figure 7).
  • Number-three (rear) bearing collapsed, showing extensive overheating and out-of-balance damage to races and adjacent seals (figure 8).
  • Rear bearing air vent and oil return lines fractured from outside of housing (figure 9).
  • Two of the three T5 thermocouples burnt away completely (figure 10).

Figure 1. Arrial 1S1 engine, serial number 15038, as removed from the aircraft.

Arrial 1S1 engine, serial number 15038, as removed from the aircraft.

Figure 2. Diffuser assembly, showing break-up of the outer housing.

Diffuser assembly, showing break-up of the outer housing. 

 Figure 3. First-stage gas-generator turbine blades oxidised and burnt over their outer length.

First-stage gas-generator turbine blades oxidised and burnt over their outer length.

Figure 4. Second-stage nozzle guide vanes extensively melted and disrupted in a localised area.

Second-stage nozzle guide vanes extensively melted and disrupted in a localised area.

Figure 5. Second-stage gas-generator turbine blades damaged and oxidised, with one blade missing. Item in upper left corner is a guide vane from the power turbine NGV.

Second-stage gas-generator turbine blades damaged and oxidised, with one blade missing. Item in upper left corner is a guide vane from the power turbine NGV.

Figure 6. Second-stage NGV housing with a large puncture and cracking from the released turbine blade.

Second-stage NGV housing with a large puncture and cracking from the released turbine blade. 

Figure 7. Power turbine NGV assembly, missing a vane (see Figure 5).

Power turbine NGV assembly, missing a vane

Figure 8. Rear bearing race and rotating air seals, showing extensive out-of-balance damage.

Rear bearing race and rotating air seals, showing extensive out-of-balance damage.

Figure 9. Rear bearing air vent line, fractured at point of connection with the bearing housing. The oil return line had failed in a similar way.

Rear bearing air vent line, fractured at point of connection with the bearing housing. The oil return line had failed in a similar way.

Figure 10. Thermocouple assembly - thermocouples at arrows burned/damaged.

Thermocouple assembly - thermocouples at arrows burned/damaged.

From these observations, the axial compressor diffuser assembly and the second stage turbine rotor were selected for further examination.

 

 
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