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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.

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

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

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

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

Figure 3. 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.

Figure 4. 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.

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.

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

Figure 4. 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).

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

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

Figure 8. 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.

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.

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

Figure 10. 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.

Publication Mode
Aviation
Publication date
Review Date