In-Flight Compressor Surge After Bleed Valve Malfunction
A high-bypass turbofan is in a step climb from FL340 to FL360. At N1 84% and EGT 720°C, the crew hears a loud bang. Within 2 seconds, EGT rises to 810°C, N1 oscillates ±6%, and fuel flow fluctuates ±300 kg/h. The crew reduces thrust to idle; the engine recovers and stabilizes. There is no FADEC fault message. A borescope shows minor HPC stage 3–4 tip rub within limits and no FOD. The bleed valve position indicator shows OPEN at idle; the valve should be closed above idle. The engine has 900 cycles since the last shop visit. The aircraft is a twin-engine ETOPS aircraft, currently 90 minutes from the nearest suitable airport.
1. Classify the event. What evidence distinguishes compressor surge from a flameout or a fuel-metering fault?
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The event is a compressor surge (rotating stall), not a flameout or a fuel-metering fault.
- A flameout would show EGT falling, N1 decaying and no recovery without a restart. Here EGT rose sharply and the engine recovered when thrust was reduced.
- A fuel-metering fault would change fuel flow and EGT but would not normally produce a loud bang with simultaneous N1 and fuel-flow oscillation. The bang is the pressure pulse from a stalled compressor stage.
- The ±6% N1 oscillation and ±300 kg/h fuel-flow fluctuation are the signature of an aerodynamic instability: the compressor alternately stalls and recovers, and the fuel control responds to the changing N1/N2.
The rapid stabilization after throttle reduction supports surge rather than a mechanical failure.
2. How could a bleed valve stuck open contribute to surge at high altitude?
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At FL360 the compressor is already operating at high pressure ratio and low corrected mass flow, close to the surge line. A bleed valve stuck open changes the stage-by-stage pressure and axial-velocity distribution. In particular, it reduces the pressure downstream of the HPC and alters the incidence on the front stages, moving the operating point toward the surge line. The VSV schedule is set for the normal bleed configuration, so the combination of a stuck-open bleed valve and high-altitude, high-power operation can push the compressor into surge.
The fact that the valve is open at idle, when it should be closed above idle, confirms a valve or control fault. The borescope tip rub is a consequence of the surge, not the root cause, provided it remains within limits.
3. State the maintenance and dispatch decision.
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Dispatch decision: no dispatch until the bleed system is corrected and the engine is checked.
Maintenance actions:
- Troubleshoot the bleed valve, its actuator and sense lines; replace the valve if it is stuck or the actuator is faulty.
- Borescope the HPC and confirm the tip rub is within the AMM limit.
- Check VSV rigging and the FADEC bleed-valve schedule.
- Ground-run the engine through idle-to-takeoff accelerations and verify the bleed valve closes and opens at the correct N1/N2.
- If the valve is replaced and the borescope is within limits, return to service with a repeat borescope at a shortened interval.
- If the rub is beyond limits or surge recurs, remove the engine.
An ETOPS dispatch with a known bleed-valve fault and a recent surge is not acceptable.