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If the intake of a running jet engine is blocked for some short duration what would be the sequence of events following it? Will it cause any damage to the engine parts?

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This explanation is for a subsonic aircraft.

If air flow is disturbed at the engine inlet, like it happens if the engine angle of attack is too high (or the crab angle too high like in this accident) or air is disturbed by another mean like the wake of a preceding aircraft, a compressor stall can occur.

The flow within the fan and compressor stages is no more homogeneous, and different mass flows exist in the compressor. These disparities are increased until the flow stops in low mass flow regions, the reason is:

  • Compressor stages are made of blades and vanes delimiting small passages between them.

  • If one passage is blocked by slow air, upstream air tries to use the next one.

  • The next passage is reached under a higher angle of attack, so this tends to stall this passage too.

    enter image description here

  • This effect propagates rapidly and is called a rotating stall.

This has been explained here: What exactly is a compressor stall?


Effects

  1. Unsteady airflow

As passages are blocked by detached air over stalled airfoils, pressure increases in the passages and upstream.

Detached airflow being very turbulent with changing velocity (including negative values), blade/vane vibrations can start. Vibrations are not good for an engine, buffeting can break a metal part if resonance occurs.

  1. Pressure decrease

The compressor being now less efficient, pressure downstream the compressor decreases in addition of being variable. The pressure variation intensity is somehow limited by the plenum chamber effect of the compressor diffuser for an axial compressor and diffuser stator after the impeller for a centrifugal compressor.

enter image description here
Location of the diffuser, source

Depending on whether a new balance can be found or not, combustion may continue while the rotating stall occurs. In that case it'll be at reduced power due to the less efficient compressor, and a lower fuel flow is required.

It may be necessary to shutdown the engine and restart it to get out of the compressor stall.

  1. Overheating

If fuel flow is not reduced in a steady rotating stall, the mixture becomes too rich, combustion temperature increases, overheating occurs in the combustion chamber and downstream in the turbine. As the materials used for the combustion chamber and the turbine are already at their highest sustainable temperature, any increase can damage them.

This can happen if the fuel control unit (FCU) doesn't limit fuel flow correctly and attempts to maintain previous thrust / fuel flow. It is suspected this was the cause of this accident.

  1. Surge

The compressor diffuser (or plenum in a centrifugal compressor) is upstream of the combustion chamber. The latter is a very high pressure region at hot temperature. If the diffuser pressure is too low compared to turbine inlet pressure, hot gas can start flowing back into the diffuser and the compressor. This is a surge.

The surge creates a loud bang and the pressure wave can damage the fan and the engine inlet. The surge can be associated with flames getting out of the engine by the exhaust pipe, or the engine inlet. There is also an asymmetrical additional thrust leading to a yaw force.

Compressor elements are not designed to sustain hot gas from the combustion chamber, they may be damaged if exposition is repeated.

Again the engine may recover itself from the surge, if there is no damage. The cycle can repeat but at some point damages are inevitable.

  1. Loss of control

If the surge is important and the aircraft is in a critical phase (e.g. rotating to take off), the yaw effect can lead to a loss of control.

The yaw effect was actually interpreted as a collision with another aircraft on the runway in this takeoff accident.

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The compressor would stall. This will cause a sudden rise in turbine temperature, which could damage the turbine(s). In the end, with no air intake, the flame will die out and the engine would stop.

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