CAUSES AND EFFECTS OF COMPRESSOR STALL AND SURGE STALL.
Compressor stall is a localised aerodynamic break down that can occur in different parts of the compressor depending on the prevailing off design conditions.
It is the result of the angle of attack of rotor blades becoming too high (positive incidence) or too low (negative incidence) to the point where stalling and a breakdown of the smooth airflow over the blades occurs.
If the stall condition in a stage or a group of stages continues until all stages are stalled, a condition called a hung stall, then the compressor airflow may reverse flow in a condition called surge.
The transition from a stall to a surge may happen so fast that you may not be able to distinguish the difference.
On the other hand, the stall may be so weak that it will only produce slight vibration or poor acceleration and then disappear. This is called a transient stall.
Severe compressor stalls may reveal themselves by a rise in exhaust gas temperature (EGT), distinct vibration, and ‘coughing’ noises from the engine, poor or no acceleration.
The distinction between a severe stall and mild surge is a matter of severity and it requires careful observation to recognize that flow reversal is actually occurring.
Engine surges are caused by aerodynamic disruption causing a complete breakdown of the airflow through the compressor that causes all the compressor stages to stall.
This will occur if the rear compressor stages choke or the compressor pressure ratio becomes excessive.
Surging is a motion of the airflow forwards and backward through the compressor.
It can be caused by foreign object ingestion, distorted aerofoils, off schedule variable inlet guide vanes or variable stator vanes, bleed valve malfunction, rapid throttle movements, over-fuelling during acceleration, variable exhaust nozzle failure.
In fact any external influence that simultaneously reduces the air mass flow and raises the compressor pressure ratio can be listed as suspect.
Surge can be indicated by a varying degree of abnormal engine noises. It could be a muffled rumble, a single abrupt bang or a repeated series of bangs of varying severity.
Other indications are rapidly increasing EGT, vibration, flames emitting from the exhaust (and possibly from the air intake as well), poor throttle response, flame out.
Surge is always accompanied by a high exhaust gas temperature (EGT), which may also rise rapidly on throttle opening.
If a surge occurs the throttle should be gently pulled back to idle and the engine shut down for investigation.
All occurrences of surge must be reported and recorded in the Tech Log together with the duration of the surge condition.