Compressor stall

A compressor stall is a situation of abnormal airflow through the compressor stage of a jet engine, causing a stall of the vanes of the compressor rotor.

All compressor stalls result in a loss of engine power. This power failure may only be momentary (occurring so quickly it is barely registered on engine instruments), or may shut the engine down completely (that is, causing a flameout). When a compressor stall affects the airflow through the entire engine it is also known as a compressor surge.

Types

There are two general types of compressor stall.

The first, less severe type of stall, the "axis-symmetric stall", is a straightforward expulsion of air out the intake due to the compressor's inability to maintain pressure on the combustion chamber.

In the second, more-severe "rotational stall", the air flow disruption of the stall causes standing pockets of air to rotate within the compressor without moving along the axis. Without fresh air from the intake passing over the stalled compressor vanes they overheat, causing accelerated engine wear and possible damage.

Causes

The most likely cause of a compressor stall is a sudden change in the pressure differential between the intake and combustion chamber. Jet aircraft pilots must take this into account when dropping airspeed or increasing throttle.

The following factors can induce compressor stall:

• Engine over-speed
• Engine operation outside specified engineering parameters
• Turbulent or disrupted airflow to the engine intake
• Contaminated or damaged engine components

Effects

Compressor stalls can result in one or more extremely loud bangs emanating from the engine as the combustion process "backfires". This may be accompanied by an increased exhaust gas temperature, and yawing of the aircraft in the direction of the affected engine.

The effects of a stall can vary. A minor stall may create an alarming noise but have little other effect. On the other hand, a violent compressor surge might completely destroy the engine and set it on fire.

The appropriate response to compressor stalls varies according the engine type and situation — but usually consists of immediately and steadily decreasing thrust on the affected engine.

As a bit of trivia, the crews of the Lockheed (now Lockheed Martin) SR-71 Blackbird were witness to arguably some of the most spectacular compressor stalls ever. These usually happened at high Mach numbers, and were normally the result of shockwave detachment on the conic diffusers on the intakes. The resulting loss of thrust on the affected engine (and its accompanying yaw moment), combined to the long fuselage of the airplane should have made for an interesting ride for the crew, specially taking into account that they had a rather short time to correct the problem before the mission or the airframe were compromised.