Compressor Stall

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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 (although definitions differ, and often the terms are used interchangeably).


Types

There are two general types of compressor stall.
The first, 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, "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

Compressor stalls are aerodynamic stalls in which the airfoils in the compressor lose their lifting capability. This often results in 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 thrust too high for the operating altitude

Engine operation outside specified design parameters

Turbulent or disrupted airflow to the engine intake

Contaminated or damaged engine components (such as damaged or wrongly positioned guide vanes)

Abrupt increases in engine thrust

Use of reverse thrust at insufficient forward speed, resulting in reingestion of turbulent airflow

One of the most common causes of first stage compressor stalls in commercial aviation aircraft is a bird strike. On take-off, while maneuvering on the ground or while on approach to landing, planes operate in proximity to birds. It is not uncommon for birds to be sucked into the intake of the engine and often can cause a first-stage compressor stall. Because birds are combustible material, a fire or flames described as "shooting" out of the engine are common reports during this type of compressor stall.

In the case of an axi-symmetric stall, an entire stage of airfoils stall. This may cause an increase in rotor speed due to the large reduction in work done by the stalled rotor stage, which results in other stages stalling and creating a domino effect in which the remainder of the compressor stages stalling. The result of this is the loss of the compressor's capability to maintain a pressure ratio and the subsequent backflow from the combustor section.

In the case of a rotating stall, a single airfoil stalls. This stall creates the aforementioned pocket of stagnant air, which then passes to the next airfoil on the rotor. The pocket of stagnant air causes this airfoil to stall, thus propagating the stall.


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.


Trivia

The Lockheed SR-71 Blackbird, a supersonic reconnaissance aircraft developed in the United States, employed special jet engines that were known for their tendency to “unstart,” that is, their tendency to produce spectacular compressor stalls, often violent enough to throw the pilot's head against the canopy of the aircraft. The stalls occurred when shockwaves over the jet intakes moved out of their proper location during high-speed supersonic flight. The stalls produced a very dramatic loss of thrust and a violent yaw moment, and required quick action by the crew to avoid compromise of the mission or airframe. Unstarts were the bane of SR-71 pilots until computer controls on the engines later in the SR-71 program significantly reduced their incidence and simplified recovery.

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