View source for Compressor stall

{{Short description|Gas turbine phenomenon}}{{More citations needed|date=March 2024}}
[[File:Comparison of normal and distorted airflow into the compressor section.png|thumb|right|250px|Comparison of normal and distorted airflow into the compressor section<ref name=phak>{{cite book |title=Pilot's Handbook of Aeronautical Knowledge |date=2016-08-24 |publisher=[[Federal Aviation Administration]] |pages=7–24}}</ref>]]

A '''compressor stall''' is a local disruption of the airflow in the [[compressor]] of a [[gas turbine]] or [[turbocharger]]. A stall that results in the complete disruption of the airflow through the compressor is referred to as a '''compressor surge'''. The severity of the phenomenon ranges from a momentary power drop barely registered by the engine instruments to a complete loss of compression in case of a surge, requiring adjustments in the fuel flow to recover normal operation.

Compressor stalls were a common problem on early jet engines with simple aerodynamics and manual or mechanical [[fuel control unit]]s, but they have been virtually eliminated by better design and the use of hydromechanical and electronic control systems such as [[full authority digital engine control]]. Modern compressors are carefully designed and controlled to avoid or limit stall within an engine's operating range.

==Types==
[[File:Axial compressor.gif|thumb|right|An animation of an [[axial compressor]] showing both the [[Axial compressor#Description|stator]] blades and the [[Axial compressor#Description|rotor]] blades]] 

There are two types of compressor stall:

===Rotating stall===
'''Rotating stall''' is a local disruption of [[axial compressor|airflow within the compressor]] which continues to provide compressed air, but with reduced effectiveness.  Rotating stall arises when a small proportion of [[airfoil]]s experience [[Stall (fluid dynamics)|airfoil stall]], disrupting the local airflow without destabilising the compressor. The stalled airfoils create pockets of relatively stagnant air (referred to as ''stall cells'') which, rather than moving in the flow direction, rotate around the circumference of the compressor. The stall cells rotate with the rotor blades, but at 50 to 70% of their speed, affecting subsequent airfoils around the rotor as each encounters the stall cell.  Propagation of the instability around the flow path annulus is driven by stall cell blockage causing an incidence spike on the adjacent blade.  The adjacent blade stalls as a result of the incidence spike, thus causing stall cell "rotation" around the rotor.  Stable local stalls can also occur which are axi-symmetric, covering the complete circumference of the compressor disc, but only a portion of its radial plane, with the remainder of the face of the compressor continuing to pass normal flow.

A rotational stall may be momentary, resulting from an external disturbance, or may be steady as the compressor finds a working equilibrium between stalled and unstalled areas. Local stalls substantially reduce the efficiency of the compressor and increase the structural loads on the airfoils encountering stall cells in the region affected. In many cases however, the compressor airfoils are critically loaded without capacity to absorb the disturbance to normal airflow such that the original stall cells affect neighbouring regions and the stalled region rapidly grows to become a complete compressor stall.

===Axi-symmetric stall or compressor surge {{anchor|Axi-symmetric stall|Compressor surge|Pressure surge}}===
'''Axi-symmetric stall''', more commonly known as '''[[Surge in compressors|compressor surge]]'''; or '''pressure surge''', is a complete breakdown in compression resulting in a reversal of flow and the violent expulsion of previously compressed air out through the engine intake, due to the compressor's inability to continue working against the already-compressed air behind it. The compressor either experiences conditions which exceed the limit of its pressure rise capabilities or is highly loaded such that it does not have the capacity to absorb a momentary disturbance, creating a rotational stall which can propagate in less than a second to include the entire compressor.

The compressor will recover to normal flow once the [[engine pressure ratio]] reduces to a level at which the compressor is capable of sustaining stable airflow.  If, however, the conditions that induced the stall remain, the return of stable airflow will reproduce the conditions at the time of surge and the process will repeat.<ref name="Kerre261">Kerrebrock 1992, p.261.</ref> Such a "locked-in" or self-reproducing stall is particularly dangerous, with very high levels of vibration causing accelerated engine wear and possible damage, even the total destruction of the engine through the breaking of compressor and stator vanes and their subsequent ingestion, destroying engine components downstream.

==Causes==
A compressor will only pump air in a stable manner up to a certain pressure ratio. Beyond this value the flow will break down and become unstable. This occurs at what is known as the surge line on a [[compressor map]]. The complete engine is designed to keep the compressor operating a small distance below the surge pressure ratio on what is known as the operating line on a compressor map. The distance between the two lines is known as the surge margin on a compressor map. Various things can occur during the operation of the engine to lower the surge pressure ratio or raise the operating pressure ratio. When the two coincide there is no longer any surge margin and a compressor stage can stall or the complete compressor can surge as explained in preceding sections.

===Factors which erode compressor surge margin===
The following, if severe enough, can cause stalling or surging.
*Ingestion of [[Foreign object damage|foreign objects]] which results in damage, as well as sand and dirt erosion, can lower the surge line.
*Dirt build-up in the compressor and wear that increases compressor tip clearances or seal leakages all tend to raise the operating line. 
*Complete loss of surge margin with violent surging can occur with a [[bird strike]]. Taxiing on the ground, taking off, low level flying (military) and approaching to land all take place where bird strikes are a hazard. When a bird is ingested by a compressor the resultant blockage and airfoil damage causes compressor surging. Examples of debris on a runway or aircraft carrier flight deck that can cause damage are pieces of tire rubber, litter and nuts and bolts. A specific example is [[Air France Flight 4590|a metal piece dropped from another plane]].<ref>The crash of Air France Flight 4590 was initiated by a piece of titanium alloy, dropped from a DC-10, on the runway.  The metal debris ruptured a tire of the Air France Concorde, and pieces of the exploding tire damaged the plane, rupturing a fuel tank and causing wing structural failure and engine failure.  While the metal debris did not cause a compressor failure, the Concorde accident is an example of a small piece of metal debris being dropped by one aircraft onto a runway and struck by another aircraft, and it is certainly possible that such a piece of debris, once deposited on a runway, might be thrown up by a wheel forward of a jet engine's intake and ingested by the engine, causing compressor damage.  Furthermore, the surges of the port engines of the Flight 4590 Concorde could be examples of compressor stall, induced by the spikes in internal engine pressure as leaking fuel was ingested into the engines (outside of throttle control) and rapidly burned.</ref>  Runways and aircraft carrier flight decks are cleaned frequently in an attempt to preclude ingestion of foreign objects.
* Aircraft operation outside its design envelope; ''e.g.'', extreme flight manoeuvres resulting in airflow separations within the engine intake, flight in icing conditions where ice can build up in the intake or compressor, flight at excessive altitudes.<ref>"Jet Propulsion for Aerospace Applications" 2nd edition 1964 Walter J.hesse Nicholas V.S. Mumford, Pitman Publishing Corporation p201</ref>
* Engine operation outside its flight manual procedures; ''e.g.'', on early jet engines abrupt throttle movements (''slam acceleration'') when pilot's notes specified slow throttle movements. The excessive over-fuelling raised the operating line until it met the surge line. (Fuel control capability was extended to automatically limit the over-fuelling to prevent surging.)
* Turbulent or hot airflow into the engine intake, ''e.g.'', use of [[Thrust reversal|reverse thrust]] at low forward speed, resulting in re-ingestion of hot turbulent air or, for military aircraft, ingestion of hot exhaust gases from missile firing.
* Hot gases from gun firing which may produce inlet distortion; ''e.g.'', [[Mikoyan MiG-27]].

==Effects==
[[File:PAK FA T 50 compressor stall on MAKS 2011.jpg|thumb|[[Sukhoi Su-57]] prototype suffering a compressor stall at [[MAKS Air Show|MAKS 2011]]]]
Compressor axially-symmetric stalls, or compressor surges, are immediately identifiable, because they produce one or more extremely loud bangs from the engine.  Reports of jets of flame emanating from the engine are common during this type of compressor stall. These stalls may be accompanied by an increased exhaust gas temperature, an increase in rotor speed due to the large reduction in work done by the stalled compressor and{{dash}}in the case of multi-engine aircraft{{dash}}[[Aircraft principal axes#Yaw|yawing]] in the direction of the affected engine due to the loss of thrust.

==Response and recovery==
The appropriate response to compressor stalls varies according to the engine type and situation, but usually consists of immediately and steadily decreasing thrust on the affected engine.  While modern engines with advanced control units can avoid many causes of stall, jet aircraft pilots must continue to take this into account when dropping airspeed or increasing throttle.

A compressor anti-stall system is a compressor bleed system that automatically dumps away unwanted air to prevent compressor stalling.<ref>{{Cite web |url=https://www.afahc.ro/ro/afases/2015/afases_2015/mecanica/Tudosie%20Alexandru.pdf |title=MATHEMATICAL MODEL FOR A JET ENGINE WITH ANTI-STALL AUTOMATIC VALVE AIDED COMPRESSOR, Alexandru-Nicolae Tudosie, Department of Electrical, Power Systems and Aerospace Engineering, Faculty of Electrical Engineering, University of Craiova, Romania |access-date=2024-05-17 |archive-date=2024-05-17 |archive-url=https://web.archive.org/web/20240517150040/https://www.afahc.ro/ro/afases/2015/afases_2015/mecanica/Tudosie%20Alexandru.pdf |url-status=live }}</ref> Other methods of stall prevention may include an anti-stall tip treatment of the casing.<ref>{{Cite web |url=https://patents.google.com/patent/EP1478857B1/en |title=EP1478857B1{{dash}}Compressor with an anti-stall tip treatment{{dash}}Google Patents |access-date=2024-05-17 |archive-date=2024-05-17 |archive-url=https://web.archive.org/web/20240517150030/https://patents.google.com/patent/EP1478857B1/en |url-status=live }}</ref>

== Notable stall occurrences ==
=== Aircraft development ===
==== Rolls-Royce Avon engine ====
The [[Rolls-Royce Avon]] [[turbojet|turbojet engine]] was affected by repeated compressor surges early in its 1940s development which proved difficult to eliminate from the design. Such was the perceived importance and urgency of the engine that [[Rolls-Royce Holdings|Rolls-Royce]] licensed the compressor design of the [[Armstrong Siddeley Sapphire|Sapphire]] engine from [[Armstrong Siddeley]] to speed development.

The engine, as redesigned, went on to power aircraft such as the [[English Electric Lightning]] fighter, [[English Electric Canberra]] bomber, and the [[de Havilland Comet]] and [[Sud Aviation Caravelle]] airliners.

====Olympus 593====
During the 1960s development of the [[Concorde]] [[Supersonic transport|Supersonic Transport]] (SST) a major incident occurred when a compressor surge caused a structural failure in the intake. The hammershock which propagated forward from the compressor was of sufficient strength to cause an inlet ramp to become detached and expelled from the front of the intake.<ref>Note: Before ejection from the intake the inlet ramp first travelled inward and struck the front face of the compressor causing considerable damage to the initial compressor stages. Despite this damage, the [[Rolls-Royce/Snecma Olympus 593|Olympus 593]] still remained operable up to around 80% power.</ref> The ramp mechanism was strengthened and control laws changed to prevent a re-occurrence.<ref>Trubshaw, Brian; Edmondson, Sally. ''Brian Trubshaw Test Pilot''. p 110. {{ISBN|0 7509 1838 1}}</ref>

=== Aircraft crashes ===
==== U.S. Navy F-14 crash ====
A compressor stall contributed to the 1994 death of Lt. [[Kara Hultgreen]], the first female [[aircraft carrier|carrier-based]] [[United States Navy]] [[fighter pilot]]. Her aircraft, a [[Grumman F-14 Tomcat]], experienced a compressor stall and failure of its left engine, a [[Pratt & Whitney TF30]] [[turbofan]], due to disturbed airflow caused by Hultgreen's attempt to recover from an incorrect [[Final approach (aeronautics)|final approach]] position by executing a [[Slip (aerodynamics)|sideslip]]; compressor stalls from excessive [[Yaw (rotation)|yaw]] angle were a known deficiency of this type of engine.

==== Southern Airways Flight 242 ====
The 1977 loss of [[Southern Airways Flight 242]], a [[McDonnell Douglas DC-9|McDonnell Douglas DC-9-9-31]], while penetrating a [[thunderstorm]] cell over [[Georgia (U.S. state)|Georgia]], was attributed to compressor stalls brought on by ingestion of large quantities of water and [[hail]]. The stalls caused blades to clash with stationary vanes in both of its [[Pratt & Whitney JT8D|Pratt & Whitney JT8D-9]] turbofan engines. The stalls were so severe as to cause the destruction of the engines, leaving the flight crew with no choice but to make an [[emergency landing]] on a public road, killing 62 passengers and another eight people on the ground.

==== 1997 Irkutsk Antonov An-124 crash ====
On December 6, 1997, an [[Antonov An-124 Ruslan|Antonov 124]] transport plane was destroyed when it [[1997 Irkutsk Antonov An-124 crash|crashed]] immediately after takeoff from [[Irkutsk_Northwest_Airport|Irkutsk-2]] Airport in Russia. Three seconds after lifting off from Runway 14, at a height of about {{convert|5|m|ft}}, the number 3 engine surged. Climbing away with a high [[angle of attack]], engines 1 and 2 also surged, causing the aircraft to crash some {{convert|1600|m|ft}} past the end of the runway. It struck several houses in a residential area, killing all 23 on board, and 45 people on the ground.<ref>{{cite web |url=http://aviation-safety.net/database/record.php?id=19971206-0 |title=ASN Aircraft accident Antonov An-124-100 08 Black Irkutsk-2 Airport |access-date=2015-07-26 |archive-date=2017-08-29 |archive-url=https://web.archive.org/web/20170829034742/http://aviation-safety.net/database/record.php?id=19971206-0 |url-status=live }}</ref>

==== Trans World Airlines Flight 159 ====
On November 6, 1967, [[TWA Flight 159]], a [[Boeing 707]] on its [[takeoff|takeoff roll]] from the then-named [[Cincinnati/Northern Kentucky International Airport|Greater Cincinnati Airport]], passed [[Delta Air Lines]] Flight 379, a [[McDonnell Douglas DC-9]] stuck in the dirt a few feet off the [[runway]]'s edge. The first officer on the TWA aircraft heard a loud bang, now known to have been a compressor stall induced by ingestion of exhaust from Delta 379 as it was passed.  Believing a collision had occurred, the copilot aborted the takeoff. Because of its speed, the aircraft overran the runway, injuring 11 of the 29 passengers, one of whom died four days later as a result of the injuries.

====Scandinavian Airlines Flight 751====
In December 1991 [[Scandinavian Airlines Flight 751]], a [[McDonnell Douglas MD-80#MD-81|McDonnell Douglas MD-81]] on a flight from Stockholm to Copenhagen, crashed after losing both engines due to ice ingestion leading to compressor stall shortly after takeoff. Due to a newly installed auto-throttle system designed to prevent pilots reducing power during the takeoff climb, the pilot's commands to reduce power on recognising the surge were countermanded by the system, leading to engine damage and total [[Turbine engine failure|engine failure]]. The airliner successfully made a forced landing in a forest clearing without loss of life.

==See also==
* [[Index of aviation articles]]
* [[Axial fan design]]
* [[Surge in compressors]]

==References==
The Jet Engine{{dash}}Rolls-Royce plc, 1995. {{ISBN|0-902121-23-5}}.

===Notes===
{{Reflist}}

===Bibliography===
{{refbegin}}
*Kerrebrock, Jack L. "Aircraft Engines and Gas Turbines", 2nd edition. Cambridge, Massachusetts: The MIT Press, 1992. {{ISBN|0-262-11162-4}}.
{{refend}}

==External links==

{{Aircraft gas turbine engine components}}

[[Category:Aviation risks]]