Compressor stall
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A compressor stall is a situation of abnormal airflow resulting from a stall of the aerofoils within the compressor. Stall is found in dynamic compressors, particularly axial compressors, as used in jet engines and turbocharged reciprocating engines.
Compressor stalls result in a loss of compressor performance, which can vary in severity from a momentary engine power drop (occurring so quickly it is barely registered on engine instruments) to a complete loss of compression (compressor surge) necessitating a reduction in the fuel flow to the engine.
Modern compressors are carefully designed and controlled to avoid or limit stall within an engine's operating range. Stall was a common problem on early jet engines with simple aerodynamics and manual or mechanical fuel control units, but has been virtually eliminated by better design and the use of hydromechanical and electronic control systems such as Full Authority Digital Engine Controls.
Types
There are two types of compressor stall:
Rotational stall
"Rotational stall" is a local disruption of airflow within the compressor which continues to provide compressed air but with reduced effectiveness. "Rotational stall" arises when a small proportion of aerofoils experience aerofoil stall disrupting the local airflow without destabilising the compressor. The stalled aerofoils 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%-70% of their speed, affecting subsequent aerofoils around the rotor as each encounters the stall cell. Stable local stalls can also occur which are axi-symmetric, covering the complete circumference of the compressor disc but only a portion of its radius, 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 aerofoils encountering stall cells in the region affected. In many cases however, the compressor aerofoils 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
"Axi-symmetric stall", more commonly known as "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.[1] 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.
Causes
Compressor stalls are aerodynamic stalls in which the aerofoils in the compressor are loaded above their lifting capability. This can arise for a number of reasons which result in either a drop in the expected compressor performance or the compressor is loaded in conditions beyond its design.
Factors affecting compressor performance
- Damaged compressor components caused by ingestion of foreign objects. One of the most common causes of compressor stalls in commercial aviation aircraft is a bird strike. On take-off, while manoeuvring 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 the disruption to the airflow and damage to the blades often causes compressor stall.
- Worn or contaminated compressor components such as eroded rotor blades, seals or bleed valves. Even dust and dirt in the compressor can reduce its efficiency and lead to a stall if the contamination is severe enough.
Factors increasing compressor loads
- Aircraft operation outside of design envelope - e.g., extreme flight manoeuvre resulting in airflow separations within the engine intake. Flight within icing conditions where ice can build up within the intake or compressor. Engine thrust requirements too high for the operating altitude. (limited with modern fly-by-wire controls)
- Engine operation outside specified design parameters - e.g., abrupt increases in engine thrust ("slam acceleration") causing a mismatch between engine components. (Occurrence reduced through the use of modern electronic control units)
- Turbulent or hot airflow to the engine intake - e.g., use of reverse thrust at low forward speed, resulting in re-ingestion of hot turbulent air, or for military aircraft, ingestion of hot exhaust gases from fired missile.
- Worn or contaminated engine components - e.g. poorly performing control unit or turbine within an engine may result in a mismatch increasing the likelihood of stall.
Effects
Compressor axially-symmetric stalls, or compressor surges, are immediately identifiable because they produce one or more extremely loud bangs from the engine. Reports of fire or flames "shooting" out of 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 yawing of the aircraft in the direction of the affected engine due to the loss of thrust. Severe stresses occur within the engine and aircraft particularly from the intense aerodynamic buffeting within the compressor.
Response and recovery
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.
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.
Notable stall occurrences
Aircraft development
Pratt & Whitney J58 engines
The Lockheed SR-71 Blackbird, a supersonic reconnaissance aircraft developed in the United States, employed Pratt & Whitney J58 turbojet engines that were known for their tendency to “hard unstart,” that is, to produce spectacular compressor stalls, often violent enough to throw the pilot's head against the canopy of the aircraft.
These were due to shock waves that moved out of their proper location within the jet's air intakes during supersonic flight. The stall of one engine produced a dramatic loss of thrust from one side, triggering a violent yaw movement, 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.
Rolls-Royce Avon engine
The Rolls-Royce Avon turbojet engine was affected by repeated compressor surges early in its development which proved difficult to eliminate from the design. Such was the perceived importance and urgency of the engine that Rolls-Royce licensed the compressor design of the Sapphire engine from Armstrong Siddeley to speed development.
The engine, as redesigned, went on to power landmark aircraft such as the English Electric Canberra bomber, and the de Havilland Comet and Sud Aviation Caravelle airliners.
Aircraft crashes
U. S. Navy F-14 crash
A compressor stall contributed to the 1994 death of Lt. Kara Hultgreen, the first female 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 and Whitney [[Pratt and Whitney TF30 [TF30]] turbofan, due to disturbed airflow caused by Hultgreen's attempt to recover from an incorrect final approach position by executing a sideslip; Compressor stalls from excessive yaw angle were a known deficiency of this type of engine.
Southern Airways Flight 242
The 1977 loss of Southern Airways Flight 242, a Douglas DC-9-31, while penetrating a thunderstorm cell over Georgia was attributed to compressor stalls brought on by ingestion of large quantities of water and hail which blocked bleed air removal from both of its 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; 62 passengers and another 8 people on the ground were killed.
Trans World Airlines Flight 159
On November 6, 1967, TWA Flight 159, a Boeing 707 on its takeoff roll from the then-named Greater Cincinnati Airport, passed Delta Air Lines Flight 379, a 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.
US Airways Flight 1549
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On January 15, 2009, US Airways Flight 1549, an Airbus A320 ditched in the Hudson River about five minutes after takeoff. The apparent cause was compressor stall in both engines after flying through a flock of birds about 90 seconds after takeoff. This same aircraft may have suffered a compressor stall on the right engine two days earlier[2][3]. After an incident in which an Airbus A321-200 experienced compressor stalls on both engines during initial climb out on December 15th 2008, an EASA Emergency Airworthiness Directive 2008-228 requested operators of CFM56-5B engines (operated on the plane that crashed into Hudson River) to monitor Exhaust Gas Temperatures (EGT) for deterioration and make sure that at least one engine shows less than 80 degrees C deterioration in its EGTs. The FAA have issued the same requirements as Airworthiness Directive AD 2009-01-01 with immediate effect.[4]
References
Notes
Bibliography
- Kerrebrock, Jack L. "Aircraft Engines and Gas Turbines", 2nd edition. Cambridge, Massachusetts: The MIT Press, 1992. ISBN 0-262-11162-4.