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AIRCRAFT ENGINES AND SYSTEMS > COMPRESSOR STALL > Flashcards

COMPRESSOR STALL Flashcards

1
Q

It may seem that the angle of attack of the rotor blades would never change since they are installed on the rotor disk at a fixed angle; however, changing the rotation speed of the rotors during engine operation and/or changing the velocity of the inlet airflow will cause the compressor’s angle of attack to change. Therefore, anything that decreases the inlet airflow or increases compressor RPM will increase the angle of attack and therefore increase the possibility of a compressor stall.

A

Angle of Attack of rotor blades on rotor fixed angle. Changing the velocity or the inlet airflow will cause the compressor AOA to change.

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2
Q

Causes of Compressor Stall?

A

CAUSES

Compressor stalls are mainly the result of airflow distortions or mechanical malfunctions.

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3
Q

INDICATIONS

Indications of a compressor stall

A

INDICATIONS
Indications of a compressor stall (Figure 3.3-2) depend on the severity of the stall. It can range from mild pulsation with minimum indications to aircraft vibration and loud bangs or noises. Since a compressor stall results in a reduction of airflow to the turbines, more fuel will be required to maintain the current thrust. This increase in fuel will increase burner and turbine temperature. Therefore, with a constant PCL position, indications of a compressor stall include RPM decay, and/or interstage turbine temperature (ITT) rise, along with possible loud noises.

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4
Q

Most likely Cause of Compressor Stall

A

Airflow distortion is the most common cause of a compressor stall. A
compressor stall is normally caused by a breakdown of the airflow through a few stages of the compressor. A compressor stall could progress until the complete unit has stalled. Airflow distortion to the compressor can be a result of the aircraft attitude and airspeed. At high aircraft angles of attack, the air entering the inlet slows and becomes turbulent, as shown in Figure 3.3-3.
Figure3.3-3 AirflowDistortion
As the airflow travels back through the compressor, the angle of the rotors and stators changes. This is to compensate for the change in direction and velocity of the air as it travels through each successive stage. At normal airflow and compressor speeds, the angle of attack is not excessive. However, if the velocity of the incoming air or the speed of the rotor blades changes, the angle of attack changes and may become excessive. Therefore, at high aircraft angles of attack, or when the incoming air is slowed or turbulent, there is the possibility of compressor stall. This is of particular concern in the landing pattern, where engine performance must be perfect.
The instances when airflow distortions may induce compressor stall include:
1. Abrupt change in aircraft attitude
2. Encountering air turbulence
3. Deficiency of air velocity or volume, caused by atmospheric conditions
4. Rapid throttle movement

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5
Q

The instances when airflow distortions may induce compressor stall include:

A
  1. Abrupt change in aircraft attitude
  2. Encountering air turbulence
  3. Deficiency of air velocity or volume, caused by atmospheric conditions
  4. Rapid throttle movement
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6
Q

MECHANICAL MALFUNCTIONS
Mechanical malfunctions are the other main cause for a compressor stall; which, depending on its severity, could result in a complete engine loss. The four mechanical malfunctions discussed here are:

A
  1. Variable inlet guide vane and stator vane
  2. Fuel control unit (FCU)
  3. Foreign Object Damage (FOD)
  4. Variable exhaust nozzle
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7
Q

REMEDY AND RECOVERY

A
  1. first reaction should be to reduce the attitude of the aircraft (possibly lower the nose) which will reduce the inlet’s angle of attack. This allows turbulent free air to enter the inlet at the proper velocity.
  2. PCL should be retarded to just below stall threshold to allow the engine to “catch up” with the inlet airflow. In addition, many aircraft will have automatic bleed valves or a procedure that will be followed to open a valve to allow airflow through the compressor. Once engine indications return to normal, the PCL may be slowly advanced to the desired setting.
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8
Q
  1. What are some indications of a compressor stall?
A
  1. Mild pulsations, engine vibration, loud bangs, drop in RPMs, rise in turbine temperature.
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9
Q
  1. What causes a compressor stall?
A
  1. Excessive compressor blade AOA.
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10
Q
  1. Other than mechanical malfunctions, what causes most stalls?
A
  1. Airflow distortions.
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11
Q
  1. What determines the angle of attack on a compressor blade?
A
  1. The angle between the chord line of the rotors and the relative wind. The relative wind is comprised of the inlet airflow and the compressor RPM.
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12
Q
  1. List some flight conditions that could cause a compressor stall.
A
  1. Abrupt attitude change or flight through turbulent air.
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13
Q
  1. How can FOD affect compressor performance?
A
  1. Adversely due to compressor blade deformation.
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14
Q
  1. What automatically compensates for acceleration and deceleration schedules to help prevent stalls?
A
  1. Fuel Control Unit (FCU).
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15
Q
  1. How can an exhaust nozzle cause a compressor stall?
A
  1. (Only applies to variable exhaust nozzle) it could fail to open, causing a back pressure and reverse flow back through the compressor.
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16
Q
  1. What are some of the ways manufacturers decrease the possibility of stalls?
A
  1. Incorporating Split-spool compressor, bleed air valves, variable inlet guide vanes, and variable exhaust nozzle.
17
Q
  1. What should a pilot do to avoid compressor stalls?
A
  1. Avoid rapid and unnecessary PCL movements. Maintain at least minimum prescribed airspeeds.
18
Q
  1. What should be done if a stall cannot be controlled?
A
  1. Execute engine failure procedures.
19
Q

COMPRESSOR STALL RECOVERY

A
  1. REDUCE THROTTLE
  2. DECREASE AOA
  3. INCREASE AIRSPEED
20
Q

COMPRESSOR STALL IS CAUSED BY?

A

DISRUPTION OF AIR FLOW

21
Q

HIGH ALT COMPRESSOR STALL INDICATIONS?

A
  1. NOISE
  2. LIGHT VIBRATION
    3 RISE IN EGT
22
Q

LOW ALT COMPRESSOR STALL INDICATIONS?

A
  1. HIGH VIBRATION
  2. BANGS
  3. VIOLENT YAW
23
Q

CAUSES COMPRESSOR STALL

A
  1. FOD (BIRD INTO ENGINE)
  2. FUEL CONTROL TO FLOW DEPENDENT OF THROTTLE SETTING
  3. CLIMBING CRITICAL A/O/A EXCEEDED IN COMPRESSOR SECTION.
  4. HOCKEY OF THROTTLES AT HIGH ALT.
24
Q

A compressor stall occurs when the compressor stage blades exceed their ________.

A

critical AOA

25
Q

Changing the rotation speed of the rotors during engine operation and/or changing the velocity of the inlet airflow will cause the compressor’s angle of attack to change.
Therefore, anything that decreases the inlet airflow or increases compressor RPM will increase the angle of attack and therefore increase the possibility of a compressor stall

A

Changing the rotation speed of the rotors during engine operation and/or changing the velocity of the inlet airflow will cause the compressor’s angle of attack to change.
Therefore, anything that decreases the inlet airflow or increases compressor RPM will increase the angle of attack and therefore increase the possibility of a compressor stall

26
Q

Therefore, anything that decreases the ________ or increases compressor RPM will increase the angle of attack and therefore increase the possibility of a compressor stall

A

inlet airflow

27
Q

As the airflow travels back through the compressor, the angle of the rotors and stators changes. This is to compensate for the change in direction and velocity of the air as it travels through each successive stage. At normal airflow and compressor speeds, the angle of attack is not excessive. However, if the velocity of the incoming air or the speed of the rotor blades changes, the angle of attack changes and may become excessive

A

As the airflow travels back through the compressor, the angle of the rotors and stators changes. This is to compensate for the change in direction and velocity of the air as it travels through each successive stage. At normal airflow and compressor speeds, the angle of attack is not excessive. However, if the velocity of the incoming air or the speed of the rotor blades changes, the angle of attack changes and may become excessive

28
Q

Compressor stalls are most likely at ______________.

A

high AOA and high power settings

29
Q

RECOGNIZE COMPRESSOR STALLS

A

LOW ALT COMPRESSOR STALL
Can be recognized by loud bangs, RPM decay, rise in EGT, yaw in a multi-engine aircraft
Since a compressor stall results in a reduction of airflow to the turbines, more fuel will be required to maintain the current thrust. This increase in fuel will increase burner and turbine temperature

30
Q

Compressor stalls are mainly the result of __________ or _______________. Airflow distortion is the most common cause of a compressor stall

A

airflow distortions

mechanical malfunctions

31
Q 
The instances when airflow distortions may induce compressor stall include:
1
2
3
4
A

1 Abrupt change in aircraft attitude
2 Encountering air turbulence
3 Deficiency of air velocity or volume, caused by atmospheric conditions
4 Rapid throttle movement

32
Q 
MECHANICAL MALFUNCTIONS FOR A COMPRESSOR STALL 
1
2
3
4
A

Mechanical malfunctions are the other main cause for a compressor stall

  1. Variable inlet guide vanes (IGV) and stator vanes – Failure to change the angle of attack will cause too much or too little airflow at low engine speed
  2. The fuel control unit (FCU), determines the proper amount of fuel to be introduced into the combustion chamber. When operating properly, the fuel control unit gradually meters fuel into the combustion chamber upon PCL inputs. Should the FCU fail, too much fuel or too little fuel may be added to the burner. An over-rich mixture (too much fuel) causes excessive burner pressure and a back-flow of air into the compressor that leads to a compressor stall
  3. Foreign object damage (FOD) is caused when an object damages the delicate blades of the compressor. Remember that the blades are airfoils with specific aerodynamic properties. Having a screw, piece of wire or loose rock strike the compressor blades at high speed will result in deformation of the blade. This will change its aerodynamic properties
  4. Variable exhaust nozzles are needed when an afterburner is used. The nozzle is closed at subsonic speeds and opened to allow the exhaust gases to expand properly when the afterburner is in operation. If the variable exhaust nozzle fails to open, an excessive back pressure will be produced which could lead to a compressor stall
33
Q

COMPRESSOR STALL AVOIDENCE

1
2
3

A

Avoid the following:

  1. Erratic or abrupt Power Control Lever (PCL) movements should be avoided, especially at low airspeeds or high angles of attack. The PCL should be advanced or retarded in a smooth fashion
  2. Maintain at least the prescribed minimum airspeed and avoid abrupt changes in aircraft attitude to allow the proper amounts of smooth air to enter the inlets
  3. Avoid flight through severe weather and turbulence
34
Q

MANUFACURER STALL PREVENTION COMPONENTS
1
2

A

Manufacturer utilized stall prevention components:

  1. Variable inlet guide vanes and variable stator vanes are installed so the angle of attack is changed at low engine peed. These variable inlet guide vanes and stator vanes are automatically positioned by the stator vane actuator (SVA) using fuel pressure via the fuel control unit. This action maintains the velocity of the air (and the angle at which it strikes the blades) within acceptable limits for low airflow conditions. It also permits high airflow with a minimum of restriction
  2. Bleed valves are installed near the middle or rear of the compressor to “bleed” (vent to the atmosphere) air and ncrease airflow in the front of the compressor at low engine RPMs
35
Q

COMPRESSOR STALL RECOVOERY

A

REDUCE PITCH, DECREASE THRUST, INCREASE SPEED.
1. Reduce the attitude of the aircraft which will reduce the inlet’s angle of attack
Improves air inlet velocity
2. PCL should be retarded to just below stall threshold to allow the engine to “catch up” with the inlet airflow
Many aircraft have automatic bleed valves or a procedure to open bleed valves to allow airflow through the compressor
3. Slowly advance PCL once engine indications return to normal
If the stall condition cannot be remedied, it may be wise to shut down the engine to prevent irreparable damage

AIRCRAFT ENGINES AND SYSTEMS (19 decks)

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