Compressor stall is a localised hiccup in which smooth airflow through the compressor is disrupted when blade stages exceed critical AoA; the imbalance between airflow supply and demand destabilises flow over compressor blades. When such a stall propagates through the entire compressor it escalates into engine surge, a system-wide upset that disturbs flow in the axial direction. A surge can cause reverse flow, loud bangs, vibrations, thrust fluctuations, and potential part damage.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is the difference between aircraft engine stall and surge?
The difference between engine stall and surge is that a stall begins when one or two blade stages exceed the critical angle of attack; at that moment the smooth airflow through the compressor is disrupted and becomes turbulent, yet the average airflow through the compressor remains steady. Because stalled operation is only a partial breakdown of airflow, some stalls are momentary and recover quickly, though careful management by the pilot is required to prevent a complete loss of thrust.
Surge is the extreme case that follows a stall. Surge occurs after a compressor stall when most or all blade stages are affected, causing a complete disruption of airflow through the compressor. The flow stalls so severely that the accumulation of compressed air downstream will no longer be sustained; the entire airflow through the engine becomes unstable and reverses direction momentarily. During surge the flow rate rapidly pulses, sometimes so violently that a loud ‘bang’ accompanies the reverse flow, and in extreme cases a sudden combustor-induced flame shoots out the back end of the machine. Because surge involves a major breakdown in compressor flow and potentially damages engine components, it creates a more serious risk to the engine and is recognized by abnormal N1 fan speed values, N1 oscillations, or no response to the thrust lever.
What causes an airplane engine compressor stall?
An airplane engine compressor stall results from a complex interaction between aerodynamic conditions, engine design, and operational factors. Stall occurs when there is an imbalance between airflow supply and airflow demand. This imbalance happens when the angle of attack of compressor blades exceeds the critical angle of attack, causing a breakdown in smooth airflow. As a result, the compressor can no longer maintain stable pressure, leading to abnormal airflow and potential engine performance loss.
Several factors trigger a compressor stall. Operating the aircraft at very high angles of attack distorts the airflow entering the engine. Inlet disturbances like turbulent or distorted airflow reduce axial velocity, increasing the risk of stall. Abrupt engine acceleration causes excessive fuel flow and increased combustion chamber pressure, reducing axial velocity and pushing the compressor beyond its limits. Abrupt engine deceleration leads to low fuel flow and decreased back pressure, destabilizing airflow.
Environmental factors like ingestion of water, hail, or ice severely disrupt airflow. The 1991 crash of Scandinavian Airlines System Flight 751, a McDonnell Douglas MD-81, resulted from ice ingestion that caused compressor stalls in both engines shortly after takeoff. Bird ingestion or foreign object damage (FOD) damages compressor blades, making them unable to compress air efficiently. Crosswinds and sideslip maneuvers disturb inlet airflow, especially during approach or takeoff.
Engine design and operational limits also play a part. Operation outside RPM design parameters affects the rotational speed of compressor blades, making them more susceptible to stall. Early versions of the Pratt & Whitney JT9D engines powering the 747-100/200 were particularly prone to compressor stalls when excessive reverse thrust was used at low speeds. Use of excessive reverse when the airplane had slowed too much for the degree of reverse used was the most common cause of stalls in these engines. Excessive yaw angles were a known deficiency in this engine type.
Design improvements help reduce the risk. Variable inlet guide vanes and variable stator vanes allow better airflow control across different engine speeds. Multi-spool engines and compressor bleeds help manage pressure imbalances. Modern digital systems like FADEC have virtually eliminated compressor stalls by continuously optimizing fuel flow and engine parameters.
Flight deck indications of a compressor stall include a loud bang, pop, or buzzing sound, along with an increase in vibration level, exhaust gas temperature (EGT), and RPM fluctuations. In response, pilots must reduce throttle on the affected engine. If the stall continues, the engine will flame out or suffer physical damage, requiring shutdown.
