Coefficient lift and Angle of Attack chart
Basic Stall Symptoms
Most aircraft accidents occur during the take-off or landing phase of the flight. Collisions with obstacles during climb out, runway overruns on landing occur every now and then. In this section of the site we will take a look at the various factors contributing to the performance of the aircraft in this part of the flight.
Recognizing an approaching stall is part of pilot training as during landing approach the aircraft is flown close to the stalling speed for the configuration the pilot has set. During training the pilot is trained to handle and act immediately upon a stall of the aircraft.
These symptoms describe the low speed non accelerated stall. Its possible to induce stall at higher speeds as in level turns and with fast pitch up accelerations where some stall symptoms will not show at all, or in very quick succession.
- Reducing airspeed
- Reducing control effectiveness
- Pre-stall buffet
- Nose high attitude (straight and level stalls)
- Stall warning device alarm
Reducing airspeed, level flight
If airspeed is reduced and the pilot wants to remains at the same altitude, the aircraft will eventually reach its maximum angle of attack (AOA) and stall if the pilot keeps trying to fly slower and slower.
As the aircraft slows down there is less airflow over the ailerons, rudder and elevator thus the amount of control deflection needed to move the aircraft is greater. This a good indication that you are approaching a slow speed stall.
Angle of Attack compared to Relative Wind
When the angle of attack (AOA) of the wing increases, the point where the airflow separates will move forward and the streamlined airflow will become turbulent and separate from the wing. This turbulent wake then meets the aft fuselage and tail section of the aircraft. This will be felt by the occupants of the aircraft as a rumble or buffet. Not all aircraft have a pronounced buffet, this depends on the form of the fuselage, size and location of elevator.
Wing design is such that the wings stalls from the root toward the wingtips so that the ailerons remain effective for as long as possible. For this to happen the angle of incidence of the wing is larger at the root and lower at the wing tips. Some aircraft have a stall fence on top of the wing, others employ a discontinuous leading edge (Kodiak from Quest Aircraft) creating a vortex over the wing at high angles of attack. Both devices prevent the stall from progressing to the ailerons so that they remain effective throughout the maneuver.
During a straight and level stall the nose high attitude is a good indication of an approaching stall, but remember that by extending flaps the nose is effectively lowered.
An aircraft can be made to stall in any nose attitude as that occurs when angle of attack of the incoming airflow and chord line becomes too large, and this fact has not much to do with nose attitude.
In some aircraft a warning device is fitted in the wing with an indication in the form of a horn or light in the cockpit. This device is set to indicate a warning around 5 kts above actual stall speed.
Lessons learned from the recent past (meaning serious aircraft accidents) has led the Federal Aviation Administration to publish AC-120-109 Subject: Stall and Stick Pusher Training. The next is an excerpt from that AC:
The goal of this AC is to provide best practices and guidance for training, testing, and checking for pilots, within existing regulations, to ensure correct and consistent responses to unexpected stall warnings and stick pusher activations. This AC emphasizes reducing the angle of attack (AOA) at the first indication of a stall as the primary means of approach-to-stall or stall recovery. Additionally, this AC provides guidance for operators and training centers in the development of stall and stick pusher event training.
Read the full PDF text at the next page: AC 120-109 Stall Stick Training