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   /Lift & Stall Effects

Coefficient lift and Angle of Attack chartCoefficient lift and Angle of Attack chart

Basic Stall Symptoms

Most aircraft accidents occur during the takeoff 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 flight training the pilot is trained to handle and act immediately upon a (full or at its initial indication) stall of the aircraft.

Stall symptoms

The symptoms below describe the low speed non accelerated stall. Its possible to induce stall at higher speeds like in level turns and with fast pitch up accelerations where some of the familiar 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 while slowing down.

Control effectiveness

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.

Pre-stall buffet

Angle of Attack compared to Relative WindAngle 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 (also creeps around at the trailing edge) and the streamlined airflow will become turbulent and finally 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. With T-tail aircraft it is less noticeable. Note that the turbulent wake starts at the wing root.

Wing design

The wing planform also determines where and how the stall develops, possible forms are rectangular, tapered, elliptical and swept. The latter you will see most on airliners. The best form is the elliptical wing, where the effective angle of attack is constant from root to tip. The high taper wing (Pipistrel aircraft and Dash-8) comes a close second where the center section is somewhat rectangular and tapered outboard sections.

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 (called twist or wash-out). You will see this too with propeller blades.

Some aircraft have a stall fence on top of the wing, others employ a discontinuous leading edge (Kodiak from Daher (former Quest Aircraft, Idaho), the Ilyushin IL-62, Cirrus SR20/22 or the Icon-A5) 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.

Nose attitude

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 and also the AoA where the wing stalls too.
Basically: 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.

Stall warning

In some aircraft (mostly certified) a warning device is fitted in the wing with an indication in the form of a horn or light in the cockpit. This device (a flip switch at the airflow stagnation point at the wing leading edge) is set to indicate a warning about 5-10 kts above actual stall speed. More on these devices in the article from AVweb about: Stall Warning Systems.

Pilot training

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 piece of text 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

Written by EAI.

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