Mar 28, 2021
Stalls are one of the leading causes of fatal aviation accidents not only in New Zealand but around the world. Most commonly experienced shortly after take-off or when descending to land – finding yourself caught in a stall is stressful for even the most seasoned of pilots. Making sure you understand the basics of stall recovery, the types of stalls and situations where a stall is likely to occur are an important part of stall prevention and successful stall recovery.
What Is An Aircraft Stall?
As we have discussed in an earlier post covering the Aerodynamic Forces In Climbs And Descents, there are four main forces on an aircraft during flight; lift, weight, thrust, and drag. All of these forces continuously work together to maintain the aircraft’s ability to continue flying.
When one of these forces is out of balance the aerodynamics of flight are disrupted and the aircraft reacts accordingly. When an aeroplane stalls during flight it means the wings are not producing enough lift to keep the aircraft flying through the air, causing it to fall.
Lift is created by the air flowing over the leading edge of the wing while being pulled downward at the trailing edge, creating equal and opposite forces. During a stall, the smooth airflow over the wing becomes turbulent and no longer produces enough lift to fully support the weight of the aeroplane. The airflow over the wing is affected by changes in the wing’s Angle of Attack (AOA).
The angle of attack refers to the angle between the oncoming air and the chord line of the wing (An imaginary straight line drawn between the leading edge and the trailing edge of the wing). To achieve constant lift, any change in the angle of attack must be matched by a change in the airspeed. If the angle of attack is too large air going over the wing becomes separated and disrupted causing the wing to stall.
3 Types Of Stalls
While all stalls have the same effect – the aeroplane falls and is unable to maintain normal flight – they are not all the same. There are 3 main types of stalls that pilots train for; Power-Off, Power-On and Accelerated. Learning how to recognize, prevent and recover from each type of stall is an important part of learning to fly.
- Power On Stall. Sometimes called ‘Departure Stalls’, Power On Stalls are those that generally occur on take-off and climb-outs with close to full engine power. Power On stalls are one of the leading causes of take-off and climb accidents.
- Power Off Stall. Sometimes called an Approach or Arrival Stall, Power Off stalls are often experienced during the approach to landing. These stalls are commonly associated with lack of speed causing a reduction in lift.
- Accelerated Stall. Sometimes referred to as a Dynamic Stall or High Speed Stall. An accelerated stall usually occurs in-flight when a certain manoeuvre abruptly increases the Angle of Attack resulting in reduced ability to produce lift. Sometimes an Accelerated Stall is intentional – such as during aerobatic manoeuvres in air shows, and sometimes they occur during normal flight if the pilot performs a manoeuvre too enthusiastically (steep turns, pull-ups etc.).
Stall Warning Signs
While we know all stalls are not the same and not all aircraft react in the same manner, there are some common signs the aircraft is heading for a stall, these are:
- Decreasing Airspeed. The number one indicator you are heading into a stall situation.
- Unresponsive Controls. A loss in the effectiveness of controls or sluggish controls particularly a loss of roll control is also a sign a stall is imminent; this is largely due to the associated reduction in airspeed.
- Activation Of Stall Warning System. The reduction in control effectiveness is usually followed by the activation of the stall-warning signal (if the aircraft is fitted with one).
- Buffeting. Buffeting is when the aircraft moves about erratically, and in the case of a stall, is caused by the turbulent airflow from the wings.
Stall recovery techniques will vary slightly for different types of aircraft but the following recovery actions are the basic fundamentals of successful stall recovery in all fixed-wing aeroplanes. While listed here one after the other, in a real-world stall situation these steps would need to be carried out in rapid succession.
One of the key factors in any stall recovery is reducing the angle of attack quickly. Decreasing the AOA will allow the wings to regain lift and normal flight to be regained. To decrease the AOA you must drop the nose by pushing forward on the flight controls (Check Forward). This is necessary even if the nose is already pitched downward due to the stall as because the aeroplane is sinking the angle of attack is still too high.
Increasing airspeed is vital to help counteract the loss of lift and get the wings flying again. Smoothly applying the maximum allowable power during a stall minimises the loss of altitude and therefore the likelihood of ground impact. As the aircraft recovers power should be adjusted and straight and level flight established (being careful to avoid a secondary stall).
Next up it is important to disconnect any automatic pilot control (if equipped), fully manual control is essential for successful stall recovery. Leaving these connect can result in the aircraft making unseen adjustments to your recovery manoeuvres.
Roll Wings Level
Correcting any roll in the wings orientates the lift vector aiding in a more effective recovery. However, both roll stability and roll control will improve considerably after decreasing the angle of attack and getting the wings flying again. Roll control is important for preventing a stall from progressing into a spin.
How To Train For Aircraft Stalls
The worst aeroplane stall accidents are often the result of an inadvertent stall at low altitudes with the recovery manoeuvres not able to be completed prior to ground contact. The best way to avoid encountering a stall situation is to be aware of the conditions where the stall is likely, and learn to avoid them. This is why the key to stall recovery lays in pilot training programs.
It is not enough to rely on technology to avoid inadvertent stalls; stall warning devices and flight envelope protection systems are not always effective under all types of stall conditions. All pilots must demonstrate the ability to recover from a stall, recognise stall conditions and avoid them by deliberately placing the aircraft into a stall during controlled flight training (or within a simulated flight environment). The more a pilot develops the muscle memory surrounding stall recovery procedures, the greater the likelihood of being able to recover successfully from an unexpected stall.
Even after your pilot training program is complete it is important to retain the instinctive reactions necessary for stall recovery. Practising under ‘slow flight’ conditions can be a good way to get a feel for aircraft flight dynamics in near stall circumstances. Slow flight means flying just above stall speed and offers the opportunity to experience first-hand how the aircraft responds leading up to stall conditions.
Mastering stall recovery techniques is an important part of learning to fly and part of an ongoing mastery of general aviation techniques. The reality is that continued flight training is always a must for any pilot whether you are just starting out or a seasoned professional. Find out more about flight training options here from one of New Zealand’s most respected flight training providers.