What Happens When An Airplane Stalls?

Stalling has been an ever-present danger since the early days of aviation. It is much better understood today, and modern aircraft have systems in place to help avoid it, but accidents still occur. This article takes a look at what causes aircraft to stall and how it can be avoided.

Even the largest aircraft can stall when the angle of attack becomes too steep. Photo: Tom Boon – Simple Flying

What is a stall?

Put simply, a stall is a reduction of lift experienced by an aircraft. It occurs when the angle of attack of the wing is increased too much. This is known as the critical angle of attack and is typically around 15 degrees (but there are variations).

In normal flight, the airflow over the shaped wings creates lift. The airfoil shape changes the airflow direction, and the downward deflection of the air causes an upward force (lift) to be exerted on the airfoil.

Increasing the angle of attack causes flow separation, where the air no longer flows cleanly over the upper surface of the wing. If this angle reaches the critical angle, the airflow is disrupted to the point where the lift generated begins to decrease.

Exceeding the critical angle will result in swirling air above the wing, and reduced lift. Photo: DLR via Wikimedia

It is the angle of attack of the wing exceeding its critical angle that causes a stall, but airspeed is also important. A ‘stall speed’ will be defined for an aircraft, rather than an angle of attack. How does this work?

If an aircraft flies slower, it required a greater angle of attack to generate sufficient lift. If the speed decreases to a certain level, this angle will reach the critical angle. At this speed, an aircraft cannot climb without causing a stall. This speed is affected by several factors, including weight, altitude, and configuration, and different stall speeds are set based on this (such as a minimum speed in landing configuration with fully extended flaps).

How increasing the angle of attack disrupts the airflow and causes separation. Image: Public domain via Wikimedia

What happens in a stall, and why is it dangerous?

An uncorrected stall will cause the aircraft to fall. The first sign for a pilot is sluggish flight controls, which become much less responsive due to the changes in airflow, and possible buffeting. Pilots will train to recognize this, but this is more relevant in smaller aircraft that are flown manually.

An early stall is easily corrected by pushing the aircraft nose down to reduce the angle of attack. This is, of course, much more serious at low altitude when taking off or landing. If not corrected, the wing loses lift, and the aircraft will start to fall.

A spin is another situation that can occur. This occurs when the aircraft has sufficient yaw at the point of stall. In this situation, one wing stalls before the other, and the difference in lift causes the aircraft to roll. This is much harder for a pilot to recover from. Training is sometimes given on smaller aircraft as part of pilot training, but in general, the focus is on preventing a spin from ever happening. Commercial airliners are not designed or tested in this area.

Warning of a stall

Any fixed-wing aircraft can stall. And all aircraft have warning systems to prevent, or alert pilots, to dangers. On a smaller, light aircraft, the most common method involves a simple flap on the leading wing edge, designed to activate a warning if the wing approaches its critical angle of attack.

A stall warning flap on a light aircraft. It will lift if the angle of attack is increased. Photo: Clint Budd via Flickr

Modern fly-by-wire aircraft will incorporate several systems to alert pilots of an approaching stall. This includes monitoring of speed and sensors to measure the angle of attack. Warnings can be given by alarm as well as mechanical ‘stick shakers’ designed to give similar warnings to manual controls.

These sensors are part of the problem that previously grounded the Boeing 737 MAX aircraft. The Maneuvering Characteristics Augmentation System (MCAS) system takes data from the angle of attack sensors. Erroneous input from one of these sensors is thought to have led to the aircraft’s nose being forced down in both tragic MAX accidents.

The angle of attack sensors are part of the problem that the grounded 737 MAX. Photo: Vincenzo Pace | Simple Flying

Stalls have caused many accidents

Despite training and warning systems, stalls do still occur. At low speed and low altitude during take-off and landing, they can be disastrous, and unfortunately, a number of crashes have occurred. Some of the most notable include:

• British European Airways Flight 548, June 1972. This is one of the most deadly crashes ever in the UK. A Trident aircraft stalled and hit the ground shortly after departure from Heathrow when the captain failed to maintain sufficient airspeed in the climb.
• Air France Flight 447, June 2003. An Airbus A330 flying from Rio de Janeiro to Paris stalled at high altitude (after the autopilot system was disabled due to airspeed measurement problems). The pilots failed to recover, and the aircraft descended to hit the ocean.
• Turkish Airlines Flight 1951, February 2009. This involved a Boeing 737-800, which crashed on landing at Amsterdam. A failed radio altimeter causes the engine power to be automatically reduced to idle, leading to a stall that pilots had no time to recover from.

Feel free to discuss aircraft stalling, spinning, or any other aspect of flight performance in the comments.

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