For new flight students and passengers, an aircraft stall can often be a source of fear. What is a stall? Will the airplane fall out of the sky? Does the engine quit?
And while stalls shouldn't be something that pilots fear, they should be taken seriously. Aircraft stalls and spins remain a leading cause of general aviation accidents - causing ten percent of general aviation accidents, according to one AOPA study. And stall/spin accidents result in more fatalities than other types of aircraft accidents. Private and commercial pilots are most likely to enter a stall, while student pilots and ATPs are less likely to stall, according to AOPA.
A 2012 advisory circular claims that loss of control accidents are a growing problem and that inappropriate reactions to stall indications are part of that problem.
What's a Stall?
Let's start with the basics. For those of you non-pilots, you need to know that an aircraft stall has absolutely nothing to do with the engine (unless we're talking about compressor stalls - an entirely different topic). Instead, an aircraft stalls when the airflow over the wing is disrupted enough to cause a loss of lift.
Stalls are dangerous because control surfaces become inadequate to control the flight, and if a recovery is not initiated, the aircraft will quickly lose altitude. And then there's that deadly spin: If uncoordinated, a stall can develop into a spin.
The FAA defines an aircraft stall as "an aerodynamic loss of lift caused by exceeding the airplane’s
critical angle of attack."
The critical angle of attack is the key phrase here. The angle of attack is the angle between the chord line of the wing (an imaginary line running from the leading edge of the wing to the trailing edge) and the relative wind. The critical angle of attack is the angle at which maximum lift is produced. An increase in the angle of attack beyond the max coefficient of lift results in a loss of lift, airflow separation over the wing and a subsequent stall.
An aircraft can stall at various airspeeds, altitudes, pitch attitudes, configurations and weights. But the critical angle of attack must be exceeded for a stall to occur.
Types of Stalls
- Power on stall: A power-on stall occurs during situations in which the aircraft power or thrust is increased quickly, such as during takeoff. Power on stalls usually occur (not always) with gear and flaps up.
- Power off stall: Power off stalls occur when the aircraft power is decreased or at idle, such as during landing. Power-off stalls tend to occur with gear and flaps down.
- Elevator trim stall: If the pilot disregards the elevator trim setting, any abrupt change in power or configuration can initiate a stall. This can happen easily during takeoff or go-arounds, when the aircraft trim tab is adjusted for the descent and a go-around is initiated. The aircraft can pitch up quickly and unexpectedly to a high angle of attack.
- Cross controlled stall: A cross-controlled stall is one of the most dangerous types, as it's an uncoordinated stall and easily transitions to a spin. A cross-controlled stall occurs when the pilot inputs aileron control in one direction and rudder pressure in the opposite direction. Cross controlled stalls are known to occur during turns in the traffic pattern.
- Accelerated stall: When excessive loads are placed on the airplane (such as during steep turns), an aircraft is capable of stalling at a higher airspeed and/or a lower pitch attitude than the pilot might be accustomed to.
- Secondary stall: Secondary stalls occur if a pilot attempts to recover from a stall too quickly by pitching up to recover from the dive before obtaining an appropriate airspeed and generating enough lift.
- Deep stall: Also called a super stall, the deep stall happens in T-tail aircraft, like this Piper Lance II or this King Air 350. It occurs when the airflow over the wing is disrupted and airflow over the tail of the aircraft is also disrupted, rendering both the ailerons and elevator/rudder ineffective at the same time. In a deep stall, recovery is difficult and sometimes, impossible.
An uncoordinated stall can result in a spin. According to the FAA Airplane Flying Handbook, a spin is an aggravated stall that results in autorotation - a downward corkscrew motion.
The spin is a result of one wing being at a higher angle of attack than the other, often descried as one wing being "more stalled than the other." The difference in angles of attack creates lift on the less stalled wing and drag on the more stalled wing.
Spins are more difficult to recover from, as altitude is lost very quickly and control surfaces may react different than the pilot expects, which is why it's important for pilots to continuously practice stall and spin recovery.