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Aircraft Spins 101

by Sarina Houston 30. May 2014 22:31
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Photo: H. Rabb/Wikimedia
As mentioned in my previous article on stalls, accidents that occur due to stall/spin scenarios are more fatal than others. According to an AOPA study, stall/spin accidents have a fatality rate of about 28 percent, higher than the overall average fatality rate of 20 percent.

A spin occurs when an airplane stalls in an uncoordinated or aggravated state. If a recovery is not initiated after an uncoordinated stall occurs, the wing that is more stalled than the other will drop and the nose will follow into a spiraling descent. The aircraft will descend rapidly in a corkscrew motion.

According to the Jeppesen Private Pilot Manual, a small airplane will descend about 500 feet for each turn in a spin, so there's not much altitude or time available for a recovery in many cases. Considering stalls and spins often occur at low altitudes to begin with, it's clear why the fatality rate is higher for these accidents.

Stages of a Spin
The FAA has outlined three stages for spins in light aircraft: incipient, fully developed and recovery.

  • Incipient: The incipient phase of a spin is the stall and spin entry, up to about 2 turns in the spin.
  • Fully Developed: When the airspeed and rotation stabilize, the spin is considered fully developed.
  • Recovery: Recovery occurs when the pilot applies rudder and aileron inputs to counter the spin and the aircraft regains lift and control function. Once the inputs are initiated to stop the spin, the aircraft can usually recover in less than one spin.

Types of Spin

  • Erect Spin: Erect spins are the most common type of spin, occurring when the aircraft rolls and yaws in the same direction and the aircraft is upright and in a slightly nose-down attitude.
  • Inverted Spin: An inverted spin occurs when the aircraft spins upside down and yaw and roll occurs in opposite directions.
  • Flat Spin: Getting its name from the flat-like pitch attitude, the flat spin occurs when the aircraft spins at a level pitch attitude around the vertical axis as a result of a yawing motion alone. Flat spins are the most difficult to recover from (and just as difficult to enter in some aircraft!)

 

Spin Recovery
Spin recovery should be initiated at the first sign of a spin. Recovery procedures are specific to the aircraft flown and are found in the pilot operating handbook of each aircraft. In light aircraft, the spin recovery procedures follow a typical pattern and can be remembered by the common acronym PARE.

P - Power: The throttle should be moved to the idle position to reduce thrust.
A - Ailerons: Ailerons should be neutralized.
R - Rudder : Full opposite rudder input should be applied until the rotation is stopped. If the aircraft is rotating to the left, right rudder should be applied. Once the spinning stops, the rudder should be neutralized.
E - Elevator: Quick forward pressure should be applied to break the stall and gain airflow over the wings. Once the aircraft gains lift, back pressure should be applied gradually so as not to stall again.

Training aircraft are stable by design. They're meant to recover from unusual attitudes without much external control input from the pilot. A Cessna 172, for example, is actually somewhat difficult to perform an intentional spin in. But this doesn't mean that pilots of training aircraft are immune to spins.

While intentional spins are not always demonstrated during training, stall and spin awareness should always be emphasized with flight students. Many pilots tend to become confident in stall recovery, but all pilots would be wise to remain familiar with spin entry characteristics and recovery procedures for their specific aircraft.

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Aircraft Accidents | Flying | Sarina Houston

How Well Do You Know Your Stalls & Spins?

by Sarina Houston 15. May 2014 10:23
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Image: Theresa Knott/Wikimedia Commons

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 airplaneā€™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.

Spins
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.

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Aircraft Accidents | Flying | Sarina Houston



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