Spin is an aerodynamic term that indicates a particular category of a stall that results in autorotation or uncommanded roll along the longitudinal axis and downward rotating path near the vertical axis. This condition is initiated intentionally or unintentionally and is highly dangerous for aircraft stability during flight. Usually, a spin occurs during a turn when the inside wing stalls first while the outside wing supports lift or both wings can stall. Each wing has a different angle of attack and lift and drag coefficients, so it is more likely for the inner wing to stall first.
How a spin occurs?
At least one wing should stall for creating a normal spin condition. The inner wing loses lift and begins to stall while the outer wing supports the lift at the same moment. This movement raises the outer wing, and the aircraft began to roll towards the wing stalled at first. The difference of lift coefficient between two wings causes aircraft to roll uncontrollably, and the difference of drag causes aircraft to yaw continuously.
Spin conditions are dangerous and may result in catastrophic consequences, especially when the aircraft is landing or take-off. Landings and take-offs are the most critical part of the flight as the pilot gets very little time to respond to these risky conditions.
What are different spin modes?
There are three different spin modes.
In upright spin mode, the aircraft begins to fall from the sky at high speed and rotate swiftly towards the stalled wing. In upright spins, the aircraft remains in a typical flight attitude. A decent spin is a sustained stall in which asymmetric lift creates a problem. Drag on the wings where one wing is higher initiate a rolling and yawing motion. The center of gravity of an aircraft moves beyond the aft limit along a helical path.
In this type of spin, aircraft began to roll and fly upside down. Dynamically inverted spin and upright spin are similar. In an inverted spin, the aircraft’s nose pitch down where rolling spins in one direction while yawing spins in the opposite direction. Most general aviation airplanes are designed to revert to a typical spin from an inverted spin. Inverted spin can be very dangerous as negative g exert pressure on the pilot and cause unconsciousness during flight.
A flat spin is the worst mode of spin among the three. The aircraft spins around the vertical axis in this spin mode, sinking towards the ground, and unfortunately has no forward speed. This type of spin is hazardous and can result in catastrophic events.
This spin mode is dangerous because the aircraft has no forward speed and due to this reason pilot is unable to use controls to recover the aircraft from stalling.
How does a flat spin happen?
In this type of spin, the noise of the aircraft is pitched up towards a horizontal attitude, and the center of gravity gets closer to the spin axis. This condition results in a slower descending rate and fast rotation, the helical screw pattern in a flat spin is more compressed. In this condition, yaw motion increases significantly with a minor rolling motion. This scenario indicates that the rudder and elevators are not moving forward in the air stream; instead, they move sideways. As a result, control surfaces are unable to operate as they are already beyond their design limitations.
As the wings stall, ailerons are not functional anymore as they are designed to operate with forwarding aircraft speed. According to aerobatics pilots, engine power moves aircraft in rotating motion, not forward motion. There is no benefit of increasing engine power in a spin because it will worsen the situation.
Weight and balance play an essential role in-flight safety, and unstable conditions affect flight controls and uncontrollable scenarios. A correctly loaded aircraft is nose heavy without reactions from flight controls. An aircraft nose sinks in routine flight maneuvering only when elevators impose tail downforce. If air is not flowing over the stabilizer, the control surface will be dysfunctional, and the aircraft’s nose will eventually sink.
If the center of gravity of an aircraft is far forward from design limits, the nose will begin to dive, and the force from elevators and rudders might not be sufficient for spin recovery. If the CG moves too aft, the nose will rise, and this pitch-up motion will cause an aircraft to stall. If the plane enters into a position where airspeed above elevator controls becomes insufficient, the pilot may not recover the plane from stalling.
What is the spin recovery technique?
Few techniques emerged from lots of research and investigations. FAA recommends a spin recovery technique known as PARE. This method is created for pilots to recall easily during a flat spin condition.
The first step requires power to be set at an idle position. As discussed earlier, engine power plays a significant role in speeding up the rotating motion. It is essential to cut the power so the aircraft can recover after getting rid of the uncontrollable rotating pattern. If the aircraft enter the left spin, power should be set to full throttle.
The next step is setting ailerons to a neutral position. A spin results from wing stall, and ailerons are the devices used for controlling aircraft by changing the angle of attack along with wingtips. As the wing is already stalling, using ailerons will make the situation much worse and uncontrollable. So, immediately neutralizing ailerons will help aircraft attain a smooth airflow over wings and aid in coming out of a stall.
Now, engine power and ailerons are not contributing to the rotating movement. A rudder is an efficient tool in this scenario as there is sufficient airflow over the vertical stabilizer. The application of rudder in the complete opposite direction can stop the spin rotation of aircraft.
Spin is an aggravated stall situation caused by a disturbance in the critical angle of attack. The most efficient way to recover from this situation is reducing the angle of attack by pushing the control column forward. By lowering the angle of attack, aircraft will begin to gain speedy airflow over wings and other control surfaces as soon as the speed gauge indicates minimal speed. The pilot can take control of the aircraft and recover it from an uncontrollable stalling situation.
Chances of failure
Chances of failure to recover from a spin are very high. It depends upon the type of aircraft and piloting skills. When there is no forward speed, it isn’t easy to get control of the airplane.
An aircraft falling from the sky with no visual faults can make a pilot very anxious. This anxiety deteriorates decision-making ability, and operating close to the ground can be hazardous in such cases. A pilot in a spin can become unconscious due to excessive negative g forces. This may result in a fatal crash as the pilot will not be able to regain aircraft controls within safe altitude.
Considering the danger of flat spins, all normal category aircraft fixed-wing and single-engine are designed with a high angle of attack on the roots than at the wingtip of a wing. This design is part of design standards given in Part 23 Airworthiness and Environmental Certifications for under 5700 kg airplanes. Such aircraft must be protected from unintentional spin. A high angle of attack on wingtips forces the root to stall first and reduces the severity of the wing sink due to stall. This design also improves the effectiveness of ailerons as the stall migrates from root to tip.
Part 23 also requires aircraft to demonstrate spin recovery. An aircraft that can perform intentionally developed spin must demonstrate spin recovery from a six-turn fully developed flat spin and, one-turn spin for an aircraft that does not support intentionally developed spin. Most of the general aviation airplanes designed for commercial or private purposes cannot perform intentional spins. Utility category aircraft and sports airplanes are designed for intentional spins and used in acrobatic performances and airshows.
1. Always double-check the aircraft loading and ensure weight and balance.
2. Make sure there are no loose movable heavy objects in cargo. In-flight movement of load can disturb the center of gravity.
3. Pilot should get the necessary training about spin recovery procedure from a flat spin.
Spin training is essentially not part of flight lessons and does not contain lessons for spin recovery techniques for typical aircraft not approved for intentional spins. However, pilots take stall entry and recovery lessons as a necessary part of basic flight lessons.
Aerobatic pilots are required to take spin training and demonstrate spin testing. Flight instructors are also required to take few lessons about spin to obtain a license. Aerobatics pilots are well prepared to take control of aircraft during a spin using power and control columns. However, a regular pilot must follow standard steps such as cutting the power and neutralizing ailerons and horizontal stabilizers.
Can a Cessna 172 flat spin?
Yes, a Cessna 172 can enter a flat spin if the tail is heavy and make the nose pitch up. It is possible to recover a Cessna 172 from a flat spin by following basic procedures. Reduce engine power to idle, neutralize ailerons and, move rudder against the direction of rotation until wings get enough airflow to take control of the aircraft.
Can a fighter jet go into a flat spin?
Yes, a fighter jet can get into a flat spin at a high angle of attack. Fighter jets can get into flat spins due to faulty weight and balance before flight or disturbance in CG. More than 30 F-14 fighter jets crashed due to spin.
What does a flat spin mean?
A flat spin is an aerodynamic condition during a flight in which the nose of the aircraft pitch up due to tail loading, and the aircraft starts falling from the sky in rotational motion with no forward speed. A flat spin is very dangerous and, in some instances, totally unrecoverable.
How do you induce a flat spin?
Aerobatic pilots intentionally perform a flat spin in an aircraft certified for intentional spins. These are the steps to induce a flat spin.
1. Reduce engine power slowly, then pull back to idle. 2. At a certain point, when aircraft comes to a stop, increase elevators slightly. 3. As the nose starts pitching down, use the full ruder. The aircraft will start spinning.
It is essential to take necessary training before performing this risky maneuver, or the aircraft will enter unrecoverable flat spin due to misuse of controls.
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