Occurring way too often, this type of intentional manoeuvre can be fun to do at altitude but deadly when it occurs close to the surface when there is not enough room to recover. During an aerobatics course you will be trained in entering the stall and spin and subsequent learn how to recover.
But there is also a type of situation where pilots sometimes get themselves into during low level maneuvering like for example sight seeing. It also happens every now and then during the takeoff and landing phase of the flight, especially in the turns from downwind to base and to final.
On this page we introduce you into the basic mechanics of these stall spin accidents and we also invite you to read our article on spinning aircraft.
Along the path to become a pilot, students are introduced to stalls/ spins and their theoretical background. Spin training was deleted many decades ago from the curriculum . Only those aspiring to become flight instructor or want to obtain an aerobatics rating are required to show proficiency in the stall/ spin exercise.
As we aviators should know by now: a wing can stall at any airspeed. When the angle between the relative air flow and wing chord is too large, the wing will stall. You can say that the maximum AoA (angle of attack) has been exceeded. Visit our pages on the basic lift formula to get a refresher on this subject.
The factors influencing stall speed are weight and balance, load factor, turns (bank angle), the amount of power and flaps or slats installed on the wings. On certain wings contamination may also influence stall speeds. See our page on stall speed factors for more details.
To execute a spin do the following basic steps: in level flight, slow the aircraft down to its published stall speed, at the stall apply rudder in the direction you want to turn the spin. That's it, really easy. At the start of the spin the aircraft is uncoordinated, this is an important factor to keep in mind.
During a straight and level turn of the aircraft the load factor of the aircraft increases: G = 1 / cos (Bank Angle) and as a result the stall speed too: Vsacc = Vs × √G. Another important factor in our story.
During a turn the lower/inner wing has a slightly lower airspeed and less lift compared to the high/outer wing. The aircraft tends to increase its bank angle due to this and the pilot must apply slight opposite pressure to hold the correct bank.
But in a descending turn the inner/low wing had a greater AoA than the outer wing and depending on the bank angle and rate of descent, more lift will be created than the outer/higher wing thus the pilot does not need to apply opposite pressure to hold the bank angle.
Should the aircraft be stalled when flying coordinated then both wings will usually show the same stall behavior without any wing drop tendency. In a climbing turn the higher/outer wing will stall first (AoA is greater). But propeller slip stream effects could masquerade that in some type of aircraft.
When flying in the standard traffic pattern or circuit (as it is called in certain parts of the world) all turns are to the left, must be coordinated and not too steep, maximum 20° - 30° bank. Airspeeds are normally around 1.5 VS requiring larger control inputs due to the lower effectiveness of the controls.
We are going to describe the situation where loss of control is going to be the most dangerous: in the pattern turning to base or to final. There are more situations, but this one occurs most often and usually with grim results.
What happens is this: during the turn to base or final the aircraft is in a non coordinated turn with excess bottom rudder causing a skidding turn. Mainly because the pilot wants to hurry up the turn due to certain external factors leaving the aircraft over banking with the nose yawing into the turn and pointing down.
Should the pilot react by holding of the bank with opposite aileron he creates a cross control situation. The portion of the lower wing with the downward deflected aileron has a higher AoA, more so if aircraft are equipped with full span ailerons (flaperons).
Due to these effects the inward and downward yaw increase and should the pilot pull back on the yoke/ control column, he/she increases the AoA for both wings even more. The airspeed will decrease as a result of the increased drag, getting it closer to the stall speed. Which is at that point lower for the low/inner wing compared to the other wing.
Within seconds the low/inner wing stalls and drops, banking the aircraft even further. The nose drops downward and the stall turns into an incipient spin... and the rest is history.
Should there be enough altitude then recovery is simple enough: ease back pressure on the yoke/ stick reducing the AoA. Counter the yaw with opposite rudder centering the coordination ball. Apply power smoothly thereby increasing elevator and rudder effectiveness. Roll the wings level and climb away.
The main aerodynamical causes are highlighted in yellow above, but the contributing factor might be a bit more difficult. With high wind speeds relative to the aircraft speed (at downwind), then at lower altitudes this high groundspeed creates the illusion or suggestion of higher airspeeds, thinking the pilot he/ she must reduce speed to remain in the pattern. Probably increasing the rate of turn with rudder...
Preventing these accidents can be easy enough: watch your AIRspeed, avoid being distracted, keep a birds eye view on the situation. Never exceed 30° bank angle, fly a larger pattern/ circuit if needed, most of all: fly accurately maintaining coordinated flight at all times.Written by EAI.