A spin in a small aeroplane is a controlled or uncontrolled manoeuvre in which the aeroplane descends in a helical path while flying in a stalled condition at an angle of attack greater than the angle of maximum lift. Spins result from aggravated stalls in uncoordinated flight. In an aggravated stall, one wing will drop before the other and the nose will yaw in the direction of the low wing. If a stall does not occur, a spin cannot occur.
Types Of Spins
An incipient spin is that portion of a spin from the time the aeroplane stalls and rotation starts, until the spin becomes fully developed. An incipient spin that is not allowed to develop into a fully developed spin is commonly used as an introduction to spin training and spin recovery techniques.
A fully developed spin occurs when the aircraft angular rotation rates, airspeed, and vertical speed are stabilized from turn-to-turn in a flight path that is close to vertical.
- A flat spin is characterized by a near level pitch and roll attitude with the spin axis near the C of G of the aeroplane. Recovery from a flat spin may be extremely difficult and, in some cases, impossible.
The primary cause of an inadvertent spin is one wing exceeding the critical angle of attack while executing a turn with excessive or insufficient rudder, and, to a lesser extent, aileron. In an uncoordinated manoeuvre, the pitot/static instruments, especially the altimeter and airspeed indicator, are unreliable due to the uneven distribution of air pressure over the fuselage. The pilot may not be aware that the critical angle of attack is about to be exceeded until the stall warning device activates. If a stall recovery is not promptly initiated, the aeroplane is more likely to enter an inadvertent spin. The spin that occurs from cross-controlling an aircraft in a skidding turn usually results in rotation in the direction of the rudder being applied, regardless of which wing tip is raised. In a slipping turn, where opposite aileron is held against the rudder, the resultant spin will usually occur in the direction of the applied rudder and opposite the aileron that is being applied.
Before flying any aircraft, in which spins are to be conducted, it is important to determine under what conditions, and with which limitations, this exercise may be carried out. The pilot should be familiar with the weight and balance limitations, operating characteristics and standard operating procedures, including spin recovery techniques, specified in the approved AFM or POH. Pilots must also be aware that some manufacturers have updated the information regarding spins for some older aircraft models. Manufacturers use various methods of revising this information.
The manual for the Piper PA-38 Tomahawk, issued in 1978 was revised in 1981 to include detailed information on the procedures to follow when spinning. Additional advice, outlining very specific spin characteristics in the case of a mishandled recovery, is very valuable to all pilots flying this aircraft.
Piper Aircraft Corporation updated the PA-28-140 Cherokee Pilot Operating Handbook through a Service Bulletin (SB) in 1982. SB 753 provides "expanded spin recovery procedures to assure that proper safety practices and procedures relative to utility category flight operations are in effect". Piper made it mandatory to retain this SB in the airplane at all times.
- Early Cessna aircraft did not include a procedure for the spin manoeuvre and Cessna has progressively updated the spin procedures included in the POHs since 1976 for C150 and since 1973 for C172 aircraft. A supplementary booklet titled 'Spin Characteristics of Cessna Models 150, A150, 152, A152, 172, R172 and 177' was issued in 1980 and sent to registered Cessna owners. This booklet outlines the differences in spin characteristics between models and even between some model years.
The procedures outlined below are suitable for most small aircraft and may be used in the absence of manufacturer's data.
The first step in recovering from an upright spin is to close the throttle completely to reduce the gyroscopic effect of the propeller and minimize the loss of altitude. The next step is to neutralize the ailerons, determine the direction of the spin, and apply full opposite rudder. Just after the rudder reaches the stop, briskly move the elevator control forward sufficiently to break the stall. Some aircraft require merely a relaxation of back pressure; others require full forward elevator control pressure. Forward movement of the elevator control will decrease the angle of attack. Neutralize the rudder when the spinning stops. The aircraft will now most likely be in a spiral dive condition, so level the wings with co-ordinated controls and gradually apply enough aft elevator pressure to return to level flight. The ailerons should be neutral prior to applying significant aft pressure to prevent torsional overstress of the wing structure. Too much or abrupt aft elevator pressure and/or application of rudder and ailerons during the recovery can result in a secondary stall and possibly another spin. In some cases, the engine will stop developing power due to centrifugal force acting on the fuel in the tanks causing fuel starvation. It is, therefore, recommended to assume that power is not available when practicing spin recovery. As a rough estimate, an altitude loss of approximately 500 feet per each 3-second turn can be expected in most small aircraft in which intentional spin is approved. Greater losses can be expected at higher density altitudes.
Scenario Based Training
In order to increase the effectiveness of training it must be linked to the real world. In situations where real demonstration and practice can be too dangerous (e.g. a low altitude stall after take-off), simulations performed at a safe altitude can impart the required knowledge and skill. But these simulations must be well thought out by the instructor. Very few accidental stalls occur from an exaggerated pitch attitude. Demonstrations using less extreme attitudes provide realistic and effective training. A low level stall or spin requires a pilot to recognize the situation and implement the appropriate skill in a timely and accurate fashion. Prompt recognition by the pilot is more likely to occur if they have been exposed to a close approximation of the situation. Describing a detailed scenario under which an exercise is to be performed sets the stage and provides that link.
Improper airspeed management resulting in a stall or a spin is most likely to occur when the pilot is distracted. Poor weather, sickness, or intermittent equipment malfunctions can result in the pilot focusing on tasks secondary to flying the aeroplane. Emergencies such as engine failures and fires can cause strong distractions at critical times such as manoeuvring for landing.