BY RICHARD GLESS AND PAUL BRAY (AOPA Air Safety Foundation, October 1995)
The stall/spin/crash accident has been with us since the days of the Wright brothers. In the early days, the terrifying "tailspin" was shrouded in mystery, thus a high frequency of this type of accident was understandable. Today, even though the spin is well understood, we are still plagued with stall/spin accidents. In 1986, stall and stall/spin accidents comprised 9.8 percent of the total and, more significantly, accounted for about 29.5 percent of the fatalities in general aviation. If today's breed of pilot understands the spin, why do we still have so many stall/spin accidents? Perhaps the answer lies in stall/ spin awareness and training or rather the lack of it.
The purpose of this pamphlet is threefold: 1) alert pilots to the stall/ spin hazard and the in-flight situations which lead to stalls and spins; 2) explain the dynamics of the spin and spin recovery; and 3) teach the most effective method of spin prevention - stall awareness.
A spin is a maneuver during which the aircraft descends rapidly in a helical movement about a vertical axis - the Spin Axis. In some ways the spin resembles a spiral dive; but there's a fundamental difference. Throughout a spiral dive maneuver you have the aircraft under full aerodynamic control; you can fly out at any time. In a spin, the aerodynamic and inertial forces are in balance - which you have to upset in order to regain control. If you have sufficient altitude at the start of the spin, fine! But if not ... you may become a statistic!
A spin is caused by two primary factors: one, the aircraft at or beyond the stall angle of attack; the other, sideslip or yaw acting on the aircraft at or beyond the actual stall point. A spin is divided into two phases - incipient and steady state. The first, the incipient phase, is that portion after stall when the aircraft commences a spin-like motion. In this phase, the aerodynamic and the inertial forces have not achieved a balance. In the second phase, the steady state or fully developed spin, the aerodynamic and the inertial forces are in balance and the attitude, angles and motions are repetitive from turn to turn.
In a spin, the view looking out of the cockpit is generally a steep, nose-down attitude, with a yawing/rolling motion about the spin axis. The airspeed is near stall airspeed. An angle of attack indicator, if the aircraft has one, shows a fully stalled condition. The turn needle is fully deflected in the direction of the spin and the rate of descent is significant. The "g" force acting on a spinning aircraft is essentially One. The spin is a recoverable maneuver in aircraft approved for spinning, but the recovery does require altitude.
Gravity, lift, thrust and drag are terms that are familiar to you. You also know the three axes of flight: roll, pitch and yaw. To understand a spin, there are some other terms that you should be familiar with:
Let's look at CI. As angle of attack increases, the coefficient of lift also increases. When angle of attack reaches a certain point, the airflow separates from the airfoil, and lift starts to decrease. As angle of attack continues to increase, lift is still generated, but decreases even more. The stall occurs at the peak. The coefficient of drag also increases as angle of attack increases. Beyond the stall point, however, drag increases even more.
When you are beyond stall angle of attack, if the aircraft experiences any rolling displacement, the up going or outboard wing will experience a decrease in angle of attack. Conversely, the downgoing or inboard wing has an increased angle of attack. The difference in angle of attack of the two surfaces is due to the vertical component of the relative wind in the rolling condition. The difference in angle of attack results in differences of lift and drag for the two surfaces; the upgoing wing is less stalled - and the downgoing wing, more stalled.
This causes a rolling and turning tendency at angles of attack beyond stall. The tendency is called autorotation and is self-feeding. The roll at stall may be initiated by adverse yaw. Let's see what that is. If you are near stall angle of attack and a wing drops, and you attempt to raise it by applying aileron alone, the aileron, going down, will increase the lift on the wing. But the increased lift increases the induced drag causing a yaw toward the down wing. This is adverse yaw. The down wing, with an increase in total drag, becomes more stalled. This produces even more roll, contributing to autorotation. To prevent autorotation, you must eliminate any slipping or turning input at the point of stall. Coordination of aileron and rudder is the key.
The Pilot's Operating Handbook for each aircraft gives the correct procedure. It is your guide for many of the facts without the fantasies in handling your aircraft.
Stall/spin is obviously more threatening under certain conditions such as low altitude. But potential stall/spin situations are part of virtually every flight you make. During preflight, check that the loading does not cause the center of gravity limits to be exceeded. With the C.G. moved aft of its proper location, you'll find that a steep climb may produce a departure stall. Even at altitude, an aft C.G. loading may result in your not having enough forward stick available to lower the angle of attack sufficiently to ensure stall/spin recovery.
Takeoffs have stall/spin potential. Just after breaking ground, and during your initial climbout, an engine failure can be disturbing. Your instinct is to try to turn back. But if you do, you may well set up a stall/spin entry. If this occurs at initial departure heights, recovery may be impossible. The solution is to lower the nose immediately to attain your best glide speed, thereby preventing a stall or loss of control. It's always better to make an unscheduled off-airport landing under control than to stall, spin and crash out of control.
Takeoffs from a short field may also be a stall-prone maneuver. In order to clear obstructions, you may pull up too much, too soon. Let the aircraft accelerate to the proper airspeed, then climb out.
Landings have their stall risks, too. For example, you may have trimmed nose-up to help maintain correct approach speed. Then you may encounter a crosswind that makes you overshoot the turn onto final. If you steepen the bank and/ or use excessive rudder pressure to turn the aircraft onto final, a slight increase in back elevator pressure may cause an accelerated stall. The solution, plan ahead - don't get trapped. If you find yourself in such a situation and recognize it - go around! But go-arounds are not free from stall/spin possibilities, either. If you have set your flaps to full down and have set trim to a nose-up condition, you are a good prospect for a departure stall. Any rudder application may provide the sideslip leading to stall/spin entry. To avoid it, when you make the decision to go around, add sufficient power, even to the maximum allowable if necessary, start the climb and reduce the nose-up trim to obtain a normal climb attitude. Use control force as necessary to control pitch attitude and heading, and bleed the flaps to a position that gives maximum lift and minimum drag. Return the aircraft to reduce control forces once you have the situation well in hand.
Finally, what do you do if you have a power loss on final? Again, instinctively you may want to apply back pressure - try to hold the airplane in the air .The safe thing is to watch your angle of attach closely as you watch for a possible touch-down spot. Flying in the pattern or on short final, at 400 to 500 feet above the surface, is no place to enter a spin that requires a 1000 to 1500 feet for recovery after you apply control forces.
There are five classic steps for recovering from a spin.
The best way to learn about a spin and spin recovery is by practice. But you must use an aircraft that has been approved for intentional spins. Is yours? Your aircraft papers and placards clearly spell it out. If your aircraft has a placard against intentional spinning - DON'T!
Except for flight instructors, in-flight spin training is not required for any pilot certificate or rating, however FAR 61 does require ground training in "stall awareness, spin entry, spins, and spin recovery techniques."
Any flight instructor should be prepared to discuss these subjects with you and many are able to conduct in-flight training as well.
Let' s consider some of the points to be covered. Specifically, you should know the answers to the following questions:
The answers to the first seven questions can be found in the F AA Flight Training Handbook (AC-61-21A) and other reference sources such as Kershner's flight Instructor's Manual. The answer to the last question and some information you may find useful in avoiding the stall/ spin/ accident are given in the following scenarios - all of which should be conducted at a safe altitude with an experienced instructor on board and in an aircraft properly certificated for spins. First, ask the instructor to demonstrate and then practice the following scenarios under his or her guidance.
After a simulated takeoff at altitude, continuously increase pitch attitude as might be done in an attempt to clear obstacles (entry is similar to a departure-type stall). This demonstration is most effective if rudder coordination is improper when the aircraft stalls. To make a right turn in a steep climb, many pilots use insufficient right rudder and excessive right aileron. This produces a slipping turn to the right (ball indicator far to the right), with the result that, if a departure stall occurs, the airplane spins rapidly to the left "over-the-top."
The tendency of the airplane to enter a spin from a left climbing turn is less pronounced since, with no rudder pressure applied by the pilot, the left yawing moment is approximately correct resulting in a coordinated turn (more or less) and the spin does not develop as rapidly after the stall break. In any event, this demonstration makes the point that a departure stall is especially critical, because of the tendency for the airplane to enter a spin rapidly to the left at a critically low altitude.
This demonstration emphasizes the need for immediate and positive reduction of pitch attitude to avoid a stall if the engine fails on takeoff or during the initial climb and illustrates the altitude loss and stall/ spin hazard associated with an attempt to make a 180. turn back to the runway. For demonstration purposes, establish a climb at best angle-of-climb airspeed, and note the altitude as power is reduced to idle. If pitch attitude is not reduced immediately, a stall or high sink rate may develop. After best glide speed is obtained, execute a 180ï¿½ turn and note the total loss of altitude. This illustrates the need to maintain flying speed and the altitude required to turn back to a runway.
These demonstrations, conducted at a safe altitude, illustrate errors that could result in a stall or spin during a poorly planned and executed turn from base to final.
Skidding turn to final approach: This scenario simulates the airplane at too Iowan altitude on the turn from base to final approach. Due to the low altitude, the pilot hesitates to use a properly banked turn and, instead, attempts to turn using excessive (bottom) rudder to yaw the airplane onto final approach. The excess rudder causes the airplane to bank and develop a nose-down pitch attitude. At this point, the pilot may be distracted by attention to ground references and be counteracting the steepening bank with opposite aileron and further nose-up elevator to oppose the down-pitching tendency. These control movements and positions can result in a stall/ spin/ crash.
Slipping turn to final approach: In this scenario, the turn from base to final is started too late to avoid overshooting the simulated runway centerline. The pilot rolls rapidly into a steep bank with insufficient rudder pressure in the direction of the turn. The steep bank will cause a nose-pitching-down tendency and increased sink rate. If opposed with aft elevator control movement, the result may be an accelerated stall and a spin over-the-top.
Following these demonstrations, the instructor should emphasize the need for proper planning in the traffic pattern, airspeed, altitude and power control and the need for properly coordinated, medium-banked turns. Further, it should be emphasized that these kinds of errors are more likely to occur in the event of engine failure shortly after takeoff; or a forced landing where the pilot is faced with a difficult situation and might attempt a rapid turn at low altitude while trying to extend a glide by using excessive up elevator control.
This scenario demonstrates how a pilot's attention may be diverted from aircraft control to visual reference to another aircraft while attempting to fly at reduced airspeed causing a stall or loss of control. In a simulated traffic pattern, reduce power and increase pitch to maintain spacing behind simulated slower traffic. This demonstrates how a pilot's attention can be diverted causing failure to notice instrument or other indication of a near-stall condition.
Simulate power loss on final approach in the landing configuration and prevent $e aircraft from losing more than 100 feet of altitude during the next 20 seconds. This maneuver simulates a pilot trying to stretch a glide following power loss on final approach. It can result in a full or partial stall.
This scenario demonstrates how an improperly executed go-around can result in an inadvertent stall or spin, particularly if the pilot delays initiation of the go-around until obstacle clearance at the departure end of the runway becomes a factor. Establish a properly trimmed descent to simulate a short-field approach at 1.2 Vso and initiate a go-around by application of full power. Failure to counteract nose-up trim setting with forward elevator pressure can result in extreme nose-high pitch attitude and a stall or spin, especially if the aircraft is loaded at aft center of gravity.
This scenario illustrates the effect of flap retraction at a speed below the flaps-up stalling speed such as might occur on a mishandled go-around. It can be demonstrated by simulating a go-around from short-field approach speed (1.2 Vso). Reduce speed in a simulated landing flare to just above stalling, apply full power and retract the flaps rapidly. The result is usually a full or partial stall.
This scenario demonstrates the aggravated left-turning tendency of an airplane in a go-around. Establish a slipping approach to the right such as would be used to compensate for a right crosswind on landing with proper approach speed and trim. This requires right aileron and left rudder. Simulate a go-around with application of full power and climb-pitch attitude but do not neutralize the rudder. Carried to extreme, this control action can result in a rapid spin entry to the left, especially if the nose-up attitude is too high.
If the thought of conducting these demonstrations frightens you, it shouldn't. If conducted in a properly certificated airplane and at a safe altitude, the spin is a routine acrobatic maneuver. There is nothing to fear - it can even be fun!
The most effective spin prevention is stall awareness. To be specific there are five cues that can warn you of an impending stall.
Your sight, hearing and feeling enable you to sense an impending stall. But you can lose your awareness very quickly if your attention is lessened or lost by distraction - the major cause of inadvertent stalls. Anything that takes your attention away from your number one responsibility, FLY THE AIRCRAFT, may lead to a stall. How do you prevent that kind of distraction? Develop a good scan pattern. It should - in fact, it must - keep your attention moving back and forth between flying the aircraft, the instruments and outside references. Remember the cardinal rule of flying: aviate, (i.e., fly the airplane), navigate and communicate in that order.
In any situation, if you become aware of an impending stall, how do you handle it? By taking three steps:
Studies show that the initial climb and the approach airspeeds that you must use during takeoffs and landings are factors in two-thirds of general aviation's reportable accidents. Do you know how your airplane reacts when near stall airspeed? To be sure, practice flying at minimum controllable airspeed and find out about your airplane's attitude, power needed versus airspeed produced, trim required, effectiveness of controls and the effects of flap extension and retraction. Practice at minimum controllable airspeed will sharpen your stall avoidance ability.
You may not be able to avoid a stall/spin threat, but you should be able to avoid the condition by recognizing it before it becomes a problem. To increase your confidence, take some spin practice with a qualified instructor in an aircraft that has been approved for intentional spinning.
Become proficient in flight at minimum controllable airspeed and reacquaint yourself with how your aircraft reacts in stall recovery.
The key to the stall/spin problem is Stall Awareness. Know the warning signs, respond to them and go ahead and do what you have to do.
And remember: no stall - no spin!
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