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Stalls and Spins

To Spin or Not to Spin

BY PHYLLIS A. DUNCAN, EDITOR (From FAA Aviation News, May/June 1992)

R ecent changes to FAR Part 61 require recreational, private. and commercial pilots applicants to have aeronautical knowledge of "... stall awareness. spin entry, spins. and spin recovery techniques in airplanes. " The following article has been adapted from the October 1991 issue of the AOPA Air Safety Foundation's Flight Instructors' Safety Report, where it was adapted from the FAA/AOPA audiovisual "Back to Basics" program, "Stall/Spin Awareness. " It contains some excellent information and diagrams for flight instructors who now have to teach about spins and for pilots who may some day find themselves "autorotating about the spin axis. " - Editor

Avoiding the stall/spin accident

The stall/spin 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 occurrence of this type of accident was understandable.

Today, however, the spin is well understood, but we are still plagued with stall/spin accidents. In fact, stall/spin accidents comprise about 8% of the total and, more significantly, account for about 25% of the fatalities and serious injuries 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 training - or, rather, the lack thereof.

What is a spin?

A spin is a maneuver during which the aircraft descends rapidly in helical movement about a "vertical" axis - the Spin Axis, which should not be confused with the vertical axis of the airplane. In some ways the spin resembles a steep spiral, but there is a fundamental difference. Throughout the spiral dive maneuver you have the aircraft under full aerodynamic control; you can fly out at any time. In a spin, on the other hand, the aerodynamic and inertial forces may be in a degree of balance which the pilot has to upset in order to regain control.

In an aircraft approved for spins, the spin is a recoverable maneuver, but the recovery does require altitude. If you have sufficient altitude at the start of the spin, fine, but if not ... you could become a statistic. However, in aircraft placarded as not approved for spins, there is absolutely no assurance that recovery from a fully developed spin is possible.

Although spin research indicates that in straight wing aircraft there are many factors which may contribute to a spin, two things must occur: One, a wing drops after becoming fully stalled, and, two, there is sideslip or yaw acting toward the low wing at or beyond the actual stall point. If allowed to continue to its natural conclusion, the lowering wing has an increasingly greater angle of attack while on the rising wing the angle of attack actually decreases. This leads to a rolling moment or rotation about the airplane's longitudinal axis. The airplane is then in a spin and descends vertically, continuing to roll and yaw until the pilot does something to interrupt it.

A spin is divided into two phases - incipient and steady state. The first, an incipient phase, is that portion after stall when the aircraft commences a spin-like motion. In this phase the aerodynamic and inertial forces have not yet achieved the necessary balance. In the second phase, the steady state or fully developed spin, the aerodynamic and 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. The airspeed is near stall. 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.

Understanding the spin

Gravity {weight), lift, thrust, and drag are terms that are familiar, as are the three axes of flight: roll, pitch, and yaw. To understand a spin, there are some other terms to become familiar with:

  • Relative Wind. This is the speed and direction of the air approaching the aircraft as it flies. The velocity of the relative wind and the airspeed of the aircraft are equal and opposite to each other.
  • Angle of Attack. This is the angle formed by the relative wind and the chord line of the airfoil.
  • Coefficient of Lift {CJ. This is a numerical representation of the lift generated by a particular airfoil at a given angle of attack at a specific airspeed.
  • Coefficient of Drag (Go). This is a number representing drag and derived from the same factors of the particular airfoil's configuration, relative wind, and angle of attack.

As the angle of attack increases, GL also increases. When the angle of attack reaches a certain point, the airflow separates from the airfoil, and lift starts to decrease. As the angle of attack continues to increase, lift is still generated, but it decreases still more. The stall occurs at the peak. Go also increases as the 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 the angle of attack. Conversely, the downgoing or inboard wing has an increased angle of attack.

The difference in the angle of attack of the two surfaces is because of the vertical component of the relative wind in the rolling condition. The difference in angles of attack result in differences of lift and drag for the two surfaces; the up going wing is less stalled, and the downgoing wing is more stalled. This differential condition 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 us review 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 that wing. But the increased lift also increases the induced drag, causing a yaw toward the down wing. The down wing, with an increase in total drag, becomes more stalled. This produces even more roll which contributes to the autorotation. To prevent this autorotation, you must eliminate any slipping or turning input at the point of stall. Coordination of aileron and rudder is the key.

Spin recovery

An airplane's pilot operating handbook (POH) contains the manufacturer's recommended spin recovery sequence in the "Emergency Procedures" section. For aircraft without a POH, consult the aircraft flight manual (AFM), appropriate section of the owner's manual, or other operations and limitations data. Spin recovery generally follows these five classic steps. First, power: Reduce it to idle. Second, ailerons: Neutralize them; they could aggravate the spin. Third, rudder: Apply it fully, opposite to the direction of the spin to slow the rotation. If you are confused about the direction, check the turn indicator. It will be fully deflected in the direction of the spin. Do NOT use the ball since its position may conflict with that of the turn needle. Fourth, as the rotation slows, push the elevator briskly forward to reduce the angle of attack and break the stall. Hold the rudder and the elevator where they are until rotation stops. It may take a full turn or even more. Fifth, as rotation stops, neutralize the rudder, and recover from the ensuing dive in the normal manner, i.e., back pressure - but not an excessive amount, or you may induce a secondary stall and another spin that may be more violent than the first.

The best way to learn about spins and spin recovery is by practice. (Note: The FAR require in-flight spin training only for flight instructor applicants in either airplanes or gliders.) But you must use an aircraft that has been approved for intentional spins. Is yours? Your aircraft's flight manual and placards clearly spell it out. If your aircraft has a placard against intentional spinning - DON'T! Advisory Circular (AC) 61-21C, Flight Training Handbook, states that:

"The pilot of an airplane placarded against intentional spins should assume that the airplane may become uncontrollable in a spin."

Certification for a normal category aircraft requires recovery from only a one-turn spin, which is merely the incipient spin phase and not a fully developed spin. That is why you must practice spinning only in an aircraft approved for spins. If you intend to practice spins, first be sure that the aircraft is properly loaded, and that you have someone with you who is experienced in spinning that aircraft, preferrably a qualified instructor.

If you want to avoid inadvertent spins, keep them from developing in the first place. How do you do that?

Stall/spin awareness

The most effective prevention of spins is stall awareness. We know that a stall is the result of an excessively high angle of attack and that it can occur at any attitude and at any speed. There are three situations in which a pilot can easily exceed an airplane's critical angle of attack: low-speed flying, high-speed flying, and in turning flight. Since it is difficult to fly without flying slowly at some point or turning, the potential for a stall is with us more than we would like. (High-speed flight near MACH 1 and at altitude involves a situation where the difference between the low-speed stall buffet and the high-speed MACH buffet is only a few knots.) Through training and practice, pilots can become "stall aware." We learn to recognize the many indications of a stall and to react with proper control response before a stall becomes fully developed. We also learn what to do to recover from a full stall.

The following five cues may warn you of an impending stall.

Vision is one, but its usefulness is limited to watching for a change of attitude. If you see that the nose is higher than it should be for the power and speed being developed, you may be about to stall. Nose attitude, however, is not an absolutely sure sign because stalls can occur in any attitude.

Hearing can give you another cue. The sounds related to flight will increase as your speed increases, as you know, but if a stall is impending, the sounds may diminish.

The third sign is kinesthesis - "muscle sense" - the response of your body to the aircraft's changes of direction and speed. You can feel it. If you have not already done so, you can develop the ability. Also, the onset of airframe buffet {less noticeable in power-off stalls) may indicate the approach of a stall.

The fourth cue is the feeling of control pressures and response. As speed is reduced, control resistance to pressure becomes less and less, and you can move the controls through a greater range of movement without a corresponding change in aircraft attitude.

Last, but not least, are your flight instruments. They warn you of impending stall, and they indicate the actual stall. An angle of attack indicator is a very accurate stall warning instrument; however, the airspeed indicator is the most common instrument.

Your senses and flight instruments are the means by which stall awareness should enable you to recognize an impending stall. But you can lose your awareness very quickly if your attention is diverted or lost by distraction - the major cause of inadvertent stalls. Anything that takes your attention away from your number one responsibility - FLYING 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 coordinating these three steps:

  • The first one is to reduce the angle of attack positively, generally by lowering the nose.
  • The second step is to apply maximum allowable power as necessary.
  • And third, coordinate your controls to regain straight and level flight and, thus, full aerodynamic control of the aircraft.

Do you know how your airplane reacts when near stall speed? To be sure, practice flying at minimum controllable airspeed at a safe altitude 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.

Where and how stall/spin accidents occur

Stalling and spinning is obviously more threatening under certain conditions, namely low altitude, but low altitudes are part of every flight you make. Another threatening condition is an improperly loaded airplane, either over-gross and/or out of balance. During preflight, check that the loading does not exceed the center of gravity limits. If it does, your aircraft may lose stability. With the center of gravity moved aft of its proper location, you will find that a steep climb may produce a departure stall. Even at altitude an aft center of gravity loading may result in your not having enough forward stick available to lower the angle of attack sufficiently to ensure stall/spin recovery.

Takeoffs also have stall/spin potentials. Just after breaking ground, and during your initial climbout, an engine failure can be disturbing. Your instinct may be 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 may be better to make an unscheduled, off-airport landing under control than to stall, spin, and crash out of control.

Takeoff from a short field may also be a stall-prone maneuver. In order to clear obstructions, you may pull up at too steep an angle of attack too soon. Let the aircraft accelerate to the proper airspeed, then climb out under control.

Landings have their stall risks, too. For example, you may have trimmed nose-up to help maintain the 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, even a slight increase in back elevator pressure may cause an accelerated stall. The solution is to plan ahead - do not get trapped. If you find yourself in such a situation and recognize it, smoothly initiate a go-around! But go-arounds are not free from stall/spin possibilities, either. If you have set the flaps to full down and have set trim to a nose-up condition, you may induce a departure stall on go-around unless you manage the go-around using the correct procedures. Any uncoordinated 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 smoothly, even to the maximum allowable if necessary, start the climb, and reduce the nose-up trim to obtain a normal climb attitude. Use control forces as necessary to control pitch attitude and heading and bleed the flaps to a position that gives maximum lift and minimum drag. Retrim 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 to try to hold the airplane in the air. If you do that, you should know what can happen. The safe thing is to watch your angle of attack closely as you watch for a possible touchdown spot. Flying in the pattern or on short final, at 400-500 feet above the surface, is no place to enter a spin that requires 1 ,000-1 ,500 feet for recovery after you apply control inputs.

You may not be able to avoid all flight situations in which a stall/spin could occur, but you should be able to avoid the stall/spin by recognizing the situation before it becomes a problem. The key to stall/spin avoidance is stall awareness. Know the warning signs, respond to them, and go ahead and do what you have to do. Become proficient in flight at minimum controllable airspeed and reacquaint yourself with how your aircraft reacts in stall recovery. To increase your confidence, take some spin instruction from a qualified instructor in an aircraft that has been approved for intentional spinning. (Besides, it is a great way to qualify for your wings in the Pilot Proficiency Award Program!) And remember, it is a fact not a fantasy - no stall, no spin!

Editor's Note: In Part 2 of this article, which will appear in the July/August issue, we will discuss in-flight spin training in more detail, including some suggested maneuvers for flight instructors to do with their students to hone their stall/spin awareness.