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Stalls and Spins

Stalls

BY RICH STOWELL (From Flight Training, November 1993.)

Last month's historical overview illustrates the ongoing challenge to understand and deal with stalls and spins. Airplane manufacturers still struggle to design airplanes that strike a satisfactory balance between maneuverability and stability while meeting the performance demands of the pilot.

Government regulators struggle to design training requirements that develop piloting skills to handle these airplanes safely after a reasonable training period. Since the stall/spin potential is a natural by-product of airplane design, it follows that pilots should learn about these phenomena.

This part of our series touches on some important aspects of stalled flight. The foundation for stall awareness is laid with one fundamental truth: angle of attack is the key parameter that determines when a wing will stall. This angle is controlled with the elevator; our own elevator inputs, therefore, determine whether or not the wing stalls. Let's acknowledge and accept that airspeed, G-load, and curved flight are controlled with the elevator as well. While it may appear the elevator serves a multi-faceted control function, all of the parameters it affects are closely linked to angle of attack.

The critical angle of attack signifies an important aerodynamic crossroad. Exceed it and airflow tears away from the wing's surface, causing a sharp drop in the coefficient of lift, a rise in the coefficient of drag, and a reduction in control effectiveness.

Angle of attack measurements are the only way to know precisely how close we are to the critical angle of attack. But since angle of attack indicators are not standard equipment on light airplanes, we are relegated to inferring our proximity to the stall from aerodynamic, mechanical, and physiological cues. None of these cues is fail-safe, but when all the cues are integrated and used synergistically, they can provide an effective stall warning, provided the pilot is paying attention to them.

Aerodynamic cues

Buffeting and an uncontrolled pitch change are aerodynamic stall warning cues designed into light airplanes. Stall buffet occurs when turbulent air spills from the wing root and strikes the tail section. The stall characteristics inherent in different wing planforms are coupled with various aerodynamic "tricks" to ensure good stall warning.

The relatively stable rectangular planform, for example, naturally stalls at the wing root before progressing outward to the wing tip. This favorable stall pattern has the advantage of a distinct buffet combined with positive lateral control at the onset of stalled flight.

A moderately tapered wing planform yields better performance than a rectangular one, but it tends to stall simultaneously throughout its span. This less favorable characteristic adversely affects lateral control and reduces the likelihood of early stall warning.

The desirable stall characteristics observed in rectangular planforms can be built into other planforms. Twisting a tapered wing slightly so that the angle of attack at the wing root is greater than at the wing tip (called "washout") and placing stall strips near the wing root are two ways of mimicking rectangular planform stall behavior. Such modifications help trigger the stall at the wing root first.

Unfortunately, aerodynamic stall warning can be thwarted by the effects of power, turbulence, and the accumulation of snow, ice, or frost on the wing. It can be defeated altogether by loading the airplane beyond its weight-and-balance limits. Skidded turns and other examples of poor flight control coordination can transform otherwise docile stall characteristics into more aggressive stall behavior. Inadvertently yawing the airplane near stalled flight may elicit dangerous stall patterns similar to those on planforms with sweep back. Swept wing designs, while highly maneuverable, suffer from violent wing tip stalls, usually without any aerodynamic warning.

Mechanical cues

Continuous horns and warning lights are the most common means of mechanical stall warning. These devices are set to activate at least 5 knots (kts) prior to stalled flight, but they do have limitations. For example, one study found stall warning lights to be practically useless during day visual flight rules (VFR) conditions, where pilot attention is primarily outside of the cockpit.

Also, bright daylight makes it difficult to distinguish between a light that's on or off. While continuous aural devices were better able to warn pilots of an impending stall, the same study found that pilots, overloaded with other distractions near stalled flight, essentially "tuned them out" 36 percent of the time.

Mechanical systems can be rendered inoperative by inlet air blockages, frozen or stuck stall-sensing vanes, or electrical problems. Stall sensors located on the left wing only are a potential weak link in the system as well. If both wings stall simultaneously, or if the left wing stalls first, as it might during a descending left turn, the stall sensor should detect the onset of stalled flight. If the right wing stalls first instead, as it could during a steep climbing turn to the left, the mechanical warning system may not activate until significant portions of both wings are stalled.

Believe it or not, the stick or yoke can be an effective stall warning tool, too! Since the elevator controls angle of attack, the direction in which it's moving determines our trend toward or away from stalled flight. The exact control position and control pressure preceding a stall depend on power setting, trim position, and the type of maneuvering. Even so, realize that for every inch the stick is pulled aft during upright flight, the angle of attack will positively increase and the airplane will move closer to stalled flight. Forward control movements will lower the angle of attack and push the airplane away from stalled flight.

Correlating elevator movements with changes in angle of attack requires a light touch on the controls. When distracted or under stress, pilots subconsciously respond by tensing up and pulling the stick or yoke further back. An unchecked pull coupled with a decreasing airspeed trend ultimately will terminate in stalled flight.

Our innate behavior must be modified and replaced with a forward elevator response. We need to react instinctively to the airflow needs of the wing, not necessarily to an undesirable picture seen over the nose. Rather than continuing along the path to an inadvertent stall/spin, a stall-aware pilot responds with forward elevator inputs when a maneuver is botched.

Reducing the angle of attack is the key factor in recovering from a stall and regaining control of the airplane. The next step is to smoothly add power as applicable to minimize the loss of altitude (See Stall and Spin Awareness Training, AC 61-67B and the Flight Training Handbook, AC 61-21A, pages 143-157).

Physiological cues

Aerodynamic and mechanical stall warning are useless if we become oblivious to our senses of sight, sound, and touch. Most stall/spin accidents occur during day VFR conditions, so responding to physiological cues plays an important role in stall awareness. Stall practice should be used to fine-tune our senses to changes in airspeed, G-load, and flight path preceding stalled flight. Witnessing the telltale signs demands focusing our attention on the airplane itself and ignoring its attitude relative to the horizon.

The next time you're out practicing stalls at a safe altitude, look at the downward trend displayed by the airspeed indicator. Really listen for a simultaneous decrease in ambient noise levels. Look at and feel the direction in which you're moving the elevator control to initiate the stall. Feel the changes in control pressures as well.

While practicing accelerated stalls, feel the G-load increase on your body as you pull back on the elevator. Watch the nose of the airplane pitch toward you before the stall, then observe it pitch away from you when airflow separates from the wing. Feel the stall buffet. Listen to the sound of turbulent air hitting the back of the airplane, and acknowledge the stall warning horn.

A high workload can hinder a pilot's ability to respond to physiological stall cues. Uncoordinated flight can confound the situation by causing sensory conflicts and interfering with stall warning signals. Concentrating on the wrong thing can prevent appropriate reactions, too.

During reduced-power stall practice, for instance, many pilots initially fight against stall warning cues in a futile attempt to keep the nose pointed "up," away from the ground looming in the windscreen. Our top priority must be to give the airplane what it needs for controlled flight and an arrested descent - a lower angle of attack and power! The only way to accomplish this is to momentarily move the stick or yoke away from your body, no matter what it looks like outside.

Stall dynamics

Once we enter stalled flight, aerodynamic forces and moments quickly try to restore the airplane to equilibrium. The airplane does not desire to be in a conventional stall, so it seeks an alternative configuration. There are actually three configurations into which the airplane could stabilize: normal flight, autorotation, or deep stall.

Deep stall is the least desirable of the three and occurs when the airplane pitches into a very high angle-of-attack, high-drag configuration. Recovering from deep stall may be impossible. Fortunately, light airplanes are designed to avoid this flight mode, but only when they are loaded within their weight and center-of-gravity limits.

While characteristically incapable of deep stall, most light airplanes are capable of autorotating (spinning), even when within their prescribed operating envelopes. Unrecoverable spin modes are possible as well, particularly if a spin progresses beyond one turn in airplanes operated in the normal category. Autorotation is excited by generating sufficient yawing and rolling moments near stalled flight. It often proves fatal when entered close to the ground. This flight mode will be discussed in detail next month, in Part III.

Normal flight certainly is the most desirable of the three configurations. The main objective of our stall practice is to develop the appropriate reflexes to return to normal flight. Three basic actions are required to resume normal flight and to avoid autorotation and deep stall. First, only operate your airplane within its approved weight-and-balance limits. Second, manipulate the rudder to prevent a coupling of yaw and roll at high angles of attack. In other words, coordinate the control inputs. Proper coordination improves our ability to interpret stall warning cues and guarantees a more docile stall pattern across the wing. Third, use forward elevator inputs to reduce the angle of attack below critical and power to arrest the descent if necessary.

It's also important to recognize that we mostly practice intentional stalls that are choreographed to help us pass a checkride. An approach-to-landing stall, for example, is initiated with the wings level, power at idle, flaps and landing gear extended, at the reference stall speed (Vso). There are other equally important, but often overlooked, considerations implicit in the performance of this specific maneuver. The airplane must be at a certain weight, under a +1.0 G loading, and have properly coordinated control inputs. If these considerations are not correctly addressed, the airplane may not react as expected.

A revealing picture

The best way to demonstrate the interaction between weight, airspeed, and stalled flight is on a V-G diagram (also called a V-N diagram). It displays all of the control functions of the elevator in one convenient place. For illustration purposes, let's restrict ourselves to positive G flight within normal category limitations.

Thus defined, the operating envelope has three distinct boundaries: a horizontal cap at the design limit load of +3.8 Gs, a vertical boundary drawn at the never exceed speed (Vne) and a curved line representing stalled flight, which varies with airspeed and G-load below maneuvering speed (Va).

The V-G diagram reinforces the concept that we cannot know how close we are to stalled flight based on airspeed alone. We must factor in the effects of weight ( or G load). Much of our practical stall experience, though, obscures weight's relevance, bolstering a faulty association with airspeed. Let's illustrate a few typical stall scenarios, plotting their progression on a V-G diagram (Figure 4). Let's assume we begin each scenario from level flight at some speed (V) between Vso and Va.

Path A represents our approach-to-landing stall practice. Airspeed is reduced to Vso under a constant +1.0 G deceleration, at which point the wing stalls. Trying to stretch a glide by pulling back on the elevator also follows Path A directly to a stall.

Path B is an often deadly route chosen by some pilots in response to an engine failure after takeoff. The airplane is banked sharply in an attempt to make it back to the airport. Airspeed decays and G-load increases in the turn (all controlled by the elevator!) as the pilot literally pulls the airplane into an accelerated stall/spin.

Path C depicts rolling into a turn at a constant airspeed. If the bank becomes too steep (too many Gs required) for the chosen airspeed, we will experience an accelerated stall. Steep turns at low speed, for example, place us at greater risk of an accelerated stall. Moving the elevator forward, toward higher speeds and lower Gs, would move the airplane away from stalled flight in each one of these cases.

Pilots who are uncomfortable with stalls often believe they are safe as long as they fly faster than Vso. The Va diagram proves otherwise. Also, few stall/spin accidents are a result of +1.0 G flight at Vso. These accidents usually occur while maneuvering close to the ground, under more than +1.0 G, at airspeeds above Vso, in uncoordinated flight. We must be aware of the bigger picture portrayed by the V-G diagram, even though our experience may be limited to one special case.

Whenever flying in the traffic pattern, be alert to the vital signs ear-marking an increase in angle of attack: a decreasing airspeed trend, or an increasing G-load trend, or decreasing speed combined with increasing G's; the elevator control moving further aft; and changes in noise levels, control pressures, and the direction in which the nose moves relative to you.

The clues are there, if you make a conscious effort to look for them. Try not to become preoccupied with the ground or extraneous duties during critical flight operations. Fly the airplane first, and stay coordinated! Otherwise, as we'll see next month, any excess yaw could be the catalyst that turns an inadvertent stall into a spin.