CFI to CFI
The persistent stall/spin
How we can stop these accidents
The stall/spin accident has been around for more than 100 years now, yet pilots continue to ignore lessons from the past.
The two most common stall/spin combinations occur during the two most critical phases of flight: the skidded base-to-final turn before landing, and the engine failure on takeoff and ill-advised attempt to return to the runway. Let's analyze the aerodynamics of these two scenarios.
Pilots wrongly associate stalls with low airspeed and recoveries with lowering the nose well below the horizon. In reality, stalls can take place at any airspeed and any attitude, and they always result from exceeding an aircraft's critical angle of attack (AOA). Pilots preparing to fly an unfamiliar airplane almost always ask, "What's the stall speed?" My usual retort is a shrug. "I don't know; what are we doing?" Too many look at the numbers in the flight manual or colors on an airspeed indicator and think, If I keep it above this, all will be well. Nothing could be further from the truth.
Consider VSO, the stall speed in the landing configuration. What exactly does that mean? It means there is a whole list of parameters that must be met for that stall speed to be accurate: power off; flaps and gear down; max gross weight; most forward center of gravity; one-G environment.
Now ask yourself, "Does this really represent my flight regime?" Of course not!
In looking at the base-to-final spin scenario and the take off/engine failure stall/spin accident, you'll see that both exhibit very similar human errors and aerodynamic misunderstandings. The good news is that each is easily correctable.
For many, looking at the equation for lift stirs nightmares. Suffice to say that as the velocity decreases, the lift coefficient--the other pilot-controlled variable--must increase to keep lift equal to weight. Airspeed bleeds off as result.
With the flaps down and the power reduced for landing, the airplane is operating under the parameters that define VSO. But turns increase the load factor and increase the weight an airplane's wings must support. Decreasing velocity must be accompanied by higher AOA.
An airplane on base, trimmed, flaps down, but overshooting the extended runway centerline is set up for an inadvertent stall/spin. The pilot is tempted to "cheat" by using excessive inside rudder to slide the nose toward the centerline. By coercing that nose around with rudder, the pilot accelerates the wing on the outside of the turn and increases lift. At the same time, the inside wing loses lift, and bank angle increases. To counter the steepening bank, the pilot applies opposite aileron, and that increases the drag on the inside wing even more. The nose begins to fall, so the pilot adds back-pressure and is in danger of exceeding the critical AOA.
Instructors in the emergency maneuvers training program at CP Aviation in Santa Paula, California, demonstrate this scenario at safe altitudes. It never fails to shock both low- and high-time pilots at how innocent things appear until the stall and spin occur. The spin begins with the nose well below the horizon, and the high AOA on the low (inside) wing puts the airplane in a position where the stall on the low wing "snaps" the airplane onto its back with a steep, nose-down attitude. Altitude loss is exceptionally rapid. Aerodynamically, when a wing is established in a turn and the pilot applies opposite aileron to level the wings, the AOA on the low wing increases. Raising the nose exacerbates the situation by increasing pitch and AOA. What recourse does the wing have but to say, I have had enough?
In reality, getting to this point inadvertently requires a series of mistakes and uncoordinated actions. Flying this way "feels" bad (see "Become One with the Airplane," p. 26). If it feels bad, why do it? Not only are you forcing the airplane into an uncomfortable position, but just by releasing it, the plane will often simply fix itself. What would that cost? A go-around?
Now let's look at the other scenario. With engine failure on takeoff, that wide strip of runway behind sings a siren song to pilots. But pilots must discipline themselves to ignore it.
Imagine a takeoff, full power perhaps at VY climb attitude, and the stillness of the air resounds as the engine fails; the urge to return to the runway can be overwhelming. But consider the aerodynamics. The angle that we use to climb at VY is very close to VBG (best glide), better known as L/D max, but still slightly steeper. Why? Because power is involved. When the powerplant quits, pilots must trade altitude for energy (airspeed).
The pilot on departure is climbing out when the engine coughs and the tach drops off. The pilot knows the runway is just a scant half-mile away and starts to turn around. Airspeed declines rapidly, induced drag caused by the higher AOA causes the airplane to sink more quickly, and the pilot adds rudder and increases the back-pressure in a futile attempt to tighten the turn while reducing the rate of descent. Sound familiar? High AOA on the low inside wing. Excessive yaw and roll coupled in the same direction: a recipe for disaster. Recognizing the aerodynamic forces at play can help pilots avoid accidents. Flying an airplane by rote numbers may make some pilots feel more comfortable, but pilots should have the flexibility to adapt to an ever-changing environment.
Patrick Dugan is an instructor with CP Aviation in Santa Paula, California. He specializes in emergency maneuvers training, aerobatics, and tailwheel instruction.
By Patrick Dugan