Flying behind the curve

The control wheel is pulled aft, and the nose drops sharply. But this is not the way a stall/spin accident typically occurs. The genesis is often much more subtle, the result of a pilot inadvertently getting behind the power curve where flight characteristics are not what he might expect. This explains why many of those fortunate enough to have survived such accidents do not realize they had actually stalled and could have possibly spun into the ground.

Although most pilots have heard of a power curve, many do not understand what it is and how it can lead to an inadvertent stall. Perhaps the best way to understand flight behind the power curve is to go there.

While maintaining a constant altitude, gradually reduce power until slowing to that indicated airspeed at which as little power as possible is needed to continue maintaining altitude. This is called the endurance speed and is used when a pilot needs to remain airborne—or loiter—for as long as possible. (Endurance speed is approximately the same as an airplane’s minimum-sink speed, the airspeed that results in the minimum rate of descent during a glide. It is slower than the best glide speed and is used to remain airborne as long as possible.)

Next, trim the airplane and adjust power to maintain the endurance speed. Then gently push on the control wheel for a few seconds. As expected, airspeed will increase, and altitude will decrease. After releasing this forward pressure, the nose will rise slightly, and airspeed will decrease toward the original trimmed speed, the result of longitudinal stability.

Once the airplane is again stabilized at its endurance speed and at a constant altitude, pull slightly aft on the control wheel, and maintain back-pressure for a few seconds. As expected, airspeed will decay, and altitude will momentarily increase—but not for long. After a loss of, say, 10 knots, use the elevator to maintain that slower airspeed without changing power. What will happen to the climb rate? If you’ve done this properly, there won’t be any. Instead, the airplane will begin to sink even though the nose is being held relatively high.

This descent rate is the result of the significant increase in induced drag associated with an increased angle of attack. Recall that the endurance speed is the only airspeed at which minimum power is required. It stands to reason, therefore, that any other speed, even a slower one, will require more power to maintain altitude. In other words, when below the endurance speed, additional power will be required; the airplane will descend unless power is increased. The airplane is said to be behind the power curve. The more the nose is raised in a misguided attempt to arrest the sink rate, the greater becomes the rate of descent.

The power curve can be visualized as a U-shaped curve with an unusually wide opening at the top. The right (or front) side of this curve represents normal flight. This is where—as expected—increasingly more airspeed requires increasingly more power, and a decrease in airspeed results in a climb (as long as power is not changed).Although most pilots have heard of a power curve, many do not understand what it is and how it can lead to a stall.The left side of the curve is the backside. This is where increasingly more power is needed to fly increasingly more slowly. It is possible to fly so slowly (before stalling) that insufficient power is available to maintain altitude. When this occurs, the pilot has no option but to lower the nose to increase airspeed and accelerate away from the backside of the power curve.

Just consider that endurance speed defines the bottom of the power curve and separates the front side from the backside. It can be difficult to grasp how things can be so bass-ackward on the backside of the curve, but they are. This is why the backside is formally known as the region of reversed command.

Do you wonder what this has to do with inadvertent stall/spin accidents? It has plenty to do with it. Operating inadvertently behind the power curve is a prelude to many such tragedies.

Noting a developing sink rate at low airspeed—while possibly with full throttle—the unsuspecting pilot reacts instinctively by pulling back on the wheel. This can be forgiven on the front side of the power curve, but not on the backside. The more the pilot pulls, the more he goes down.

Next comes the stall and possibly a developing spin, but only if the Earth has not yet risen sufficiently to interrupt the developing sink rate.

www.BarrySchiff.com