THE REGIONS OF NORMAL AND REVERSE COMMAND
Velocities above maximum endurance are referred to as the region of normal command.
The region of normal command is characterized by airspeed stability. Assume an airplane is in equilibrium at point B on the attached figured. A decrease in airspeed (for whatever reason) results in a thrust or power excess that will eventually accelerate the airplane back to the original airspeed at point B. An increase in airspeed from point B (for whatever reason) results in a thrust or power deficit that slows the airplane back to the original airspeed.
In the region of normal command, velocity and throttle setting for level flight are directly related. To fly in equilibrium at a faster airspeed, more TA/PA is needed than at a slower airspeed. To fly slower, less TA/PA is needed.
Velocities below maximum endurance are referred to as the region of reverse command. The region of reverse command is characterized by airspeed instability. Assume an airplane is in equilibrium at point A in the second attached figure. A decrease in airspeed (for whatever reason) results in a thrust or power deficit that will eventually slow the airplane to the point of stalling (assuming a level flight attitude is being maintained). An increase in airspeed (for whatever reason) from Point A results in a thrust or power excess that accelerates the airplane away from point A. The airplane eventually reaching equilibrium at point B.
In the region of reverse command, velocity and throttle setting for level flight are inversely related. Once stabilized at a faster airspeed in equilibrium flight, TA/PA will be lower than when stabilized at a slower airspeed. Simply stated, the slower an airplane flies in the region of reverse command, the more thrust and power is needed.
A complete knowledge of this flight region is particularly important because most aviation accidents occur while operating in the region of reverse command. Whenever an airplane is taking off or landing, it is flying in or near this region. A very dangerous situation for an inexperienced pilot is trying to slow down in the region of reverse command. If the pilot increases back pressure to increase angle of attack and decrease velocity, this will causes thrust and power required to increase, creating a deficit. Once the airspeed bleeds off, the deficit causes the airplane to descend. The inexperienced pilot tends to pull back on the control stick in order to keep from descending. This causes the airplane to move further into the region of reverse command, creating a greater deficit. Eventually the deficit will be so great that even full throttle is not able to overcome it. Since this usually occurs during landing, there is not enough altitude to recover. This is the origin of the phrase “behind the power curve.” An experienced pilot knows that in order to maintain level flight as an airplane slows down in the region of reverse command, the throttle must be increased. Increasing angle of attack will only aggravate the situation.
