Occasionally, they pose questions about the science of flight and I’m in my happy place when I get to dive in, research, and present an answer. Recently AOPA member Joel Neuman wrote to share a conversation among flight instructors regarding flight in the region of reverse command when pitch controls airspeed and power controls altitude.
Neuman took the position that “on final approach, especially on short final, most training airplanes are flying in the region of reverse command.” Other flight instructors, however, argued that “you weren’t quite there yet.” The question Neuman posed to me is, “Where is there, the transition from normal command to reverse command?” In slowing from cruise flight to approach and landing, I believe a pilot is there well before entering the airport traffic pattern and I’ll explain.
The law of conservation of energy implies that the total energy in a closed system remains constant. For an airplane those forms of energy are potential (altitude), kinetic (proportional to the square of velocity), chemical (fuel), and that energy lost into the airmass (think drag). While glider pilots make expert use of an external energy source in the form of updrafts from thermals, we’ll consider an aircraft that flies in still air so that conservation of energy applies to this system.
Throughout flight, the fuel supply consistently decreases (assuming no in-flight refueling) and the airmass energy increases as drag opposes the aircraft motion. If the aircraft gains one form of energy, such as altitude or airspeed, conservation of energy implies that another form must be sacrificed. For example, flight at constant altitude and airspeed requires that the rate at which fuel is burned exactly compensates for the rate at which energy is lost because of drag. For this discussion, we’ll keep the flaps up, cowl flaps open, propeller on a constant rpm setting, the ailerons neutral and rudder deflections consistent with coordinated flight. The remaining variables will be throttle setting and elevator deflection using the yoke. The pilot can use the trim mechanism to reduce the workload in holding the yoke in a fixed position.
For a fixed throttle setting, the pilot applies pressure to the yoke to set an elevator deflection that causes the wing to fly at a specific angle of attack. That angle of attack then determines the airspeed at which the aircraft flies as shown on the first image. The higher the angle of attack, the lower the airspeed. (Beyond the stall angle of attack, the relationship is more complicated and those angles of attack are not represented in the figure.) The best rate of climb airspeed for this throttle setting, VY, is that which corresponds to the greatest vertical speed and any other airspeed will result in a lower vertical speed. The front side of the power curve corresponds to airspeeds greater than VY and the back side of the power curve to those less than VY.
The throttle controls the rate at which chemical energy is depleted and the pilot can choose how it’s converted to other forms of energy. Assuming the airspeed is held constant, increasing the throttle will always increase the vertical speed. The figure above shows the power curve for three different fixed throttle settings: 26 in manifold pressure, 18 in manifold pressure, and idle.
Front side of power curve
Consider an aircraft that is in cruise flight at 4 degrees angle of attack as in the first graph. In this case, both power and airspeed are constant so the rate at which fuel is burned exactly compensates for the rate at which energy lost to drag. Now suppose that a vertical gust thrusts the aircraft up to a higher altitude. The pilot can then reduce the throttle slightly and the aircraft will descend as shown above. As the aircraft approaches the assigned altitude, the original power setting can be resumed. By timing the power application well, airspeed can remain constant through the maneuver. Negative altitude excursions can be handled analogously by briefly adding power.
Alternatively, such altitude excursions can be handled solely by using the yoke. If a vertical gust causes an increase in altitude, the graph on the first image shows that pushing forward on the yoke will cause the aircraft to lose altitude. When the desired altitude nears, the pilot can introduce back-pressure on the yoke to return to the original angle of attack. During the correction, the airspeed will increase and then settle down to the original airspeed corresponding to the 4-degree angle of attack. This is the way an autopilot handles such altitude excursions.
Back side of the power curve
Now consider an aircraft in level slow flight, as in the first graph, where the wing flies at 12 degrees angle of attack. If a vertical gust moves the aircraft to a higher altitude, decreased power will allow the aircraft to descend back to the desired altitude, just as it does on the front side of the power curve. Increased power can similarly correct for negative deviations in altitude.
Now suppose that a gust increases the aircraft’s altitude and the pilot corrects only using the yoke. Unlike on the front side of the power curve, he needs to pull back on the yoke control (or push uncomfortably forward) for the aircraft to descend back to the desired altitude. If the angle of attack is already near the critical angle of attack, it would be easy for the wing to stall. In this case, losing altitude isn’t a problem but we are looking for a technique that inspires confidence in passengers.
The abundance of time normally spent in cruise flight can lead to sloppy habits and lull pilots into the belief that the yoke controls altitude (or climb/descent profile) and throttle controls airspeed. After all, leveling off at cruise altitude is typically done by holding the yoke to maintain the desired altitude while the airspeed stabilizes. If a faster cruise speed is desired, the pilot applies more power while continuing to hold the yoke pressure necessary to keep constant altitude.
In his classic text Stick and Rudder, Wolfgang Langewiesche explains that the “elevator doesn’t elevate” so he refers to those control surfaces as the flippers. He emphasizes that in any flight regime, the throttle controls the vertical speed and the flippers control the angle of attack that results in a given airspeed. Higher airspeeds mean that a pilot can get away with yoke controlling vertical speed and throttle controlling airspeed. Perhaps it’s the front side of the power curve that should be labeled the region of reversed command. I’m not out to buck that long-standing tradition but it’s an interesting way of thinking about things that is hardly new.
Let’s return to the original question regarding where along the airspeed spectrum should yoke control airspeed and throttle control vertical speed? In other words, when are we there? Above VY a pilot can get away with either technique or a combination. But even at VY control deflections either way cause the vertical speed to decrease. I teach my students that, at an airspeed used for cruise climb or ones used to descend toward the pattern, both above VY, it makes sense to hold a fixed airspeed using the flippers and let the throttle govern the vertical speed. For me, there is well on the front side of the power curve.