Pitch and power

The late great Bob Hoover demonstrated mastery of flight as he gracefully flew stock aircraft, often with engines shut down and no power to assist. While the rest of us may never learn to fly like Hoover, we can aspire to understand energy management as he did.

As a designated examiner, many of the issues I see on practical exams emanate from poor energy management and a fundamental lack of understanding of proper corrective measures. It’s a topic that receives little to no attention in many ground school courses.

Two years ago, the FAA updated the Airplane Flying Handbook (FAA-H-8083-3C) to include a new chapter on energy management. It’s been a long time in coming; after all, the topic constitutes a significant part of Wolfgang Langewiesche’s 1944 classic Stick and Rudder. But better late than never, and I’m happy to see an entire chapter devoted to such an important subject.

I confess that I’ve never found FAA manuals to be page-turners, so diving into the new edition of the Airplane Flying Handbook took a backseat to other tasks—like cleaning my house gutters or dusting under the fridge—which seemed more compelling at the time. But earlier this summer, I found myself on a commercial airliner headed to Europe and piloted by a captain with a strong commitment to keeping the seatbelt sign illuminated. Desperate for some diversion, I opened the copy of the Airplane Flying Handbook I keep on my iPad and set to work. The exposition is a bit verbose and could benefit from additional editing, but learning to visualize an airplane as an energy system is important for every pilot.

To describe the system, consider an aircraft contained inside a region of air particles that surrounds its flight. The aircraft’s kinetic energy (mv²/2 where m is the mass) is proportional to the square of its velocity, so the airspeed indicator is the best way to measure this energy. Its potential energy (wh where w is the weight and h is the altitude above a fixed plane) is proportional to altitude and is measured on the altimeter. Kinetic energy plus potential energy constitutes the aircraft’s mechanical energy.

The aircraft’s engine uses fuel to provide power (chemical energy per unit of time). Drag (induced and parasitic, for example) stirs and heats the air and releases energy in the aircraft’s wake. The net energy (chemical minus drag) can be used to increase airspeed, altitude, or a combination of the two by adjusting the elevator control. The conservation of energy law implies that the total energy of the system remains constant even though one form might be traded for another. The pilot controls the total energy with the throttle and distributes it using the elevator control to meet airspeed and altitude goals.

In constant-speed cruise flight, the kinetic and potential energies remain constant, and the energy produced by the engine matches that energy released into the atmosphere because of drag. While mechanical energy remains constant, the pilot can use the elevator control to exchange potential energy for kinetic energy. Langewiesche calls this the “law of the roller coaster.” Pushing forward on the yoke increases the airspeed and descent rate and pulling back on the yoke has the opposite effect.

The collection of parallel curves in the figure below describes the roller coaster effect. For example, the pilot flying at 150 knots at 4,000 feet can push the yoke forward and achieve 250 knots around 2,000 feet with no change in mechanical energy (A to B). Moving from one point to another along one of these curves can be accomplished using only the elevator.

Moving across the lines, however, means moving to a different energy state and necessitates a change in throttle setting (A to C). Given the limitations of the powerplant and lift production, only the combinations of airspeeds and altitudes inside the collection of power curves are reasonable for sustained flight. While it may be possible to move briefly outside the power envelope, sustained flight is impossible. The Airplane Flying Handbook calls this graph an energy control map.

Basic rules that follow from the energy control map are:

1. To move to a state with a different total mechanical energy value, start by adjusting the throttle and follow with necessary pitch changes.
2. If the total mechanical energy is appropriate, then initiate corrections by using the elevator control.

The Airplane Flying Handbook provides two scenarios that demonstrate the utility of an energy-centric approach to piloting an aircraft. Upon reaching an undesired energy profile, the pilot should decide whether the error is one of total energy which requires a throttle movement or merely one of energy distribution, which can be solved using the elevator.

Scenario 1: An airplane on an instrument approach descends below the desired vertical profile. If the target airspeed has been maintained, then the problem is one of total energy and the pilot should correct by increasing the throttle setting while maintaining constant airspeed. This advice is consistent with the notion that, for this phase of flight, the elevator controls airspeed and throttle controls altitude.

Scenario 2: An aircraft departs a high density-altitude airport, and the pilot is surprised by the attendant poor performance, so he verifies the gear and flaps are retracted and adjusts the mixture to optimize climb rate. With the aircraft pointed toward high terrain, the pilot continues to pull back on the yoke until the airplane stops climbing and the stall horn sounds. In this case, the pilot has guided the aircraft to an energy state outside the envelope (A to C in figure above). In this case, it will not be possible to move to a higher energy state without first pushing forward on the yoke, at the expense of some altitude, to reach an airspeed for which the climb rate is positive (C to D). This assumes, of course, that the pilot takes corrective action while such a trade is possible.

It should be noted that potential energy is considered with respect to a fixed plane and kinetic energy with respect to the air mass. The Airplane Flying Handbook continues, “In contrast, changes in agl-altitude and groundspeed are affected by external factors, such as varying elevation and wind, which the pilot cannot alter.” This statement is curious because the pilot certainly can affect these by choosing a route that involves or avoids high terrain and goes with or against the wind.

At the risk of extending an already long chapter, the Airplane Flying Handbook discussion of energy management would benefit by including terrain and air mass effects. Pilots should realize that flying over high terrain throws away potential energy and must accept a lower energy state until lower terrain is reached.

Additionally, motion of the air can alter an aircraft’s energy state. Updrafts are the equivalent of free potential energy—as glider pilots know well—and downdrafts have the opposite effect. Headwinds and tailwinds alter the kinetic energy when measured with respect to the ground. While we can’t orchestrate favorable winds for a given route, we can choose to depart and land into the wind with the lowest possible kinetic energy with respect to the ground. Landing into the wind is especially important if there is an engine failure over less-than-ideal terrain.

The energy management chapter in the Airplane Flying Handbook introduces valuable topics and opens the door for further thought. Certainly, no one will reach for this resource in an emergent situation or even as part of preflight planning. But a rainy day or long overseas flight can provide the perfect opportunity for pilots to learn and adopt an energy-centered mindset in piloting aircraft.

Catherine Cavagnaro owns Ace Aerobatic School in Sewanee, Tennessee, and is the Gaston Swindell Bruton professor of mathematics and computer science at Sewanee: The University of the South.