Features

November 2016 Issue




Energy Management Basics

It’s about establishing the right power setting, airspeed and pitch angle to obtain desired performance.

Editor’s note: This is the first of a two-part series on flight energy management, describing what it is and how we can use it to enhance safety. Look for the second part in December’s issue.

Herbert Raab/Vesta—Creative Commons

In my experience as a flight instructor, many civilian-trained pilots have little to no understanding of energy management (EM) concepts. I often find myself advising pilots to maintain their energy, particularly in the traffic pattern. That’s because a classic accident sequence involving a pilot’s failure to manage his or her energy works like this: flying too slowly and/or too low on final approach and attempting to arrest the descent by increasing pitch alone. Instead, a well-founded understanding of energy management mandates increasing power to add energy to the aircraft to prevent airspeed decay and a possible stall.

But too many responses to my encouragements about energy management take the form of “What are you talking about?” That’s unfortunate, because managing energy is one of the keys to establishing and maintaining a safe flight path. With all that in mind, let’s take a look at what we mean when we talk about energy management.

Definitions

Energy can be defined as the capacity for performing work, where work equals force times distance. Stated in aviation terms, work is needed to move an aircraft horizontally over a designated distance and vertically to a target altitude. To accomplish this work, two basic forms of flight energy are needed: kinetic energy (EKIN)—represented as airspeed—and potential energy, EPOT, which is one way we should think of altitude. Engine power/thrust is the initial energy source of EKIN and EPOT, and determines the continuous maneuvering capabilities of an aircraft.

Energy increases as we add power and decreases by reducing power and/or increasing drag, as described in Table 1, on the opposite page. At the same time, an important concept is that an aircraft may be considered an “energy system,” with pilot-controlled EM parameters as described in Table 2. An aircraft’s “energy instruments” may be thought of as the airspeed indicator for EKIN, altimeter for EPOT, vertical speed indicator (VSI) for the rates of accumulation and depletion of EPOT, manifold pressure/tachometer for engine power, and fuel gauges for the amount of chemical energy available for conversion to power by the engine.

For purposes of operating a powered aircraft, we may define EM as using flight and power controls—as measured by the energy instruments—to establish and maintain an appropriate and safe energy state for all phases of flight.

Energy Conservation

The Law of Energy Conservation, a basic tenet of physics, states that energy can neither be created nor destroyed. Instead, it changes from one form into other forms. The extent to which its form changes tells us how much of an aircraft’s total energy is EPOT and EKIN for a given flight condition and time. It explains how an aircraft can convert EPOT to EKIN, or vice versa. During such a conversion, one form of energy decreases while the other increases proportionately.

A classic example is the January 2009 so-called Miracle on the Hudson, US Airways Flight 1549, during which Capt. Chesley (Sully) Sullenberger and First Officer Matt Stiles put on a well-publicized, dramatic display of EM. They converted their EPOT of approximately 3000 feet of altitude to EKIN after bird ingestion caused failure of both engines powering their Airbus A320.

Their altitude at the time of engine failure represented a limited amount of EPOT available for conversion to EKIN. They accomplished that conversion by regulating angle of attack (AoA) and pitching the airplane appropriately. They converted EPOT to EKIN at the proper rate, i.e., they flew at the airplane’s best-glide speed (VG) while descending and maneuvering for an off-airport emergency landing, maintaining appropriate final approach and touchdown airspeeds throughout.

(The Airbus A320 family’s automation provides its crew with a unique airspeed that maximizes the glide ratio—distance an aircraft glides per unit of altitude loss in wings-level flight—in a given aircraft configuration. This airspeed is called the “green dot” airspeed, indicated by a small green circle on the airspeed indicator tape and depends on the aircraft being in a “clean” configuration, i.e., flaps, slats and landing gear fully retracted. At altitudes below 10,000 feet, the green dot airspeed primarily depends on aircraft weight, and typically ranges from 200 to 235 KIAS).

The tables below define and describe the various forms of energy with which pilots should be familiar. According to Airbus, “the level of energy of an aircraft is a function of the following primary flight parameters and of their rate of change (trend):

• Airspeed and speed trend;
• Altitude and vertical speed (or flight path angle);
• Aircraft configuration (i.e., drag caused by speed brakes, slats/flaps and/or landing gear); and,
• Thrust level.

One of the tasks of the pilot is to control and monitor the energy level of the aircraft (using all available cues) in order to:

• Maintain the aircraft at the appropriate energy level throughout the flight phase:
— Keep flight path, speed, thrust and configuration; or,
• Recover the aircraft from a low-energy or high-energy situation, i.e., from:
— Being too slow and/or too low; or,
— Being too fast and/or too high.

Controlling the aircraft energy level consists in continuously controlling each parameter: airspeed, thrust, configuration and flight path, and in transiently trading one parameter for another.”

Energy Mismanagement

The door swings both ways, of course, and available energy can be mismanaged. In an all-engines-out scenario, mismanagement occurs when EPOT is converted to EKIN at an insufficient rate: Airspeed is reduced to less than VG as the pilot inappropriately increases AoA and pitches up too much, increasing induced drag. Doing so is an energy-depletion maneuver, which we might employ when the airplane is above its landing gear extension speed and we need to get the gear down. In extreme cases, energy mismanagement results in a precipitous decrease in EKIN, leading to exceeding critical AoA, stalling and loss of control.

Energy mismanagement also occurs when EPOT is converted to EKIN at too fast a rate. This can happen when a pilot inappropriately pitches down too much, resulting in a rapid decrease in altitude and increase in airspeed greater than VG. In this situation, limited and valuable EPOT is needlessly wasted at too rapid a rate, precious time to look for a desirable landing site is unnecessarily wasted and landing with too much EKIN (i.e., too hot on final) predisposes to losing control on the runway, or beyond it.

From these two examples, it should be clear that efficiently managing the flight energy we’ve accumulated depends on accurately establishing and maintaining an appropriate power setting and pitch angle. But what if efficient energy management isn’t our primary consideration? What if we need to abruptly change our energy state, right now, in our normal operations? How could such a need arise, and how would we go about it?

aircraft energy management strategy

An example of an opposite EM strategy—converting EKIN to EPOT—arises when flying a chandelle, a maneuver familiar to commercial pilots and flight instructors. The maneuver was named by French aviators in WWI who described it as monter en chandelle (to climb vertically).

Starting at design maneuvering speed (VA) or slower and at an appropriate entry altitude, the maneuver consists of a minimum-radius climbing turn through a 180-degree heading change, ending at an airspeed near the airplane’s power-on stalling speed (VS1) and at a higher altitude. Energy is managed by appropriately increasing AoA and pitching the airplane throughout the maneuver so EKIN decreases continuously and gradually while simultaneously EPOT increases continuously and gradually.

As an energy-management exercise, it’s easy to mismanage the chandelle, by increasing AoA excessively, for example, causing EKIN to be converted to EPOT at too rapid a rate during the climb. Airspeed erodes rapidly to VS1 and critical AoA can be exceeded, leading to a power-on stall.

Management Strategies

As an example of an efficient management of energy versus an inefficient one, let’s concentrate on a short-field approach and landing for a moment, specifically in a Cessna 172S Skyhawk SP. The Pilots Operating Handbook (POH) says: “For a short field landing in smooth air conditions, approach at 61 KIAS with full flaps using enough power to control the glidepath. After all approach obstacles are cleared, smoothly reduce power and hold the approach speed by lowering the nose of the airplane.”

In the described procedure, AoA is manipulated to convert EPOT, combined with appropriate power, to maintain EKIN at 61 KIAS while flying a stabilized approach. If AoA is increased too much, induced drag increases inappropriately, resulting in a precipitous decrease in airspeed/EKIN (energy depletion maneuver) and, thus, a decrease in lift. The airplane will drop below the glidepath.

In this low energy state, the only way to recover and recapture the desired glidepath is to add power to increase airspeed/EKIN, which then acts to increase lift and restore altitude/EPOT. The pilot must coordinate changes in AoA with appropriate changes in power to maintain airspeed/EKIN on the glidepath. When nearing the desired touchdown point, it’s imperative to properly combine AoA, airspeed/EKIN and power to maintain an appropriate energy state. Touchdown should occur at minimum controllable airspeed/EKIN (i.e., any further increase in AoA or wing loading, or decrease in engine power output, results in a stall) with the airplane in a pitch attitude resulting in a power-off stall when the throttle is closed.

Air combat maneuvering (ACM) employed by military fighter pilots includes specific maneuvers and proper combinations of power, EKIN, EPOT, and drag to gain a tactical advantage and outmaneuver and attain a position from which to attack an opponent. Proper ACM relies on sound energy management techniques—e.g., diving to convert EPOT into EKIN (boom) and then climbing away to convert EKIN into EPOT (zoom). An airplane with considerable energy is maneuverable; an airplane with insufficient energy for its mass is less maneuverable and is “easy prey.”

One example of less-efficient ways to manage energy is the chandelle, summarized above. Another is the forward slip, as may be employed when flying that very same short-field approach and landing. This time, though, we want to deplete our EPOT/altitude by increasing the rate of descent and steepening the glidepath without increasing EKIN/airspeed. Of course, we want to do all this while tracking along the runway’s extended centerline. A forward slip is desired in this instance, in which the airplane is flying sideways because the longitudinal axis of the airplane is yawed at an angle to the flightpath. The rate of turn is too little for the bank angle, accomplished with opposite control inputs.

This results in the relative wind striking the side of the fuselage and lowered wing, and a marked increase in drag. It’s a textbook energy depletion maneuver, and explains how an airplane in a slip can descend rapidly without increasing EKIN/airspeed. Energy is managed by regulating AoA to appropriately pitch the airplane to convert EPOT to EKIN to maintain desired airspeed (for example, 1.3 VSO), bank angle to control the rate of descent, and rudder to maintain heading and yaw the airplane’s longitudinal axis.

Energy management concepts are interwoven in aerobatic flight, soaring and air combat (above). For example, at the beginning/bottom of flying an inside loop EKIN/airspeed is highest and EPOT/altitude is lowest. By appropriately increasing AoA and pulling up into the loop, EKIN decreases gradually while simultaneously EPOT increases gradually. EKIN at the bottom of the loop is converted to EPOT at the top of the loop. As the airplane “floats” over the top, EKIN is lowest and EPOT is highest. As the airplane completes the second half of the loop, EKIN increases gradually and EPOT decreases gradually back to their original values. The EPOT gained at the top of the loop is reconverted back to EKIN at the bottom.

Concepts and Training

A foundational understanding of energy management really hasn’t been sufficiently emphasized in civilian pilot training programs in the United States. Fortunately, this is about to change. The new private pilot airman certification standards (ACS) that went into effect last summer require applicants to demonstrate EM understanding in 10 areas of normal flight operations, including short- and soft-fields, forward slips to a landing; go-arounds/rejected landings; steep turns; and emergency descents, approaches and landings.

The EM-centered approach to flying I’ve tried to present here helps pilots focus on the fact we’re all really energy managers when it comes to flight, who monitor and manage the aircraft’s energy state to obtain desired performance. If some of these concepts prove difficult, EM training from an experienced flight instructor can help.

In my view, EM is one of a pilot’s most important tasks, from takeoff to landing, because without energy, a pilot has nothing. Next month in this space we’ll explore what I mean, as we tackle EM’s relationship to loss-of-control accidents.

Dr. Banner is a flight instructor at University Air Center, Gainesville Fla., a professor at the University of Florida, and an instructor pilot in the U. S. Air Force Auxiliary/Civil Air Patrol. A CFII and MEI, he has some 5000 hours of flight time and owns a GCBC Citabria.