“Energy management” is not a precise term. It is used in different ways to express different things, so it almost always requires clarification. Often, a speaker will say “energy management” and expect the listener to understand without any further explanation.
Aviation safety is too important to tolerate vague phrases like “energy management” that facilitate misunderstandings.
The Big Ambiguities
Energy management often talks about kinetic and potential energy as the two kinds of energy. But:
- Kinetic energy is defined as mass times ½ speed squared. We can simplify by talking about weight and not mass, but what kind of speed? Indicated airspeed, true airspeed, or groundspeed? It matters. Airplanes flying through the air have kinetic energy measured with true airspeed but, upon ground contact, dissipate kinetic energy measured with groundspeed.
- Using that mass/weight simplification, we can define potential energy as weight times height. But height above what? If it is height above ground, properly called absolute altitude, then potential energy will increase and decrease rapidly when flying over buildings and streets in a city. Or if the reference is mean sea level, then an airplane at 5,500 feet will have the same potential energy flying high above low-lying Florida as it will on short final approach to Denver.
- And a complete discussion gets involved: drag dissipates airspeed, thrust can offset that dissipation or can increase speed or altitude, and rotary wing aircraft store energy in the rotation of the rotor.
So if the objective of using “energy management” is to communicate concepts accurately and precisely to help pilots fly their airplanes, details must be clarified.
…and we’re not talking about sailplanes.
Here are key ideas that a full discussion, much too long for this article, of “energy management” would bring out:
- “Energy management” sometimes just means airspeed, as in “energy to flare” or “enough energy for an aerobatic maneuver.”
- In aerobatics, “energy management” can also mean minimizing speed loss by not pulling too hard on the stick, pulling too many Gs, and creating excessive induced drag.
- “Energy management” can, however, be meaningful when distance to and altitude above a fixed point are part of the discussion, such as Bob Hoover’s engine out performances in the Shrike Commander, a sailplane on final glide, or descending to a landing. But that distance to a reference point is normally implied, not stated. Even in this case, “energy management” is not self-explanatory but has to be expressed in terms of airspeed and altitude.
Airplane Flying Handbook (FAA-H-8083-3C)
Can an official FAA publication help understand the concept, or is it mired in trendiness? This publication from the year 2020 contains the Chapter 4, “Energy Management: Mastering Altitude and Airspeed Control.” As an FAA document, with the title implying that the handbook is about flying and hence for pilots, you’d expect it to be about flying, and to be accurate, relevant, and easy to read. It’s not. It is theoretical, full of abstractions, charts and diagrams that complicate things far beyond easy understanding.
How bad is it? All these terms appear in Chapter 4 to explain “energy management:” Energy Balance Equation, Energy Error, Energy Distribution Error, Total Energy Error, Energy Exchange, Energy Height or Total Specific Energy (ES), and Energy System. Maybe it’s just me, but I’ve gone a half century as a pilot and as an engineer without encountering any of these terms.
Does the following statement help pilots fly the airplane, or understand the process?
“Energy management can be defined as the process of planning, monitoring, and controlling altitude and airspeed targets in relation to the airplane’s energy state…”
As if to prove my point, that sentence had to revert to airspeed and altitude for explanation.
Then there’s the phrase, “The elevator is the energy distribution controller,” an abstraction that requires considerable explanation. Or the “energy map” (Figure 4-7) showing specific excess power, another abstraction. What pilot thinks in terms of “energy map” when flying?
When I was working on my Ph.D. at MIT, my thesis advisor, an MIT Ph.D. himself, a retired navy Captain, fighter pilot, and CFII, told me that for teaching ground school, even to MIT students, not all of whom were techies, to speak their language. Word for word, he said to me, “The wing is the thing.”
A Significant Error in the Airplane Flying Handbook (FAA-H-8083-3C)
My expectation was that with all its abstractions, The Airplane Flying Handbook would have its facts straight. Not so. The handbook states that kinetic energy is associated with indicated airspeed. This is flat out wrong on two counts:
- Indicated air speed is what is shown on the airspeed indicator. It does not take into consideration any errors in the pitot/static system or in the instrument. Calibrated airspeed is the correct term for indicated airspeed with errors removed.
- But that’s not right, either. The kinetic energy of an airplane relative to the air is derived from true airspeed, the actual speed of the airplane through the air.
For example, jets at altitude will have an indicated airspeed about half of the true airspeed. A jet cruising at 480 knots has 480 knots worth of true airspeed-referenced kinetic energy, not 240 knots indicated worth.
However, at low altitude, the indicated and true airspeed are often close to each other, so the indicated/true airspeed error is not always significant.
The Meat of the Issue
- Pilots fly airplanes with respect to airspeed and altitude, the quantities used in Airplane Flight Manuals. AFMs do not specify aircraft performance with respect to energy. Neither do speed and altitude operating restrictions, nor do airspace restrictions nor ATC instructions reference energy management.
- Concepts are most useful when they apply directly to displayed information and available controls. Unnecessary abstraction requires translation from instrument readings to concept to decision making and back to control inputs. These translations are extra workload and decrease the pilot’s ability to handle other tasks, such as maintaining situational awareness.
- While an airplane in flight possesses both kinetic and potential energy, there are no cockpit instruments which directly measure either one and display those measurements in units of energy.
- In the cockpit, the stick/control wheel moves the elevators, and the throttle adjusts engine power. While both are used to affect airspeed and altitude, neither one directly controls only kinetic or only potential energy.
- There is no cockpit guidance on how much altitude rate will give how much airspeed change, or vice versa. Rather, this “energy management” tradeoff is done ad hoc.
- Old fashioned explanations are simpler, more direct ways to teach pilots what they need to know without getting as abstract as energy management. In other words, the pilot is the customer, so speak the pilot’s language, not the dialect of the instructor. (The point of this article is clear expression of concepts, not a discussion of flight mechanics, so the many varieties of airspeed/altitude/throttle explanations will not be discussed here.)
The bottom line is that energy management is an abstraction that is not operationally necessary for flying an airplane. It can be useful to help some people understand concepts, but it is more likely a stumbling block.
Video (satire): Flying with Kinetic and Potential Energy
Here’s a trip around the pattern, recorded live, referring only to kinetic and potential energy, not airspeed and altitude. Yes, it can be done, and technically, it makes sense, but is it helpful? Note that no direct quantitative energy measurements were available or stated, and that airspeed and altitude were the primary information sources when flying the pattern but were never acknowledged.
The Last Word, from Gilbert and Sullivan
In the much loved and still regularly performed 1885 comic opera, The Mikado, self-absorbed Pooh-Bah presciently describes the misuse of “energy management” when he says, “Merely corroborative detail designed to give artistic verisimilitude to an otherwise bald and unconvincing narrative.”
In other words, say what you mean! Don’t hide important thoughts about airspeed and altitude behind fancy phrases like “energy management” and expect meaningful communication to result.