Since the total specific energy, ES, has the units of height (e.g., feet), it is usually called energy height. It also gets this name from the
fact that energy height is the maximum height that an airplane would reach from its current altitude, if it were to trade all its speed for
altitude. Figure 4-6 shows lines of constant total specific energy or energy height. Different positions of an airplane along a given
energy height line have the same total energy regardless of their location on the line (e.g., A and B).
Thus, even though the airplane in point A is cruising at 100 knots and 6,000 feet, it has the same total specific energy expressed in
height (6,500 feet) when cruising at 240 knots and 4,000 feet (B). This also means that the airplane in either position, A or B, would
be able to “zoom” to the same maximum altitude of 6,500 feet by trading all its speed for altitude. The lines of constant energy height
can be used as idealized trajectories to depict an airplane moving from one energy state to another solely through energy
exchange (e.g., A to B). If the airplane rapidly exchanges altitude and airspeed, it would follow along the energy height line while, in
the short term, maintaining constant total energy.
In addition to showing energy height lines, the energy map can also depict available specific excess power (P S) contours, as well as
energy trajectories of an airplane moving from one energy state to another. [Figure 4-7] The airplane can move along energy height
lines by simply exchanging energy (e.g., A to B). However, to move across energy height lines, the airplane needs to increase or
decrease total energy while distributing the energy change between altitude and airspeed. Thus, the ability of an airplane to go from
one energy height to another (e.g., from A to positions C, D, or E) is a function of specific excess power ( PS), measured in rate
of change in distance or height (e.g., feet per minute).
Examine the energy positions depicted in Figure 4-7. The airplane in position A is flying at 4,000 feet and 150 knots with a total
energy equivalent to 5,000 feet. Since positions C, D and E are located at higher energy heights (11,000, 9,500, and 6,500 f eet
respectively), the only way for the airplane to reach them from position A is by increasing its total energy (i.e., increasing
thrust above drag, or PS > 0). The reverse is also true. If the airplane is in position C, D or E, the only way for it to get back to
position A is by decreasing its total energy (i.e., decreasing thrust below drag, or PS < 0). In other words, the rate at which the
airplane can move from one energy height to another —e.g., how swiftly it can climb/descend at a steady speed, or accelerate/
decelerate in level flight —is a function of specific excess power, which can be positive (PS > 0) or negative (PS < 0) depending on
whether the airplane needs to move to an energy height that demands more or less total energy.
At the edge of the energy envelope, where available PS = 0 at full throttle, the airplane can no longer climb while maintaining
airspeed or accelerate without descending. Inside this envelope, inner contours increase in value, reaching a “peak” where available
PS is maximized. Notice that P S at full throttle is maximized at a specific airspeed (V Y) decreasing in value at slower or faster
airspeeds. At VY then, the airplane can attain the maximum rate of climb while maintaining airspeed or the maximum acceleration
without descending [Figure 4-7].
Figure 4-7 . Energy map depicting specific excess power (P S) contours (shown in feet per minute) and energy trajectories for a
hypothetical airplane.
Three Basic Rules of Energy Control
An “energy-control” map can help visualize the basic energy control rules. [Figure 4-8] The energy-control map depicts not only the
trajectories of an airplane transitioning from an arbitrary initial energy state (1) to other target states (2, 3, 4, 5, 6, and 7), but also the
changes in energy caused by the throttle (blue/red arrows) and the elevator (green arrows). In other words, it allows pilots to visualize
the basic control rules for moving an airplane from any state to another. The edge of the sustainable energy state envelope (where PS
= 0 at full throttle) is also illustrated.
Note that the line of constant total energy (dashed line) that divides the area in the map requiring more total energy (blue area) from
that which requires less energy (red area) is depicted relative to the arbitrary initial energy state (1). The throttle adds (blue arrow) or
subtracts (red arrow) the amount of total energy demanded by the new target energy state, while the elevator (green arrows)
distributes the correct amount of total energy between potential and kinetic energies. By balancing the simultaneous actions of the
controls, the airplane can follow the desired energy trajectory.
As illustrated in Figure 4-8, moving the airplane from position 1 to the energy states in 2 and 3 calls for a higher throttle setting to
increase total energy by the same amount (in this example, positions 2 and 3 are located at the same higher-energy height).
The difference between these two energy trajectories (1-t o-2 and 1-t o-3) lies in the way the total energy change is distributed
by the elevator through changes in pitch attitude. As can be seen in Figure 4-8, changes in total energy by adjusting throttle setting
(blue/red arrows) extend across lines of constant total energy (dashed line), while changes in energy distribution by adjusting
the elevator deflection (green arrows) extend along the lines of constant total energy (equal energy height). Appropriate changes in
total energy via the throttle and/or changes in energy distribution via the elevator, depicted by their respective energy “arrows,”
determine the direction of a given energy trajectory between two energy states. To visualize this effect, compare the
trajectory from 1-to-2 with that from 1-to-3 and notice the way the corresponding elevator energy arrows (left green arrow =
up-elevator; right green arrow = down-elevator) are positioned in relation to the throttle energy arrow (blue arrow = increased
throttle).
Figure 4-8. The energy-control map helping to visualize the basic energy control rules.
Thus, transitioning to a higher altitude at a constant speed (1-t o-2) requires increased throttle and up-elevator to stay on speed, while
transitioning to a faster airspeed at a constant altitude (1-to-3) demands increased throttle and (gradual) down-elevator to stay on path,
re-trimming as needed to relieve elevator control pressures.
Transitioning to a lower altitude at a constant speed (1-t o-4) requires decreased throttle and down-elevator to stay on speed, while
transitioning to a slower airspeed at a constant altitude (1-t o-5) demands decreased throttle and (gradual) up-elevator to stay on path,
re-trimming as needed to relieve elevator control pressures.
Finally, transitioning to a higher altitude by trading speed for altitude (1-t o-6) requires up-elevator without initially changing throttle
setting, while transitioning to a faster airspeed by trading altitude for speed (1-t o-7) requires down-elevator without initially changing
throttle setting. In both cases, at the end of the energy exchange maneuver, the elevator will need to be re-trimmed and throttle
setting adjusted to match drag at the new speed in order to maintain total energy constant while remaining at the new altitude-airspeed
target.
As can be visualized in Figure 4-8, there are three general energy control rules for coordinating the throttle and elevator to move the
airplane from one energy state to another:
Rule #1: If you want to move to a new energy state that demands more total energy, then:
Throttle: increase throttle setting so that thrust is greater than drag, thus increasing total energy;
Elevator: adjust pitch attitude as appropriate to distribute the total energy being gained over altitude and airspeed:
a. To climb at constant speed, pitch up just enough to maintain the desired speed;
b. To accelerate at constant altitude, gradually pitch down just enough to maintain path.
Upon reaching new desired energy state, adjust pitch attitude and throttle setting as needed to maintain the new path-speed
rofile.
Rule #2: If you want to move to a new energy state that demands less total energy, then:
Throttle: reduce throttle setting so that thrust is less than drag, thus decreasing total energy;
Elevator: adjust pitch attitude as appropriate to distribute the total energy being lost over altitude and airspeed:
a. To descend at constant speed, pitch down just enough to maintain the desired speed;
b. To slow down at constant altitude, gradually pitch up just enough to maintain path.
Upon reaching new desired energy state, adjust pitch attitude and throttle setting as needed to maintain the new path-speed
rofile.
Rule #3: If you want to move to a new energy state that demands no change in total energy, then:
Throttle: do not change initially, but adjust to match drag at the end of maneuver as needed to maintain total energy constant;
Elevator: adjust pitch attitude to exchange energy between altitude and airspeed:
a. To trade speed for altitude, pitch up;
b. To trade altitude for speed, pitch down.
Upon reaching new desired energy state, adjust pitch attitude and throttle setting as needed to maintain the new path-speed
rofile.
Note that control rules 1 and 2 allow the elevator to distribute the change in total energy in different ways. For example, using rule
1.a the pilot may choose to adjust the pitch-up attitude to climb at a slower (or faster) airspeed. Other situations may require
combining two control rules. One example is when, at maximum cruise airspeed in level flight, thrust has reached its maximum limit
(i.e., PS = 0) but the target energy state is at a higher altitude and total energy within the airplane’s envelope. At maximum
level airspeed, there is no excess thrust available to increase the airplane’s total energy needed to climb. One solution is to initially
trade kinetic for potential energy (rule 3.a), slowing down to an airspeed where drag is reduced below thrust, thus allowing the
airplane to increase its total energy and climb at that slower airspeed (rule 1.a).
Mitigating Risks from Mismanagement of Energy
Besides learning the proper use of the controls for normal energy management tasks, pilots should be equipped with the ability to
identify, assess, and mitigate two major risks associated with mismanagement of energy: 1) unwanted deviations from the
desired energy state; and 2) unintentional, irreversible deceleration and/or sink rate causing depletion of mechanical energy. The
first risk involves unintended altitude-airspeed deviations (refer to Managing Energy Errors section). The second risk entails
unforeseen, continuous airspeed and/or altitude loss coupled with little or no available excess power in a given flight condition (refer
to Preventing Irreversible Deceleration and/or Sink Rate section).
Two Energy Management Scenarios
Two flight scenarios illustr ate the two major risks associated with failur e to manag e the airplane's energy s tate and how a pilo t can
identify, assess, and mitigate those risks.
Scenario 1
Unintentionally descending below th e desired glideslop e on fin al approach to landing and failing to mak e th e proper correction.
[Figure 4-9] To bring the airplane back to the desired glideslope, should the pilot pitch up, throttle up, or both?
Figure 4-9. Descending below the desired glideslope.
Scenario 2
Flying toward rising terrain and not being able to fly up and over it before impacting terrain. [Figure 4-10] Note the rising terrain all
along the departure corridor. What can the pilot do to prevent an impending crash?
Figure 4-10. Departing from Runway 33, Aspen/Pitkin County Airport (KASE), elevation 7,820 feet.
For both scenarios, this section will demonstrate how proper energy management can provide the pilot with the skill to manage the
associated risks and avoid tragic results.
Managing Energy Errors
In addition to learning effective techniques for maintaining stabilized path-speed profiles (e.g., tracking the glideslope)
and transitioning from one profile to another during flight (e.g., leveling off from a descent), pilots should develop skills for
managing unwanted deviations in vertical flight path and airspeed—returning the airplane to its target energy state. Since many
inflight “energy crises” start as undetected, ignored or poorly managed path-speed deviations, pilots need the skills to recognize,
correct and prevent these deviations.
Although the intention is to correct altitude and airspeed deviations, the pilot is always acting on the airplane’s energy state. Thus, it
is important to translate altitude-speed deviations into energy errors. [Figure 4-11] Because the airplane’s total energy is distributed
over altitude and airspeed, there are two types of energy errors: 1) total energy errors and 2) energy distribution errors.
Figure 4-11. An energy state matrix that translates the main altitude-speed deviations into energ y errors relative to the desired energy
state (5).
Monitoring the altimeter (or other flight path reference) and airspeed indicator allows the pilot to distinguish these two types of
energy errors. In total energy errors, the airplane has too much energy (blue boxes) or too little energy (red boxes) . The pilot will
notice that altitude and speed deviate in the same direction (“lower-and-slower” or “higher-and-faster”). On the other hand, in energy
distribution errors the airplane may have the correct amount of total energy (green boxes) but its distribution over altitude and speed is
incorrect. Here, altitude and speed deviate in opposite directions (“higher-and-slower” or “lower-and-faster”). In this case, the
pilot deals with relative deviations—not absolute altitude and speed.
Following energy management principles, total energy errors are corrected by increasing or decreasing energy using the throttle,
while energy distribution errors are corrected by exchanging energy between altitude and speed using the elevator. To correct a
combination of total energy and distribution errors, both controls need to be used simultaneously. Figure 4-12 summarizes the control
skills needed to correct total energy and energy distribution errors.
Scenario 1 [Figure 4-9] is a good example to illustrate energy errors and the skills needed to correct and avoid them. Figure 4-13
actually depicts three possible scenarios (B, C, and D) where an airplane on final approach to land has descended below its intended
flight path. Should the pilot pitch up, throttle up, or both? It depends. The airplane is lower than desired, but the pilot should check
the airspeed as well. Relative to the target airspeed, the actual speed may be slower (B), faster (D), or on target (C). In all three cases,
the goal is to return the airplane to its correct energy state (A), following a deviation in altitude and/or airspeed.
Lower-and-slower (B) is fundamentally different from lower-and-faster (D). The former requires advancing the throttle forward to
regain total energy (3 in Figure 4-12), while the latter requires pulling back on the yoke/stick to null the energy distribution error (9
in Figure 4-12).
Figure 4- 12. The control skills needed to correct total energy and energy distribution errors identified in Figure 4-11 with an
additional column giving caution to the “very slow” condition where careful AOA management is needed in addition to energy
management.
Note that in the scenario depicted in B in Figure 4-13 , advancing the throttle forward to increase energy would only succeed if
excess thrust is available (P S > 0). This may not be the case if the pilot has badly mismanaged energy and slowed down to a
speed where induced drag is so high that even applying full throttle would result in no surplus energy (see column "Cautions When
Very Slow" in Figure 4-12). Depending on the flight condition, available excess power at full throttle may be negative (P S < 0).
In this case, the only recourse is to first trade altitude for speed by pushing forward on the yoke/stick, reducing AOA and
induced drag, and only then advancing the throttle forward to regain total energy. But if the airplane is too close to the ground, there
may not be enough room to reverse the negative energy rate and prevent the airplane from striking the ground.
Now consider the scenario depicted in C in Figure 4-13, where the airplane has descended below the desired flight path but is flying
at the correct speed. Here, even though there is no speed deviation, the pilot is faced with a combination of total energy and
distribution errors. Regaining altitude without changing speed requires advancing the throttle forward while easing aft on the yoke/
stick (6 in Figure 4-12). In other words, decoupling altitude and airspeed (i.e. changing one without changing the other) demands
the use of both controls simultaneously.
In all cases, path and speed should be monitored carefully as they are corrected, adjusting pitch attitude and throttle setting as
appropriate. Once short-term deviations are corrected, the airplane will need to be trimmed for long-term control to maintain the
desired path-speed profile (5 in Figure 4-12).