Airplane Flying Handbook (FAA-H-8083-3C)
Chapter 4: Energy Management: Mastering Altitude and Airspeed Control
Introduction
This chapter is all about managing the airplane’s altitude and airspeed using an energy-centered approach. 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 in order to:
1. Attain and maintain desired vertical flightpath-airspeed profiles.
2. Detect, correct, and prevent unintentional altitude-airspeed deviations from the desired energy state.
3. Prevent irreversible deceleration and/or sink rate that results in a crash.
Importance of Energy Management
Learning to manage the airplane’s energy in the form of altitude and airspeed is critical for all new pilots. Energy management
is essential for effectively achieving and maintaining desired vertical flight path and airspeed profiles, (e.g., constant airspeed climb)
and for transitioning from one profile to another during flight, (e.g., leveling off from a descent).
Proper energy management is also critical to flight safety. Mistakes in managing the airplane’s energy state can be deadly.
Mismanagement of mechanical energy (altitude and/or airspeed) is a contributing factor to the three most common types of fatal
accidents in aviation: loss of control in-flight (LOC-I), controlled flight into terrain (CFIT), and approach-and-landing accidents.
Thus, pilots need to have:
1. An accurate mental model of the airplane as an energy system.
2. The competency to effectively coordinate control inputs to achieve and maintain altitude and airspeed targets.
3. The ability to identify, assess, and mitigate the risks associated with mismanagement of energy.
Viewing the Airplane as an Energy System
The total mechanical energy of an airplane in flight is the sum of its potential energy from altitude and kinetic energy from airspeed.
The potential energy is expressed as mgh, and the kinetic energy as ½ mV². Thus, the airplane's total mechanical energy can be stated
as:
mgh + ½ mV²
Where,
m = mass
g = gravitational constant
h = height (altitude)
V = velocity (airspeed)
A flying airplane is an “open” energy system, which means that the airplane can gain energy from some source (e.g., the fuel tanks)
and lose energy to the environment (e.g., the surrounding air). It also means that energy can be added to or removed from the
airplane’s total mechanical energy stored as altitude and airspeed.
A Frame of Reference for Managing Energy State
At any given time, the energy state of the airplane is determined by the total amount and distribution of energy stored as altitude and
airspeed. Note that the pilot’s frame of reference for managing the airplane’s energy state is airplane-centric—being a function of
indicated altitude and indicated airspeed, and not height above the ground or groundspeed.
The indicated altitude displayed in the altimeter and its associated potential energy are based on the height of the airplane above a
fixed reference point (mean sea level or MSL), not on the height above ground level (AGL), which changes with variations in terrain
elevation. Likewise, the indicated airspeed displayed in the airspeed indicator and its associated kinetic energy are based on the speed
of the airplane relative to the air, not on the speed relative to the ground below, which varies with changes in wind speed and
direction.
Note that changes in indicated altitude and airspeed are attained through forces resulting from the pilot’s direct manipulati on of the
controls. These direct control inputs determine the airplane’s ability to climb/descend or accelerate/decelerate. In contrast,
changes in AGL-altitude and groundspeed are affected by “external” factors, such as varying terrain elevation and wind, which
the pilot cannot alte r. Of course, the pilot should manipulate the airplane’s energy in such way as to minimize any risks
associated with terrain or wind. For example, the pilot may seek to manipulate energy state so as to maximize the airplane’s energy
gains and minimize energy loses when faced with rising terrain. A safer heading may also be an option.
Once airborne, the airplane gains energy from the force of engine thrust ( T) and it loses energy from aerodynamic drag ( D). The
difference between energy in and out (T – D) is the net change, which determines whether total mechanical energy—stored as altitude
and airspeed—increases, decreases, or remains the same.
When thrust exceeds drag ( T – D > 0 ), the airplane's total mechanical energy increases. The pilot can store the surplus energy as
increased altitude or airspeed. For example, if the pilot decides to put all the surplus energy into altitude, the airplane can climb at a
constant airspeed. [Figure 4-1A] If the pilot opts to place all the surplus energy into airspeed, the airplane can accelerate while
maintaining altitude. [Figure 4-1B]
When drag exceeds thrust, (T – D < 0), the airplane's total mechanical energy decreases. The pilot has two sources of stored energy to
tap into. For example, the pilot may choose to let the airplane descend at a constant airspeed [Figure 4-1C)] or slow down while
maintaining altitude [Figure 4-1D] as stored energy is withdrawn to deal with the energy deficit. When energy gained equals that lost
(T – D = 0 ), all thrust is spent on drag. In this case, the total amount of mechanical energy and its distribution over altitude and
airspeed does not change. Both remain constant as the airplane maintains a constant altitude and airspeed. [Figure 4-1E]
Energy can also be exchanged between altitude and airspeed. For example, when a pilot trade s airspeed for altitude, as altitude
increases, airspeed decreases. In other words, when energy is exchanged, altitude and airspeed always change in opposite directions
(absent any other energy or control inputs). As one goes up, the other one come s down. Also note that even though the distribution of
energy over altitude and airspeed may change dramatically during energy exchange, the total amount of mechanical energy can
remain the same at the end of the exchange maneuver [Figure 4-1F], as long as thrust is adjusted to match drag as the latter varies
with changes in airspeed.
Figure 4-1 A-F. Examples of typical energy transactions.
Managing Energy is a Balancing Act
Since the airplane gains energy from engine thrust (T) and loses energy through aerodynamic drag (D), energy flows
continuously into and out of the airplane while in flight. Usually measured as Specific Excess Power (PS), or rate of energy change,
the net energy flow is a direct function of the difference between thrust and drag.
PS = (T – D)V/W
Where,
T = Thrust
D = Drag
V = velocity (airspeed)
W = aircraft weight
More importantly, there is a fundamental relationship between changes in the airplane’s total energy resulting from this net energy
flow on one hand, and changes in the energy stored as altitude and airspeed on the other. This fundamental relationship can b e
summarized through the airplane’s energy balance equation. [Figure 4-2]
Figure 4-2. The energy balance equation.
The left side of the energy balance equation represents the airplane’s net energy flow, while the right side reflects matchin g changes
to the energy storage. Thus, changes to the airplane’s total energy affect the left side of the equation, while the right side shows
possible changes in energy distribution between altitude and airspeed.
Note that a change in total energy resulting from the difference between thrust and drag (left side) always matches the change in total
energy redistributed over altitude and airspeed (right side). Although rate of energy change, expressed as specific excess power (P S),
varies during flight—becoming positive, negative, or zero—both sides of the equation are inexorably balanced regardless of whether
the airplane is accelerating, decelerating, climbing, descending, or maintaining constant altitude and airspeed. (Note: This
simplified balance equation does not account for long-term changes in total mechanical energy caused by the reduction in
aircraft weight as fuel is gradually burned in flight. Although the effect of weight loss on total energy becomes critical when
solving long-term aircraft performance problems such as range and endurance, it is negligible when considering short-term flight
control problems.)
Of course, the pilot controls the change in total energy on the left side of the equation, as well as the distribution of any changes in
energy over altitude and airspeed on the right side. How the pilot coordinates the throttle and elevator to achieve and maintain desired
altitude and airspeed targets as well as avoid energy "crises" is at the core of energy management and is elaborated in the rest of the
chapter.
Role of the Controls to Manage Energy State
An energy-centered approach clarifies the roles of the engine and flight controls beyond the simple “pitch for airspeed and power for
altitude” by modeling how throttle and elevator inputs affect the airplane’s total mechanical energy. From an energy perspective, the
problem of controlling vertical flight path and airspeed becomes one of handling the airplane’s energy state—the total amount
of energy and its distribution over altitude and speed. Thus, rather than asking what controls altitude and what controls airspeed, a
pilot can now ask what controls total energy and what controls its distribution over altitude and airspeed.
Primary Energy Role of the Throttle and Elevator
The throttle, by increasing or decreasing engine thrust against drag, regulates changes in total mechanical energy. As illustrated
above, changing total energy is a function of both thrust and drag ( T – D). However, drag mainly varies long-term due to airspeed
changes, or by using high lift/drag devices which can only increase drag. Therefore, changes in total energy are normally initiated by
changing thrust, not drag. When the throttle setting makes thrust greater than drag, an increase of total mechanical energy is the result.
When the throttle setting makes thrust less than drag, a decrease of total mechanical energy is the result. Once the desired path-speed
profile is established, the throttle sets engine thrust to match the total energy demanded by vertical flight path and airspeed combined.
The throttle then is the total energy controller.
On the other hand, the elevator is an energy exchanger and distribution device whose primary job is to allocate changes in total
energy between vertical flight path and airspeed by adjusting pitch attitude. Here, once the chosen path-speed profile is achieved, the
elevator sets the appropriate pitch attitude to maintain the demanded distribution of total energy over vertical flight path and airspeed.
Thus, the elevator is the energy distribution controller.
The throttle and elevator then are really energy state controls —neither one controls altitude nor airspeed independently since these
two variables are inherently coupled through the airplane’s total mechanical energy. Instead, to control altitude and airspeed
effectively, the pilot coordinates the use of both devices to manage the airplane’s energy state.
The reservoir analogy [Figure 4-3 ] illustrates the energy-based role of the throttle and the elevator. In this analogy, the throttle
controls the “valve” regulating the net total energy flow while the elevator controls the “valve” regulating the distribution of energy
into and out of the altitude and airspeed “reservoirs.” Referring back to the energy balance equation [Figure 4-2], it becomes clear
then that the throttle controls the left side of the equation (total energy) and the elevator controls the right side (energy distribution).
As illustrated in Figure 4-3, when the throttle increases thrust above drag ( T – D > 0 ) the airplane gains total energy, and when the
throttle reduces thrust below drag (T – D < 0) the airplane loses total energy. The elevator then distributes this increase or decrease in
total energy between altitude and airspeed. Finally, when the throttle adjusts thrust equal to drag ( T – D = 0 ), there is no change in
total energy, but the energy stored as altitude and airspeed can be exchanged between the two reservoirs using the elevator, while total
energy, at least short-term, remains constant.
Figure 4-3. The reservoir analogy illustrating the primary role of the throttle and elevator to manage the airplane’s energy state.
Additional Role for the Elevator
On the front side of the power required curve, where the airplane cruises at high speed (1 in Figure 4-4) and a low angle of attack
(AOA) with little or no excess power or excess thrust (A in Figure 4-4), pulling back on the yoke or stick (elevator up) will result in a
brief energy exchange climb, causing the airplane to slow down from 1 to 2 toward the center of the power curve [Figure 4-4]. This
decrease in airspeed results in a reduction in total drag; hence available energy in the form of positive excess power (P S > 0) where
thrust exceeds drag (T – D > 0). With this excess power (B in Figure 4-4) the airplane can now climb at a constant airspeed or turn in
level flight while maintaining a constant airspeed at an increased load factor.
On the backside of the power required curve, where the airplane flies at low speed (3 in Figure 4-4) and high AOA with little or no
excess power or excess thrust (C in Figure 4-4), pushing forward on the yoke or stick (elevator down) will result in a brief energy
exchange descent, causing the airplane to accelerate from 3 to 2 toward the center of the power curve [Figure 4-4]. This increase in
airspeed results in a reduction in total drag; hence available energy in the form of positive excess power (P S >0) where thrust exceeds
drag (T – D > 0). With this excess power (B in Figure 4-4) the airplane can now climb at a constant airspeed or turn in level flight
while maintaining a constant airspeed at an increased load factor. This role of the elevator is critical to prevent unintentional,
excessive deceleration or sink rate as illustrated later in the chapter (refer to Preventing Irreversible Deceleration and/or Sink Rate
section).
Figure 4-4. The front side and backside of the power required curve, the power available curve, and the relative excess power
available (power available - power required) at different speeds.
While the elevator can assist the throttle in changing T – D and PS through changes in airspeed via energy exchange as described
above, occasionally the elevator can directly increase the “D” in T – D at any given speed during a level turn, thus helping the
airplane rapidly bleed off total energy. As the airplane banks, load factor (lift/weight) increases because total lift has to increase to
pull the airplane into the turn while simultaneously balancing its weight. This is accomplished by pulling back on the yoke (or stick)
to increase AOA which results in increased induced drag and power required at any given speed. This action will quickly slow
the airplane down and decrease total energy more rapidly than by just reducing the throttle setting to idle. This additional role of the
elevator is shown on the power curve. [Figure 4-5]
Figure 4-5. The effect of increased load factor on total drag and power required at different airspeeds.
Applying the respective role of the controls to manage the airplane’s energy state leads to a set of simple “rules” for proper throttle-
elevator coordination to effectively control vertical flight path and airspeed. What are these basic rules of energy control?
Rules of Energy Control
The central principle encapsulating the role of the throttle and elevator for managing the airplane’s energy can be summed up
as follows: coordinated throttle and elevator inputs control the airplane’s energy state. Modifying a popular adage, the principle can
be restated as “pitch plus power controls energy state.” This central principle serves to guide a set of general energy control rules to
achieve and maintain any desired vertical flight path and airspeed targets within the airplane’s energy envelope.
Visualizing the Airplane’s Ability to “Move” Between Energy States
To better understand the basic rules of energy control, a pilot needs to visualize an airplane’s energy state and its ability to switch
from one energy state to another. In other words, how does an airplane “move” from an initial altitude and airspeed to any other target
altitude and airspeed within its flight envelope, and how does the pilot control the process? A map should help, and in this case, it
charts the status of the aircraft in terms of energy.
In a navigation map, such as an aeronautical sectional chart, the geographic position of an airplane is determined by two variables —
latitude and longitude. Likewise, in an “altitude -airspeed” or “energy” map the energy position of an airplane, its energy state,
is defined by two variables—altitude and airspeed. [Figure 4-6]
Figure 4-6. The altitude-speed “map” showing lines of constant energy height.
The position of an airplane in the altitude-airspeed map represents its total specific energy or E S (which is simply the sum of its
potential and kinetic energies divided by aircraft weight) as determined by its current altitude and airspeed.
ES = h + V²
2g
Where,
g = gravitational constant
h = height (altitude)
V = velocity (airspeed)