Figure 4-13. Energy error management.
The above approach-t o-landing scenario is just one example illustrating the risk of mismanaging altitude-speed deviations.
Pilots need to be able to identify, assess, and mitigate altitude and/or airspeed deviations during any phase of flight, including traffic
pattern operations, take-offs and climbs, cruise flight, descending flight, and any procedure or maneuver involving turns.
Clearly, skills for promptly correcting path-speed deviations can enhance flight safety but the pilot should also be aware of the risk of
unrecoverable depletion of the airplane’s mechanical energy, especially as the airplane approaches the edges of its flight envelope
where available excess power is zero.
Preventing Irreversible Deceleration and/or Sink Rate
During normal flight, the airplane experiences many instances of negative energy rates (negative specific excess power or PS < 0)
while decelerating at a constant altitude or descending at a constant airspeed; these are intended energy bleed rates. However, one of
the greatest dangers from mismanaging the airplane’s energy state is encountering unintended, excessive deceleration and/or sink rate
coupled with little or no positive excess power available under a given flight condition. Failure to recover above a certain critical
altitude results in depletion of mechanical energy. Regardless of what the pilot does past that point, the airplane will hit the ground.
To help pilots understand the risk of unintended energy depletion, let’s take a closer look at Scenario 2 [Figure 4-10]. This flight
scenario illustrates a situation that is all too common in general aviation: flying toward rising terrain and not being able to fly up and
over it before impacting terrain.
As shown in Figure 4-10, there is rising terrain all along the departure corridor. The scenario is as follows:
1. A pilot of a normally-aspirated, twin-engine airplane departs out of Rocky Mountain Metropolitan airport
(KBJC) in the morning on a nice summer day and flies into Aspen/Pitkin County airport (KASE).
2. The pilot enjoys the scenery around Aspen, eats lunch, and decides to return home in the early hot
afternoon.
3. The pilot departs KASE off of runway 33. At full throttle/power the airplane takes longer to accelerate but
rotates at the normal speed.
4. The pilot pitches to the normal pitch target, retracts the gear, and initiates a climb.
5. The pilot notices the airplane isn’t performing as desired. The pilot checks to see if the gear is up and
adjusts the mixtures to try and get a little more power.
6. The terrain is rising, the pilot gradually pitches up, and the airplane starts losing airspeed.
7. The airplane quits climbing.
8. The stall horn begins to sound.
The above scenario is hypothetical, but there have been very similar situations that have ended tragically.
The airplane in the above scenario has encountered an unintended deceleration and impending sink rate that could rapidly become
irreversible. This can be shown in two ways, using the traditional power curve [Figure 4-14] and the energy map [Figure 4-15]:
Figure 4-14. The energy depletion scenario viewed in the power required and available curve. Compared with the power available
curve depicted in Figure 4-4, note the lower power available curve at this high elevation (7,820 feet at the departure airport) and
higher density altitude than standard during a hot afternoon.
As illustrated in the airplane's power required and available curves [Figure 4-14], the airplane slows down, going from speed 1 where
it is climbing (A: power available greater than power required), to speed 2, where it stops climbing (B: power available equ al to
power required), and continuing to speed 3 where the stall horn sounds (C: power available less than power required). The
energy map [Figure 4-15] tells the same story from a total mechanical energy standpoint: the airplane has positive PS at point 1 and
climbs to point 2 where it stops climbing since PS = 0, then continues to point 3, where the PS < 0 and the stall horn sounds.
Figure 4-15. The energy depletion scenario viewed in the energy map. Specific excess power (P S) contours are labeled in units of feet
per minute.
The question then is: what does the pilot do to recover from this predicament? The answer is proper energy management. The
airplane needs to move to a different place on the energy map that will allow the airplane to begin climbing. So, what does that mean?
⦁ As can be seen in Figure 4-12, the pilot is in a scenario akin to that at the desired altitude, but with
cautions when very slow.
⦁ The pilot then has to do something that is not intuitive; consider gaining speed at the expense of some
altitude initially to improve climbing performance with full throttle.
⦁ Once the airplane accelerates to an airspeed in which the PS > 0, it can begin to climb again.
The above recovery scenario is shown in the energy map Figure 4-16, which illustrates the important role of the elevator in
assisting the pilot to recover from unintentional and dangerous deceleration and/or sink rate (refer to Additional Role for the Elevator
section).
The airplane needs to gain speed at the expense of some altitude, moving from point 3 where the PS < 0 to point 4 where the PS > 0.
The airplane can then initiate a constant airspeed climb to point 5, at the desired target altitude and airspeed [Figure 4-16]. Note that
the desired target climb airspeed in the presence of rising terrain may be VX, the speed for best angle of climb. VX is slightly slower
than VY, the speed for best rate of climb, and will result in a lower climb rate but steepest climb angle. Once the airplane has
recovered from the unintentional airspeed loss and begins climbing at VX, the pilot should assess the situation and make an important
decision to mitigate further risk—either continue climbing or do something else. Should the airplane not have the needed
performance to safely clear the rising terrain on its intended course, the pilot has at least another available option: make a 180
degree turn and return to land at the departure airport until temperature and density altitude conditions improve.
Figure 4-16. The energy loss scenario recovery viewed in the energy map. Specific excess power (PS) contours are labeled in units of
feet per minute.
The above rising terrain scenario is just one example illustrating the risk of irreversible deceleration and/or sink rate. Pilots need to be
aware that unintentional depletion of mechanical energy can happen in various instances, especially as the airplane approaches
the slow edge of its energy envelope at low altitude, where available specific excess power (P S) is zero. Examples include unstable/
slow approaches to landing; high-drag go-arounds where the pilot neglects to raise the gear and/or flaps; and steeper-than-normal
turns in the traffic pattern. Note that irreversible sink rates do not necessarily involve exceeding the critical AOA resulting in a stall
and spin. The airplane can be unstalled and still experience unrecoverable sink rates near the high-speed edge of its energy
envelope, where available specific excess power (P S) is also zero. Two examples are high-speed steep spirals following botched steep
level turns, and high-speed dives too close to the ground.
The bottom line? Should the airplane ever experience unintended excessive negative energy rates with little or no excess powe r
available under a given flight condition, the pilot needs to use proper energy management allowing a prompt recovery and a suitable
follow-up action.
Review of Terms and Definitions
The terms and definitions specific to this chapter appear below.
Aircraft Energy Management
The process of planning, monitoring and controlling altitude and airspeed targets in relation to the airplane’s energy state. Note that
this definition is concerned with managing mechanical energy (altitude and airspeed) and addresses the safety (flight control) side of
energy management. It does not address the efficiency (aircraft performance) side of energy management, which is concerned with
how efficiently the engine generates mechanical energy from fuel and how efficiently the airframe spends that energy in flight.
Energy System
A flying airplane is an open energy system. That means that the airplane can gain energy from some source (e.g., fuel) and lose
energy to the environment (e.g., surrounding air). In addition, energy can be added to or removed from the airplane’s total mechanical
energy stored as altitude and airspeed.
Total Mechanical Energy
Sum of the energy in altitude (potential energy) and the energy in airspeed (kinetic energy).
Kinetic Energy
Amount of energy due to the airspeed, expressed as ½mV², where m = airplane’s mass, and V = airspeed.
Potential Energy
Amount of energy due to the altitude, expressed as mgh, where m = airplane’s mass, g = gravitational constant, and h = altitude.
Energy State
The airplane’s total mechanical energy and its distribution between altitude and airspeed.
Energy Exchange
Trading one form of energy (e.g., altitude) for another form (e.g., airspeed).
Energy Balance Equation
According to this equation, the net transfer of mechanical energy into and out of the airplane (a function of thrust minus drag) is
always equal to the change in its total mechanical energy (a function of altitude and airspeed). Note that this simplified definition 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.
Power Available
The airplane’s rate of energy gain due to maximum available engine thrust at a given airspeed. Expressed as TV, where T =
engine thrust and V = airspeed. Usually measured in horsepower, foot-pound per minute, or foot-pound per second.
Power Required
The airplane’s rate of energy loss due to total drag at a given airspeed. Expressed as DV, where D = total drag and V = airspeed.
Usually measured in horsepower, foot-pound per minute, or foot-pound per second.
Specific Excess Power (PS)
Measured in feet per minute or feet per second, it represents rate of energy change —the ability of an airplane to climb or accelerate
from a given flight condition. Available specific excess power is found by dividing the difference between power available and power
required by the airplane’s weight.
Energy Height or Total Specific Energy (ES)
Measured in units of height (e.g., feet), it represents the airplane’s total energy per unit weight. It is found by dividing the sum
of potential energy and kinetic energy by the airplane’s weight. It also represents the maximum height that an airplane would reach
from its current altitude, if it were to trade all its speed for altitude.
Energy Error
An altitude and/or airspeed deviation from an intended target expressed in terms of energy. Depending on the airplane’s total amount
of energy and its distribution between altitude and airspeed, energy errors are classified as total energy errors, energy distribution
errors, or a combination of both errors.
Total Energy Error
An energy error where the total amount of mechanical energy is not correct. The airplane has too much or too little total energy
relative to the intended altitude-speed profile. When this error occurs, the pilot will observe that altitude and airspeed deviate in the
same direction (e.g., higher and faster than desired; or lower and slower than desired). An example would be an airplane on
final approach that is above the desired glide slope and at a faster airspeed than desired.
Energy Distribution Error
An energy error where the total mechanical energy is correct, but the distribution between potential (altitude) and kinetic energy
(airspeed) is not correct relative to the intended altitude-speed profile. When this error occurs, the pilot will observe that altitude and
airspeed deviate in opposite directions (e.g., higher and slower than desired; or lower and faster than desired). An example would be
an airplane on final approach that is above the desired glide slope and at a slower airspeed than desired.
Irreversible Deceleration and/or Sink Rate
Unrecoverable depletion of mechanical energy as a result of continuous loss of airspeed and/or altitude coupled with insufficient
excess power available under a given flight condition. Failure to recover above a certain critical AGL altitude results in the airplane
hitting the ground regardless of what the pilot does.
Chapter Summary
Every pilot is an energy manager —managing energy in the form of altitude and airspeed from takeoff to landing. Proper energy
management is essential for performing any maneuver as well as for attaining and maintaining desired vertical flightpath and airspeed
profiles in everyday flying. It is also critical to flight safety since mistakes in managing energy state can contribute to loss of control
inflight (LOC-I), controlled flight into terrain (CFIT), and approach and landing accidents. The objectives of this chapter are for pilots
to: 1) gain an understanding of basic energy management concepts; 2) learn the energy role of the controls for managing the
airplane’s energy state; and 3) develop the ability to identify, assess, and mitigate risks associated with failure to manage the airplane’s
energy state.