Energy Management
#FlySafe GA Safety Enhancement Topic
The General Aviation Joint Safety Committee (GAJSC) has pinpointed a crucial area for enhancing aviation safety: aircraft performance and aerodynamics knowledge, especially in the context of energy management. Whether you are aware of it or not, you are an energy manager. To better understand what this means and how it relates to safety, let’s delve into the fundamentals of aircraft performance awareness, the intricacies of flight aerodynamics, and the critical role of energy management in ensuring safe flying experiences.
Aircraft Performance Awareness
Before we explore the nuances of flight dynamics, let’s review the basics. An aircraft operates along three axes of motion while balancing four primary forces: thrust, drag, lift, and weight or gravity. A good resource on the four forces of flight can be found in the article “May the Force(s) Be With You” in the Mar/Apr 2016 issue of FAA Safety Briefing.
Exploring Flight Aerodynamics
Drag, the resistance encountered during flight, can be classified into two types: parasitic drag and induced drag. Parasitic drag arises from the air’s resistance to the aircraft’s movement, while induced drag is generated as a consequence of lift production. Notably, drag increases exponentially with speed, emphasizing the importance of efficient energy management to maintain optimal flight conditions.
Aircraft as an Energy System
Energy comes in two varieties: kinetic and potential. Kinetic energy is associated with motion, and such things as the production of sound, and heat. Potential energy is associated with position such as altitude. By manipulating the flight controls, pilots can convert potential energy to kinetic energy and vice versa. We can think of an aircraft in flight as an open energy system. It can gain energy from a source such as fuel or altitude and it can lose energy to the environment, principally through drag.
Visualizing an aircraft as an open energy system provides valuable insights into its operational dynamics. Energy, in the forms of kinetic and potential energy, governs the aircraft’s motion and altitude. Pilots can manipulate flight controls to convert between these energy states, thereby optimizing performance and maneuverability.
Energy Management Strategies
Looking at it another way, energy earned from thrust can be spent in various ways. Obviously, a large portion of our energy income must be spent on drag, but excess energy income can be saved as altitude and airspeed. Having a substantial energy savings account is very useful when it comes to dealing with situations such as loss of power.
Let’s consider an illustration of common energy transactions from the latest edition of the Airplane Flying Handbook.
After takeoff, as we climb at a constant airspeed, we’re making deposits to our altitude account. While cruising at constant altitude and airspeed, we’re neither depositing nor withdrawing from our energy accounts. If we accelerate at a constant altitude, we’ll increase the balance in our airspeed account. But remember — drag increases as a square of speed, so we’ll quickly get to the point where there’s not enough power to go faster. Deceleration at constant altitude with no increase in power will debit our airspeed account.
We can also transfer from one account to another like moving money between your checking and savings accounts. Climbing while decelerating will transfer assets from our airspeed to our altitude account. This brings us to a very useful energy exchange.
With power fixed, or worse, unavailable; we can always draw on our altitude account to maintain airspeed and control while maneuvering to land. Controlling speed will be important here. Too fast and we’ll be giving up altitude too quickly. Too slow and we won’t travel as far.
Knowing how many miles you can glide per 1,000 feet of altitude is very useful. A rule of thumb for Cessna 152s and 172s is 1.5 nautical miles per 1,000 feet of altitude above ground level (AGL). Remember this is altitude above ground level — not sea level. Consult your pilot operating handbook for best glide speed and distance information and experiment with your flight instructor to see how far you can glide in your airplane.
While we’re at it, let’s briefly discuss power off approaches and landings. To ensure success in forced landings, we must be well acquainted with the best glide speed. As so often is the case, the best glide speed depends on what you’re trying to do. If distance is what you want, you’ll need a speed and configuration that will get you the most distance forward for each increment of altitude lost. This speed is often referred to as the best glide speed and, on most airplanes, it’ll be roughly halfway between Vx and Vy. The speed will increase with weight so most manufacturers will establish the best glide speed at gross weight for the aircraft. That means your best glide speed will be a little lower for lower aircraft weights. Not all manufacturers publish a best glide speed, but here’s a chart with examples from some who do:
Minimum sink speed is what you’ll need if you’re looking to maximize your time in the air. It’s slightly slower than maximum range speed and rarely cited in the POH. You and your instructor can figure out what it is for your aircraft.
To determine the minimum sink speed for your airplane, you can do some experiments on a dual flight with your instructor. Start at Vy or the manufacturer’s recommended best glide speed with power off — you did remember the carb heat didn’t you? — and note speed versus sink rate as you adjust pitch to reduce airspeed. For the most useful results, you should do this as close to typical mission weight as possible. Remember optimal glide speeds will be slower as aircraft weight decreases.
It’ll be easy to identify minimum sink speed. It’s the speed to fly for the lowest rate of descent. Maximum glide range speed will be a little higher than minimum sink. Here you’re looking for the highest speed forward for the lowest rate of descent.
Once you’ve identified minimum sink and maximum range glide speeds, you’ve got a couple of good numbers to use for the future. The next big challenges are to know your height above the terrain and to glide the airplane to a point from which you can make a successful forced landing. Power off approaches and landings are the best ways to train for the main event.
It’s helpful to note that we’ve just been discussing emergency energy management. With power unavailable, we’re redistributing energy in our altitude account to maintain the optimum airspeed for our situation.
Thus, the left side controls changes to the airplane’s total energy while the right side regulates the distribution of the resulting change in energy over altitude and airspeed.
A change in total energy resulting from the difference between thrust and drag always matches the change in total energy redistributed over altitude and airspeed.
When we think about energy as money, the left side of this energy balance equation would represent the airplane’s “net income,” while the right side would reflect matching changes to the airplane’s “savings accounts.” The left side controls changes to the airplane’s total energy, while the right side regulates the distribution of the resulting change in energy over altitude and airspeed. A change in total energy resulting from the difference between thrust and drag always matches the change in total energy redistributed over altitude and airspeed.
Efficient energy management involves judiciously balancing energy inputs and outputs throughout the flight. By maintaining awareness of their energy “accounts,” pilots can make informed decisions, particularly in emergency scenarios. Strategies such as power-off approaches and landings, as well as identifying best glide speeds, are crucial for mitigating risks and ensuring safe outcomes.
Instrumentation and Decision Making
Most aircraft are equipped with sophisticated energy management instrumentation, and pilots manage energy through manipulation of these flight controls. The throttle regulates total energy, and the elevator controls energy distribution to and from the airspeed and altitude accounts. Pilots can measure energy regulated by the throttle by referencing the tachometer, manifold pressure, and torque meters. By referencing airspeed indicators and altimeters, they can measure the exchange of energy, controlled by the elevator.
Most gliders have instrumentation too. Airspeed indicators and altimeters measure the balance in airspeed and altitude accounts. The rate of climb (ROC) indicators, typically extremely sensitive instruments, measure power. When climbing at a constant airspeed, the ROC will be indicating the amount of lift. The energy in our altitude account can be tapped when we encounter areas of sink.
Sophisticated instrumentation plays a pivotal role in monitoring and managing aircraft energy states. Airspeed, altitude, rate of climb, and power indicators provide real-time data that informs pilot decision-making. By leveraging this information effectively, pilots can optimize performance and respond effectively to changing flight conditions.
Keep Energy in Mind
It is a best practice for all pilots to keep your energy state in mind. Knowing your energy account balance will inform your decision making. This is especially useful when coping with emergencies. Make deposits to your energy accounts whenever possible. Like money in the bank, it’s great to know you can cover planned and unplanned expenses. You’re in the best position to cover emergency expenses with plenty of altitude and airspeed. But if you’re slow and close to the ground, your forced landing options may be reduced to bracing for impact and landing straight ahead.
By mastering the principles of flight aerodynamics, energy exchange, and instrument utilization, pilots can enhance their situational awareness and mitigate risks effectively. Through ongoing training and adherence to best practices, we can collectively contribute to a safer aviation environment for all.
Resources
- FAA’s Airplane Flying Handbook, Revision 3C, Chapter 4
- “On the Money — Getting to Know Your Airplane as an Energy System,” FAA Safety Briefing, Jan/Feb 2020
- Best Glide Speed — #FlySafe GA Safety Enhancement Topic
- 10 Tips for Safer Takeoffs and Landings — #FlySafe GA Safety Enhancement Topic
- Angle of Attack Awareness — #FlySafe GA Safety Enhancement Topic
📅 FAASTeam Events (April 2024)
- 👨✈️ In-person/Fly-in Seminar — April 16, 10:00 Eastern, Triple Tree Aerodrome, S.C. (🛩️SC00)
- 👨✈️ In-Person/Fly-In Seminar — April 16, 13:00 Eastern, Oakland County International Airport, Mich. (🛩️PTK)
- 👨✈️ In-Person/Fly-In Seminar — April 16, 18:30 Mountain, Caldwell Executive Airport, Idaho (🛩️EUL)
