May the Force(s) Be with You

Using the Four Forces to Maintain Aircraft Control

Source: FAA Safety Briefing Mar/Apr 2016, By Susan Parson

Even if you aren’t a Star Wars fan, you undoubtedly know about “The Force.” You’ve probably found yourself wielding this handy meme in conversation once or twice, perhaps to wish someone good luck (“May the Force be with You”) or to encourage greater effort (“Use the Force, Luke!”).

If you are a pilot, you do a lot more than just talk about The Force. You use it every time you take to the skies. In fact, you use all four of the forces you first met in ground school: lift, weight, thrust, and drag. Though it may not have been presented quite this way, in both ground school and flight training you learned (I hope) that the pilot’s job is to manage the Four Forces in order to comply with the Pilot’s Prime Directive: to maintain aircraft control (yes, I know I am mixing sci-fi movie metaphors). You learned that in straight-and-level unaccelerated flight, lift is equal to weight and thrust is equal to drag. And you learned techniques to manage the Four Forces, both individually and collectively, in order to maintain aircraft control.

The problem is that too many of us continue to violate the Pilot’s Prime Directive, to the point that loss of control has acquired its own acronym: LOC. As you’ve seen elsewhere in this issue, there is a fatal accident involving LOC about every four days. LOC — specifically, loss of control in flight (LOC-I) — is the number one cause of GA fatal accidents, which take around 450 lives every year.

We have to do a lot better in managing the forces. That’s the reason for the ongoing government/industry #FlySafe campaign, and that’s why we chose to focus the March/April 2016 FAA Safety Briefing issue, which traditionally opens the year’s flying season, on ways to avoid LOC. Since LOC can occur in every phase of flight, we have structured this issue’s articles to look at maintaining aircraft control in the takeoff/departure, cruise, and approach/landing segments. But let’s start by considering the role each of the Four Forces plays in aircraft control.

The Force - Lift

For a pilot, the force is lift. Lift equals life, because it keeps the airplane aloft.

As you learned in ground school, a scientist named Bernoulli discovered the inverse relationship between the velocity of a moving fluid (like air) and its pressure. When air flows over an obstacle like a wing (airfoil), the speed of the air moving over the wing increases as its pressure decreases. The pressure differential between the upper and lower airfoil surfaces is what creates lift.

A wing is specially designed to create the pressure differential and produce lift in an efficient way. Up to a point, the pilot can increase the pressure differential (and thus the lift) by flying faster, or by increasing the angle of attack (AoA). The AoA is the angle between the wing chord line (an imaginary line through the center of the airfoil) and the relative wind (that is, the direction of the air striking the wing).

When it comes to aircraft control, proper management of AoA is a Very Big Deal. If the pilot increases AoA beyond the “critical” AoA, which is a set value for any given airfoil, the moving air breaks away from the top of the airfoil. This disruption causes loss of lift and, if the pilot fails to immediately and decisively reduce AoA, the result is an aerodynamic stall. If the pilot does not reduce AoA to recover from the stall, or if the pilot aggravates the situation by using rudder in a way that stalls one wing more than the other, the result can be a spin and another LOC-I accident.

It seems simple enough to “just” manage AoA, and hopefully the proliferation of cost-effective AoA indicators will make this task easier still. However, too many pilots still focus on airspeed. Pilots tend to associate lift and loss of lift (stalls) primarily with airspeed for several reasons. First, there is a clear relationship between lift and speed. Lift is proportional to the square of the airplane’s speed, so doubling the speed will quadruple the lift. Second, for every AoA, there is a corresponding airspeed required to maintain altitude in steady, unaccelerated flight. An airplane flying at a higher airspeed can maintain level flight with a lower AoA, while an airplane flying at a slower airspeed must have a higher AoA to generate enough lift for level flight. Third, maneuvers practiced in early flight training, such as demonstration of the effect of airspeed changes and stalls entered from a wings-level attitude, tend to emphasize the relationship between AoA and airspeed. Finally, the term “stall speed,” which refers to the speed at which the wing reaches critical AoA in a wings level unaccelerated (1g) condition, further reinforces this association.

It is important to understand, however, that airspeed is not the only consideration. Factors such as gross weight, load factor, center of gravity loading, and configuration (e.g., flap setting) have a direct effect on stall speed; therefore, it is possible to stall the wing at any airspeed, in any flight attitude, and at any power setting.

Just to consider weight as one of these factors: because lift must equal weight, an airplane that is heavier must generate more lift in order to maintain level flight. For any given airspeed, a heavier airplane must be flown at a higher AoA in order to generate sufficient lift for level flight. Since an airfoil always stalls at the same AoA, an airplane with additional weight (e.g., passengers, fuel, baggage) flies at an AoA closer to the critical value. The same thing happens in a level turn, where additional lift must be generated by increasing AoA so that its vertical component balances weight. Again, that increase in AoA puts you closer to its critical value. Here’s the bottom line on managing lift: never exceed the critical AoA.

Weight

Weight is the force of gravity. It acts in a downward direction, toward the center of the Earth. Weight includes both the airplane itself and its useful load. While your ability to control the weight of the airplane itself is limited, almost everything else — how many passengers, how much fuel, how many bags — is up to you. It is also up to you to load the airplane without violating the fore and aft center of gravity (cg) limits.

Properly managing the force of weight is essential to maintaining aircraft control. For example, consider the consequences of loading an airplane beyond its aft center of gravity limit. In a worst case scenario, this situation could make it impossible to lower the nose and recover from a departure stall.

Thrust

Generated by some kind of propulsion system, thrust is the force that moves an airplane through the air in level flight. While modern airplane engines are remarkably reliable, engine failures do occur. As any competent glider pilot will tell you, it is absolutely possible to maintain aircraft control without engine-produced thrust. For a powered airplane, though, loss of engine power can result in loss of control if the pilot fails to follow the proper procedures for such an event. Make it a point to practice simulated engine-out approach and landing procedures on a regular basis.

Drag

Drag is the force that acts opposite to the direction of motion through the air, and it results from both friction (“parasite drag”) and when some of your lift points aft (“induced drag”). For purposes of this discussion, we’ll focus on induced drag, which is an inescapable by-product of lift. Whenever an airfoil is producing lift, the pressure on its lower surface is greater than on the upper surface. As a result, the air tends to flow from the high pressure area below the tip upward to the low pressure area on the wing’s upper surface. These pressures tend to equalize around the wingtips, resulting in a lateral flow that creates vortices circulating counterclockwise about the right tip and clockwise about the left tip. The downwash flow they create bends the lift vector aft and creates induced drag.

While induced drag creates a performance penalty for the aircraft producing it, the real issue is how wingtip vortices — aka “wake turbulence” — affect airplanes that encounter them. Flying into the wake turbulence generated by a larger/heavier aircraft can result in an upset — an unintentional exceedance of the pitch, bank, and airspeed parameters associated with normal operations. An upset — which can result from any number of environmental, mechanical, or human factors — is usually unexpected. A pilot who reacts with abrupt muscular inputs or by instinct can quickly aggravate an abnormal flight attitude and cause a potentially fatal LOC accident.

Becoming a GA Jedi

In the Star Wars construct, a Jedi is “a Force-sensitive individual” who studies and uses its mystical energies for the good of the order. To prevent LOC-I accidents and adhere to the Pilot’s Prime Directive of maintaining aircraft control, we pilots would do well to become GA Jedi. To develop from “Padawan” to Jedi Master:

• Make yourself “Force(s)-Sensitive” by increasing knowledge and understanding of the Four Forces of Flight. You need to understand how each one works, and how to manage them both individually and collectively to maintain aircraft control.
• Seek focused, disciplined training and practice on all aspects of aircraft control.
• Learn all you can about upset prevention, a term that refers to pilot actions to avoid a divergence from the desired airplane state. Awareness and prevention training can help you avoid incidents, because early recognition of an upset scenario coupled with appropriate preventive action often can mitigate a situation that could otherwise escalate into a LOC accident.
• Consider investing in upset recovery training, which aims to instill the pilot with the proper actions and behaviors to promptly return an airplane that is diverging in altitude, airspeed, or attitude to a desired state.

May the Forces be with you!

Learn More

Advisory circular (AC) guidance is available at www.faa.gov:

AC 61-67C Stall and Spin Awareness Training

AC 120-109A, Stall Prevention and Recovery Training

AC 120-111, Upset Prevention and Recovery Training

Susan Parson is editor of FAA Safety Briefing. She is an active general aviation pilot and flight instructor.