Introduction
The flight controls manipulate the aircraft’s path, affecting its pitch, roll, and yaw. Understanding their function is fundamental to operating an aircraft.
Attention and Motivation
Understanding the airplane’s flight controls goes beyond mere operation; it’s about connecting with the aircraft, feeling every response, and anticipating its behavior. This lesson is designed to deepen pilots’ connection to their aircraft.
Objectives
After this lesson, the learner will be able to describe:
- The primary and secondary flight controls’ purpose, location, direction of movement, effect, and proper operation.
- How to use the trim devices to relieve flight control pressures.
Teaching Strategy
Use a model airplane to point out each flight control and demonstrate how the airplane will react when used (e.g., roll, yaw, or pitch).
To show the effect of a flight control:
- Draw a side view of the mounting surface (e.g., a wing or horizontal stabilizer), if applicable, on a marker board.
- Draw the movable control (e.g., an aileron or elevator).
- Point out the mounting location (e.g., pivot point) and indicate the flight control’s direction of movement with arrows.
- Draw lines of airflow and point out the low and high-pressure areas.
- Show how the movement of the flight control distributes high and low-pressure areas.
- Finally, add a trim tab, if applicable.
Follow the classroom presentation with an actual demonstration of the flight controls on the training airplane. Allow the learner to move each flight control and observe the control surface’s movement.
Lesson Briefing
Aircraft Specific Training
- How to remove and install the flight control gust locks, if applicable
- The location of each flight control and its direction of movement
- How to inspect each control surface during a preflight inspection
- Limitations of the movement of the flight controls during a preflight inspection
Risk Management
- Failure to detect flight control malfunctions or failures
- Improper management of a flight control failure
- Unfamiliar flight control systems such as a side stick
Case Studies
A Cessna 182 on a post-maintenance test flight makes a successful landing with the trim control installed in the reverse direction:
- Aircraft: Cessna 182T
- Date: December 12, 2014
- NTSB Video: https://youtu.be/Hl14kAqacsI
After losing all flight controls, a DC-10 crash lands by using differential throttle inputs:
- Aircraft: United Airlines Flight 232
- Location: Sioux City, IA
- Date: July 19, 1989
- AOPA Video (Tribute): https://youtu.be/OWhCNxXwpZk
Resources
- Airplane Flying Handbook (FAA-H-8083-3):
- Chapter 3, Basic Flight Maneuvers
- Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25):
- Chapter 6, Flight Controls
- AOPA Video: Safety Tip: Box the Controls
Schedule
- Lesson Briefing (0:15)
- Flight Control Demonstrations – Ground Operation (0:10)
- Review and Assessment (0:15)
Equipment
- Whiteboard, markers, and erasers
- Airplane models
- Airplane manuals containing flight control descriptions
- An airplane for ground demonstrations
Lesson Debriefing
This lesson concludes with a combined informal assessment and review of the main points.
Additionally, the instructor ensures:
- All of the learner’s questions are resolved.
- The learner is made aware of his or her performance and progress.
Completion Standards
This lesson is complete when the lesson objectives are met and the learner’s knowledge is determined to be adequate for the stage of training. Ultimately, the learner must meet or exceed the Airman Certification Standards.
Lesson Content
Primary Flight Controls
Pilots use the primary flight controls for the immediate control of pitch, roll, and yaw. They include the ailerons, elevator (or stabilator), and rudder. Cables or rods usually connect the control surfaces to the pilot’s controls.
Elevator
The elevator controls the movement of the airplane about its lateral (pitch) axis. The elevator is hinged to a fixed horizontal stabilizer. Together, the horizontal stabilizer and the elevator form a single airfoil.
Control movements:
- When forward pressure is applied to the pitch control, the elevator moves downward. Upward lift increases on the horizontal tail surfaces, causing the nose to pitch down.
- When back pressure is applied to the pitch control, the elevator moves upward. Downward lift increases on the horizontal tail surfaces, causing the nose to pitch up.
Stabilator
A stabilator is a one-piece horizontal stabilizer used by some airplane manufacturers in replacement of the elevator. It also controls the airplane’s movement about its lateral (pitch) axis.
Control movements:
- When forward pressure is applied to the pitch control, the trailing edge of the stabilator moves down. Upward lift increases on the stabilator, causing the nose to pitch down.
- When back pressure is applied to the pitch control, the trailing edge of the stabilator moves up. Downward lift increases on the stabilator, causing the nose to pitch up.
Because stabilators pivot around a central hinge point, they are highly sensitive to control inputs and aerodynamic loads. Antiservo tabs are incorporated on the trailing edge to decrease sensitivity. In addition, a balance weight is used. The balance weight may project into the empennage or may be incorporated on the forward portion of the stabilator tips.
Ailerons
The ailerons control the movement of the airplane about its longitudinal (roll) axis. They are used to bank the airplane. The horizontal component of lift turns the airplane when the wings are banked.
One aileron is usually located on each wing’s outer trailing edge, and they are typically interconnected. Lowering the aileron on one wing raises the aileron on the other. The wing with the lowered aileron rises because of its increased lift, and the wing with the raised aileron lowers because of its decreased lift.
Control movements:
- When the bank control is moved to the right, the left wing’s aileron moves downward, and the right wing’s aileron moves upward. Lift increases on the left wing, causing the airplane to bank to the right.
- When the bank control is moved to the left, the left wing’s aileron moves upward, and the right wing’s aileron moves downward. Lift increases on the right wing, causing the airplane to bank to the left.
Types of Ailerons
To reduce the effects of adverse yaw and aileron control forces, manufacturers have engineered the following systems:
- Differential ailerons
- Frise-type ailerons
- Coupled ailerons and rudder
- Flaperons
Differential-Type Ailerons
For a given movement of the control stick or wheel, the differential-type aileron system raises one aileron a greater distance than the other aileron is lowered. In this case, since the raised aileron has more surface area exposed to the airflow (thus increased drag) than the lowered aileron, the adverse yaw is reduced.
Frise-Type Ailerons
The Frise-type aileron (pronounced “freeze”) helps equalize drag and reduce adverse yaw. When utilized, the leading edge of the upward-moving aileron (which has an offset hinge) projects down into the airflow and creates drag. Despite the improvement, some rudder pressure is needed when the ailerons are moved.
The Frise-type aileron also forms a slot so that the air flows smoothly over the lowered aileron. This makes the aileron more effective at high angles of attack.
Interconnected Ailerons and Rudder
Some airplanes have a rudder-aileron interconnect in the form of a spring or bungee cord. The reason may be to improve lateral (roll) stability or correct for aileron drag (adverse yaw).
Lateral Stability (Roll) Purposes: The movement of the rudder causes one of the ailerons to move slightly down. For example, a right rudder input will cause a right roll input, and a left rudder input will cause a left roll input. Aileron inputs do not cause rudder deflection.
Note: This system was used on early Cirrus aircraft (SR20 and SR22). It was removed in later models by increasing wing dihedral.
Adverse Yaw Purposes: The movement of the ailerons deflects the rudder in the direction of the turn. Pilots can overpower the interconnecting force to slip the aircraft.
Note: At least one airplane, the Ercoupe, was designed with fully coupled ailerons and rudder. Slipping this aircraft is not possible.
Flaperons
Flaperons combine both aspects of flaps and ailerons. In addition to controlling an aircraft’s bank angle, flaperons can be lowered together to function much the same as a dedicated set of flaps. The pilot retains separate controls for ailerons and flaps. A mixer is used to combine the separate pilot inputs into this single set of control surfaces called flaperons.
Rudder
The rudder controls the movement of the airplane about its vertical axis. Its purpose is to counteract yaw-induced changes, such as adverse yaw and effects of the propeller.
The movable rudder is hinged to a fixed vertical stabilizer. The vertical stabilizer’s purpose is to keep the aft end of the airplane behind the front end (like the feathers of an arrow).
The rudder’s action is very much like that of the elevators, except that it moves in a different axis; the rudder deflects from side to side instead of up and down.
Control movements:
- When pressure is applied to the left rudder pedal, the trailing edge of the rudder moves to the left. A horizontal force is exerted to the right (opposite direction) on the tail. The resultant force yaws the nose to the left.
- When pressure is applied to the right rudder pedal, the trailing edge of the rudder moves to the right. A horizontal force is exerted to the left (opposite direction) on the tail. The resultant force yaws the nose to the right.
Ground Steering
The rudder can steer the ground but requires sufficient airflow from the propeller slipstream to yaw the airplane in the desired direction. A steerable nosewheel is often connected to the rudder pedals to aid in steering.
Yaw Dampening
A yaw damper is sometimes installed to correct the airplane’s tendency to oscillate around the vertical axis while flying a fixed heading. Near-continuous rudder input is needed to counteract this effect. The yaw damper can be part of an autopilot system or a completely independent unit.
Secondary Flight Controls
Secondary flight controls may consist of spoilers, wing flaps, leading-edge devices, and trim systems.
Wing Flaps
Wing flaps are movable panels on the inboard section of the trailing edges of the wings. They are hinged so that they may be extended down into the flow of air beneath the wings to increase both lift and drag.
Flaps work primarily by changing the airfoil’s camber, which increases the wing’s lift coefficient (CL).
Extending the wings flaps:
- Increases drag.
- Lowers the stall speed.
- Increases the angle of incidence.
- Increases the wing’s washout (wing roots stall first on planes with inboard flaps).
Benefits of the Flaps
Wing flaps can:
- Shorten the takeoff and landing distance required.
- Increase forward visibility by allowing a lower-pitch attitude.
- Generate more lift at slower airspeed, enabling the airplane to fly slower.
- Produce greater drag which permits a steeper angle of descent during landing.
Effect of the Flaps
Flap deflection of up to 15° primarily produces lift with minimal drag. The increased camber from flap deflection moves the center of pressure (lift) rearward, producing a nose-down force. The nose-down pitching moment, however, is offset by the airplane’s tendency to balloon up.
Flap deflection beyond 15° produces a large increase in parasite drag. Most high-wing airplanes tend to pitch nose up because the resulting downwash increases the airflow over the horizontal tail.
Types of Flaps
Plain flaps provide a simple means of changing the camber of the wing. A higher camber produces the same amount of lift at a slower airspeed.
Split flaps also produce the same amount of lift at a slower airspeed due to an increase in camber, but because of the turbulent air pattern produced behind the airfoil, it also creates more drag than the plain flap.
Slotted flaps have a gap between the wing and the flap. When the flap is lowered, high-energy air moves through the slot and over the flap’s upper surface. The slot’s high-energy air accelerates the upper surface boundary layer and delays airflow separation, providing a higher CL than the split or plain flaps.
Fowler flaps travel both aft and down on tracks ti increase both the camber of the wing and the effective wing area. This combination allows slower and more stable flight than any other flap.
Flap Operating Limitations
When the flaps are extended, the airspeed should be at or below the airplane’s maximum flap extended speed (VFE). If they are extended above this airspeed, the force exerted by the airflow may result in damage to the flaps. If the airspeed limitations are exceeded unintentionally with the flaps extended, they should be retracted immediately regardless of airspeed.
Trim Devices
Trim devices relieve the pilot from holding constant pressure on the flight control. An improperly trimmed airplane can cause tension and fatigue, distract the pilot, and contribute to abrupt and erratic flightpath changes.
A trim system typically consists of flight deck controls and small hinged devices attached to the trailing edge of one or more of the primary flight control surfaces.
Types of Trim Devices
Trim tabs are small, hinged surfaces on the trailing edge of an aileron, rudder, and/or elevator. Their purpose is to help relieve control pressures during straight-and-level flight and other prolonged flight conditions. This is accomplished by deflecting the tab in the direction opposite to that in which the primary control surface is moved. The force of the airflow striking the tab helps hold the primary control in position.
Balance tabs look and perform like trim tabs. The essential difference between the two is that the balancing tab is coupled to the control surface by a rod so that when the primary control surface is moved, the tab is automatically moved in the opposite direction. Airflow striking the tab counterbalances some of the air pressure against the primary control surface. Like trim tabs, they are adjustable.
Antiservo tabs work like balance tabs except that they move in the same direction as the trailing edge of the stabilator instead of moving in the opposite direction. They decrease the sensitivity of the stabilator by preventing the control surface from moving to a more fully deflection position. They also function as a trim device to relieve control pressure and maintain the stabilator in the desired position.
Servo tabs, or flight tabs, are similar to trim tabs but are moved in conjunction with a cockpit control. In some aircraft, it is the only control that is connected to the cockpit control. The force of the airflow on the servo tab moves the primary control surface. Servo tabs are used by some large aircraft as a backup control in the event of a hydraulic system failure.
Ground adjustable tabs are fixed, metal tabs mounted on the rudder of many small airplanes. They are bent on the ground to apply a trim force to the rudder. Trial and error determine the correct displacement. Adjustments usually are not considered a maintenance function, but pilots should first consult the airplane’s maintenance manual, if available, or a mechanic.