Lesson 2.4 Primary Flight Controls & the Three Axes

Primary Flight Controls & the Three Axes

In the earlier lessons we studied the forces that act on the airplane and how the wing produces lift. Now we examine how the pilot actually controls the airplane in flight.

Reference:

FAA Pilot’s Handbook of Aeronautical Knowledge (PHAK)

Chapter 6 – Flight Controls

https://www.faa.gov/sites/faa.gov/files/08_phak_ch6.pdf

The Three Axes of the Airplane

An airplane moves around three axes that intersect at the center of gravity:

Longitudinal Axis (nose to tail) Movement around this axis is called roll. Controlled by the ailerons.
Lateral Axis (wingtip to wingtip) Movement around this axis is called pitch. Controlled by the elevator (or stabilator).
Vertical Axis (vertical through the center of gravity) Movement around this axis is called yaw. Controlled by the rudder.

It is important to remember and understand these concepts. The FAA tests on this, particularly the relationship of each set of flight controls which give the pilot control around which axis. i.e. "Control about the Longitudinal axis is controlled by the ailerons. This provides Lateral Stability". Examiners frequently query students about the points 4,5, and 6 listed here. Examiners are good instructors. A query on any of these 6 topics will lead into the rest of the examiners questions pertaining to the flight control sections.

Longitudinal Axis > Lateral Stability
Lateral Axis > Longitudinal Stability
Vertical Axis > Directional Stability

Flight Controls: Primary & Secondary

When we step into the cockpit, we need to understand exactly how our inputs translate to aircraft movement through the air mass. The FAA breaks flight controls into two distinct categories: Primary and Secondary. For the checkride, you must know the difference.

Primary Flight Controls are required to safely control the aircraft. They directly manipulate the airplane about its three axes of flight. If a primary control fails, your ability to maintain basic control is immediately compromised.

Secondary Flight Controls are there to improve performance, reduce pilot workload, or optimize the aircraft's flight path. They include things like flaps, leading-edge devices, spoilers, and trim systems. They make the airplane easier or safer to fly, but they are not what actively steers the wings through a dynamic change of attitude. Trim tabs are used to take the physical load off of the controls in the cockpit. They allow the pilot to trim for the correct amount of rudder, elevator, and aileron input for each particular phase of flight. This takes the load off of the control yoke.
Elevator trim is what is most frequently used. With an airspeed change, power change, configuration change (flaps, gear) and as fuel burns off, the pilot will be adjusting the elevator/stabilizer trim. This is very evident in larger aircraft. In light aircraft the pilot is frequently adjusting the elevator trim.
In large multi-engine aircraft aileron trim also comes into steady play in the simulator as engines fail. Rudder trim also becomes used throughout this scenario. Yaw, and watching the inclinometer are a steady part of an engine out scenario. "step on the ball". :)

Adverse Yaw

When the ailerons are deflected, the downward-moving aileron produces more lift and also more drag than the upward-moving aileron. This extra drag pulls the nose in the opposite direction of the intended turn. This is called adverse yaw. The pilot counters adverse yaw with rudder.

Lesson 2.4 Primary Flight Controls & the Three Axes

The Three Axes of the Airplane

Adverse Yaw

Key Points to Remember