Empennage: PHAK Chapter 2

Handbook Reference

PHAK Ch 2

2.empennage. Empennage

The empennage, often called the tail section, includes the entire tail group of an airplane. On a conventional configuration, it consists of fixed surfaces — the vertical stabilizer and horizontal stabilizer — and movable surfaces — the rudder, the elevator, and one or more trim tabs. The empennage provides stability and control about two of the three axes of flight: the vertical axis (yaw) and the lateral axis (pitch).

Fixed Surfaces

Vertical stabilizer (fin): Mounted vertically on the aft fuselage, the vertical stabilizer provides directional (weathervane) stability about the vertical axis. When the airplane yaws, the relative wind strikes the side of the fin, generating a side force that streamlines the nose back into the relative wind.
Horizontal stabilizer: Mounted horizontally on the aft fuselage, it provides longitudinal stability about the lateral axis. Because most general aviation airplanes are designed with the center of gravity (CG) forward of the center of lift, the horizontal stabilizer typically produces a downward (tail-down) force in cruise to balance the airplane in pitch.

Movable Surfaces

Rudder: Hinged to the trailing edge of the vertical stabilizer and controlled by the rudder pedals, the rudder controls yaw — rotation about the vertical axis. Pressing the right pedal deflects the rudder right; the resulting aerodynamic force pushes the tail left and yaws the nose right. The rudder is used to coordinate turns, counter adverse yaw, maintain runway alignment in crosswind takeoffs and landings, and counter asymmetric thrust or P-factor.
Elevator: Hinged to the trailing edge of the horizontal stabilizer and controlled by fore-and-aft movement of the control yoke or stick, the elevator controls pitch — rotation about the lateral axis. Aft yoke deflects the elevator up, increasing the tail-down force and pitching the nose up; forward yoke does the opposite.
Trim tabs: Small auxiliary surfaces hinged to the trailing edge of a primary control (usually the elevator). The pilot adjusts the tab from the cockpit so that, in steady flight, the airplane holds the desired attitude with no control pressure. The tab deflects opposite the desired control deflection — for example, trimming nose-up moves the trim tab down, which forces the elevator up.

Alternative Empennage Designs

A second common arrangement is the stabilator, a one-piece horizontal surface that pivots about a central hinge and acts as both stabilizer and elevator. Because a stabilator is more sensitive than a conventional elevator, it incorporates an anti-servo tab on the trailing edge. The anti-servo tab moves in the same direction as the stabilator, increasing the control force the pilot feels and providing the proper "feel" while also serving as a trim tab.

Other variations include:

T-tail: Horizontal stabilizer mounted atop the vertical fin, keeping the tail out of the propeller wash and wing wake. T-tails offer cleaner airflow at cruise but can be susceptible to a deep stall at high angles of attack, where the wing wake blankets the horizontal tail and reduces pitch authority.
V-tail: Two surfaces set in a V configuration combine the functions of horizontal and vertical stabilizers; the movable ruddervators combine elevator and rudder action.
Cruciform tail: Horizontal stabilizer mounted partway up the vertical fin.
Twin tail: Two vertical stabilizers, often used to keep the rudder in the propwash on twin-engine designs.

Trim Systems

Trim systems reduce pilot workload by relieving control pressures. Common types include:

Trim tabs — small adjustable tab on the elevator (most common in light singles).
Balance tabs — move opposite the control surface to reduce the force needed to deflect it.
Anti-servo tabs — move with the control surface to increase feel and prevent over-control (used on stabilators).
Servo tabs — pilot moves the tab directly; aerodynamic force on the tab moves the primary control (typical on larger aircraft).
Ground-adjustable tabs — bent on the ground; used on rudders of many trainers to compensate for left-turning tendencies in cruise.

Function in the Three Axes of Flight

The empennage is responsible for control about two of the three axes:

Pitch (lateral axis) — controlled by the elevator or stabilator.
Yaw (vertical axis) — controlled by the rudder.
Roll (longitudinal axis) — controlled by the ailerons on the wings, not the empennage.

A preflight inspection of the empennage should confirm freedom of movement and proper deflection of all hinged surfaces, security of hinges and attach points, condition of any static wicks, condition of trim tabs and their actuating rods, and the absence of dents, skin wrinkles, loose rivets, or obstructions in control surface gaps.

Oral Exam Questions a DPE Might Ask

Q1What surfaces make up the empennage and what do they control?

The empennage includes the vertical stabilizer with its rudder (controls yaw about the vertical axis), the horizontal stabilizer with its elevator (controls pitch about the lateral axis), and any trim tabs. It does not control roll — that's the ailerons on the wings.

Q2What's the difference between an elevator and a stabilator, and why does a stabilator have an anti-servo tab?

An elevator is a hinged surface on a fixed horizontal stabilizer, while a stabilator is a one-piece surface that pivots as a whole. Because the stabilator is more sensitive, it uses an anti-servo tab that moves in the same direction as the stabilator to increase control feel and prevent over-controlling, and it also functions as the trim tab.

Q3How does a trim tab work to relieve elevator control pressure?

The trim tab is a small surface on the trailing edge of the elevator that the pilot adjusts from the cockpit. It deflects opposite the desired elevator movement — for nose-up trim, the tab moves down, which aerodynamically forces the elevator up and holds the pitch attitude with no control pressure from the pilot.