Empennage (Tail) Structure

Overview

The empennage — the tail assembly of the aircraft — comprises the horizontal stabilizer, elevator, vertical stabilizer (fin), and rudder. These surfaces provide the longitudinal (pitch) and directional (yaw) stability and control essential for safe flight. While smaller in area than the main wings, the empennage operates under significant aerodynamic loads and must do so throughout the flight envelope, including during high-angle-of-attack manoeuvres, crosswind landings, and engine failures that impose large yawing moments. The structural design of the empennage therefore requires careful attention to fatigue, aeroelastic behaviour (flutter prevention), and damage tolerance.

Modern commercial aircraft empennage structures have progressively adopted composite materials. The Airbus A300, certified in 1974, was the first commercial aircraft with a composite primary structure — specifically the vertical fin. Today, virtually all modern commercial aircraft empennage components are constructed primarily from CFRP.

How It Works

The horizontal stabilizer generates a downward (or occasionally upward) aerodynamic force that balances the pitching moment created by the wing and fuselage, maintaining the aircraft in trimmed level flight. On most modern jets, the entire horizontal stabilizer pivots about its root to provide pitch trim — this trimmable horizontal stabilizer (THS) configuration is more efficient than a fixed stabilizer with a large elevator because it allows the entire surface to operate at its most aerodynamically efficient angle. The elevator then provides additional pitch control authority for manoeuvring.

The vertical stabilizer provides directional stability — the aircraft's tendency to align with the relative wind — and the rudder provides yaw control, essential for correcting engine failure asymmetry, crosswind landings, and coordinated turns. The fin must be sized for the critical engine-failure case at the most forward centre of gravity, which determines the required fin area for most large commercial aircraft.

Key Components

• Vertical fin box: Two-spar CFRP box structure providing the primary load path for lateral aerodynamic loads and torsion; attached to the rear fuselage through large metal root fittings.
• Horizontal stabilizer box: Similar two-spar structure; on most jets, pivots about the rear spar to provide trim; actuated by jackscrews driven by hydraulic or electric motors.
• Elevator: Hinged trailing-edge surface on the horizontal stabilizer, driven by hydraulic actuators; provides pitch control.
• Rudder: Hinged trailing-edge surface on the vertical fin, driven by hydraulic actuators; provides yaw control.
• Rear fuselage frames: Heavy structural frames transmitting empennage loads into the fuselage; the rear pressure bulkhead and adjacent frames are particularly critical.
• Fairings: Composite fairings at the wing–body junction and fin–fuselage junction, reducing aerodynamic drag.

Aircraft Applications

The T-tail configuration — where the horizontal stabilizer is mounted at the top of the vertical fin rather than at the base — is used on some aircraft (MD-80, ATR 72) to place the horizontal tail in undisturbed airflow above the wing wake. However, T-tails introduce a phenomenon called deep stall, where at very high angles of attack the horizontal tail enters the wing wake and becomes aerodynamically ineffective. The Boeing 727 and DC-9/MD-80 families manage this risk through stick-pusher systems that prevent the aircraft from reaching the critical angle of attack. Conventional low-tail configurations (Boeing 737, Airbus A320) avoid this issue entirely.

Advantages and Limitations

Advantages: CFRP empennage structures offer significant weight savings (the A310 composite fin saved approximately 400 kg versus aluminum); superior fatigue resistance is particularly valuable for the fin, which is subjected to continuous small lateral loads from atmospheric turbulence; corrosion immunity is important in the tail where drainage can be difficult; and composite construction allows smooth aerofoil contours with minimal manufacturing steps.

Limitations: The 2001 American Airlines Flight 587 accident, in which the A300-600's composite vertical fin separated from the fuselage following excessive rudder inputs, highlighted that composite empennage structures can fail differently than metal designs — without the plastic deformation and warning signs that accompany metal failure. Subsequent regulatory action established limits on pilot rudder input cycling. Damage tolerance inspection of composite tail structures requires specialized ultrasonic and thermographic techniques.