A T-tail is an aircraft tail stabilizer arrangement in which the horizontal surfaces (tailplane and elevators) are mounted to the top of the vertical stabilizer. Traditionally, the horizontal control surfaces are mounted to the fuselage at the base of the vertical stabilizer. The resulting arrangement looks like a T when viewed from the front or back, hence the name.
The tailplane surfaces are kept well out of the airflow behind the wing, giving smoother flow, more predictable design characteristics, and better pitch control. This is especially important for planes operating at low speed, where clean airflow is required for control. deHavilland Canada's line of larger STOL aircraft all use this arrangement for this reason.
The effective distance between wing and tailplane can be increased without a significant increase in the weight of the aircraft. The distance between the two planes gives the moment, or "leverage", by which the tailplane can control the aircraft's pitch attitude - with a greater distance, smaller, lighter tailplanes and elevators can be used.
The tail surfaces are mounted well out of the way of the rear fuselage, permitting this site to be used for the aircraft's engines. This is why the T-tail arrangement is also commonly found on airliners with rear-mounted engines, including trijets. The Douglas DC-9, Bombardier CRJ200, Boeing 727, Fokker 100, Vickers VC-10, Hawker Siddeley Trident, BAC 1-11, Tu-134, Tu-154, Il-62, and MD-80 all used the T-tail for this reason.
The horizontal stabilizer is kept farther away from the ground, which helps reduce damage to it by objects on the ground when taking off or landing. This is not normally a large concern for full scale ('real') planes, but for model airplanes it can be, especially if the plane has no landing gear. This benefit is also shared by V-tails and cruciform tails.
The aircraft will tend to be much more prone to a dangerous deep stall condition, where blanking of the airflow over the tailplane and elevators by a stalled wing can lead to total loss of pitch control. The F-101 Voodoo suffered from this throughout its service life. For similar reasons, T-tailed aircraft can be much more difficult to recover from a fully-developed spin.
The vertical stabilizer must be made considerably stronger and stiffer to support the forces generated by the tailplane. Unless expensive composite materials are used, this inevitably makes it heavier as well.
The T-tail configuration can cause several maintenance concerns as well. The control runs to the elevators are more complex and elevator surfaces are much more difficult to casually inspect from the ground.
Because of concerns about being able to clear the tail, the first high-speed aircraft with a T-tail, the Lockheed F-104 Starfighter, was at first fitted with a downward-firing ejector seat. For later models of this aircraft, capabilities of ejection seats improved, so it was changed to an upward-firing one, to overcome problems in low-altitude escapes.
Due to a lack of airflow over the elevator from a forward mounted engine (piston or turboprop), low speed control is reduced and low speed operation is more difficult for aircraft not designed for low speed operation.