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A T-tail is an empennage configuration in which the horizontal surfaces (tailplane and elevators) are mounted to the top of the vertical stabilizer. The resulting arrangement looks like the capital letter T when viewed from the front or back, hence the name. This differs from the traditional configuration in which the horizontal control surfaces are mounted to the fuselage at the base of the vertical stabilizer.
The tailplane surfaces are kept well out of the airflow behind the wing, giving smoother flow, more predictable design characteristics, and better pitch control (excepted in tractor propeller case). 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. T-tail configuration also allows high performance aerodynamics and excellent glide ratio as the horizontal tail empennage is less affected by wing slipstream, has a better effective aspect ratio, less interaction drag than a cruciform tail and a more efficient vertical tail (the horizontal tail plate effect increases the lift slope of the vertical tail). Therefore the T-tail configuration is especially popular on gliders.
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, Embraer ERJ 145, Boeing 717, Boeing 727, Fokker 100, Vickers VC-10, Hawker Siddeley Trident, BAC 1-11, Tu-134, Tu-154, Il-62, and McDonnell Douglas MD-80, McDonnell Douglas MD-90, all used the T-tail for this reason.
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.
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. The loss of Alaska Airlines Flight 261 was directly attributed to lax maintenance due to the complexity of the T-tail.
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.
See also 
- Hoerner and Borst, Fluid Dynamic Lift, Directional Characteristics, T-tail page 13-11
- "Gloster Javelin - History". Thunder & Lightnings.