**Update: 05/10/21 @ 1200 – Updated aircraft types with T-tail **
Take a close look at aircraft tails, and you will notice one major difference between some aircraft. While most modern airliners have a lower tailplane (or horizontal stabilizer), some smaller and regional aircraft have a higher so-called T-tail. This has relevance for the aircraft it is used on.
Tails and stabilizers
Firstly, a quick recap on the purpose of the aircraft tail. The tail provides stability and control for the aircraft in flight.
The vertical tail fin (with the airline logo on it) is technically called the vertical stabilizer. In addition to this, there is a horizontal stabilizer. This is the small wing-like protrusions from the main tail, or rear of the fuselage. The horizontal stabilizer is where you will find the elevators – they control the pitch of the aircraft. The tail, of course, also houses the rudder.
In all large commercial jets, the horizontal stabilizers are the base of the tail, in line with the fuselage. However, on many other aircraft, you will find them at the top of the tail fin. The shape of this gives rise to its name of T-tail.
Which aircraft have T-tails
To start with, most current Boeing and Airbus aircraft do not have this. The exceptions are the Boeing 717 and the 727. The 717 is formerly, of course, the McDonnell Douglas MD-95. Both of these aircraft have fuselage-mounted engines.
Historically, many jets with fuselage-mounted engines had T-tails. This includes the BAC One-Eleven, the DC-9, MD-80, and MD-90. Vickers VC-10, and the Fokker F28, F70 and F100. The Tupolev Tu-135 and Tu-154, and Ilyushin Il-62 jets all have T-tail as well.
Many current small and mid-size jets still have T-tails. Such as the Bombardier CRJ Series, Embraer ERJ, and the BAe 146 / Avro RJ. Also, several business jet series (including Learjet and Gulfstream) have T-tails.
And looking even larger, a number of military and transport aircraft have used them. Including the Lockheed C-5 Galaxy, Boeing C-17, Airbus A400M, and Illyushin Il-76. For transports, the high tail often serves also for extra clearance when loading.
Purpose of the T-tail
Placing the horizontal stabilizer at the bottom or top of the tail makes little difference in theory. Both positions will enable the tail to function correctly and allow the elevators to do their job.
The reason why on some jets it is placed higher is to do with airflow. Placing them higher on the tail keeps them out of the disturbed airflow behind the wing and engines. With fuselage-mounted engines, this is always necessary.
A high horizontal stabilizer also aids short-field performance. The disturbed airflow over a lower stabilizer can make control more difficult at low speeds. Better control at lower speed is an important part of short take-off and landing capability.
Lowering it on modern jets
Why then do modern large jets not have T-tails? There are several reasons. Firstly, a lower horizontal stabilizer is simpler to install and maintain, and the vertical tail does not need to be as strong. And secondly, there is no real need to boost the short-field performance with more powerful engines and operation from standard runways.
But perhaps most important is the avoidance of a deep stall. At a high angle of attack, disrupted airflow over the wings flows over the high horizontal stabilizer, leaving the aircraft with no pitch control. This was experienced with a BAC One-Eleven test aircraft in 1963. Since then, modifications were introduced (including stick shakers and warnings of an approaching stall) on other aircraft that used a T-tail. But lowering it would avoid this problem entirely.
There is a lot more to discuss about tail design and development. A full discussion of aerodynamics and the performance of different aircraft types is beyond the scope of this article! Feel free to discuss more specific details in the comments.