A Delta Connection Bombardier CRJ-900 with two rear-mounted engines. Photo by Andrew Cohen CC 2.0

Ever noticed that some airliners have a sleek-looking T-tail and engines mounted at the rear? This design is found most commonly on smaller aircraft such as regional and private jets. Some people hate it, while others think it looks sleek and stylish. Now you may be wondering why some aircraft have engines at the back and others don’t. There are actually several reasons why aircraft manufacturers would place engines at the rear on aircraft like the Boeing 717, 727, McDonnell Douglas MD-80, and Bombardier CRJ series to name a few.

Under-wing clearance

Agreeably the main reason why an aircraft would have its engines mounted at the rear is because there isn’t enough clearance underneath the wing to mount the engines without having to install massive landing gear. Take a look at the image of the Bombardier CRJ-900, and notice how little space there is underneath the main wings. This would explain why rear-mounted engines are common only on smaller aircraft. If you were to install the engines underneath the wings, chances are they would be able to touch the ground, which is for obvious reasons completely unacceptable. Therefore, the only sensible location for the engines would be at the back of the fuselage.

Noise considerations

An obvious benefit of rear-mounted engines is the reduction in engine noise, which is disturbing to and disliked by passengers. Moving the engines back to the rear and away from the main cabin would reduce the amount of noise making its way into the cabin and increase passenger comfort. While this wouldn’t necessarily be the primary reason to have rear-mounted engines, it is definitely a consideration.

Lower risk of damage

The engines are always the most expensive part of an airliner, and therefore airlines want to keep them safe to prevent costly maintenance bills. Wing-mounted engines are more likely to suffer damage than aft-mounted engines, as they are closer to the ground and can collide with ground objects or suffer scrapes during landing. Aft-mounted engines are farther from the ground and somewhat shielded by the wing and fuselage.

Cons of rear-mounted engines

Every pro has a con that comes with it, and thus aft-mounted engines also have their drawbacks. In case you haven’t noticed, all aircraft with¬†engines at the back also have T-tails. This is because there wouldn’t be enough space for a standard vertical stabilizer if the engines are also at the back. The issue with T-tails is that they can potentially enter something called a deep stall, which is a type of stall that pilots cannot recover from.

Illustration of a deep stall by Wikimedia user GRAHAMUK CC 3.0

A deep stall typically occurs when the aircraft pitches up at too high an angle of attack. During a deep stall, airflow to the tail-mounted stabilizer is disrupted by the main wing, rendering the elevators useless and disabling pitch control. This phenomenon led to the crash of British European Airways Flight 548, which was operated by an aircraft with a T-tail. Scary as it may sound, a deep stall isn’t as big of an issue as you may think, and aircraft manufacturers make it so that it is extremely hard to enter one, but nevertheless possible.

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