Modern air travel is an engineering marvel. However, there are still
significant differences between aircraft. One of the major impacts on the capabilities
results from the placement of the most expensive parts of the plane.
Let’s take a closer look at the exceedingly rare rear-mounted engine.
Today, you can mostly find aft-mounted engines on private business jets and small
regional jets like the CRJ series. But why did tail-mounted engines go
out of fashion for commercial jets in favor of the under-wing engine design?
Tail-mounted engines work much as any powerplant. They take in lots of air, compress it,
add fuel and ignite it. They then expel the resulting gases out the back at a very high speed.
This force then propels the aircraft forward. However, one of the differences between a
tail-mounted and a wing-mounted engine is that the fuel source has to travel much further.
This is because the fuel tanks are still located in the wings, even on a rear-engined plane.
In turn, this means that there is a need for more piping to get the fuel to where it needs to go.
It could also potentially become an issue if there’s a problem with
the aircraft’s fuel pump system. As the fuel source is located below the engines,
the tail-mounted engines cannot make use of gravity to self-feed.
Meanwhile, one of the advantages of having rear-mounted engines is that they are less
susceptible to foreign object damage (Fod). This specifically entails debris such as gravel,
grass, and dirt that could be sucked up from the ground during take-off and landing,
as the engines sit much higher than their pod-under-wing counterparts.
Not having to accommodate massive engines underneath the wings also allows for the fuselage
to sit lower to the ground. This was, of course, highly beneficial at a time when airports did not
enjoy all the modern comforts such as jet bridges and luggage belt loaders. It is also one of the
reasons business jet producers still prefer this design as it allows planes to land at much smaller
airports with less developed infrastructure. Rear engines can also more easily use reverse
thrust to taxi backward, not needing the assistance of an aircraft tug vehicle.
This ‘powerback’ procedure was used by carriers such as American Airlines with their McDonnell
Douglas MD-80s as late as the mid-2000s, lack of rear-view mirrors notwithstanding.
Other than differences in on-ground characteristics and performance, having
the engine in a different location on the aircraft naturally affects the aerodynamics of the plane.
The nose of a rear-engine aircraft pushes down during take-off. This means that the handling
of the aircraft by the pilots also differs. Furthermore, as the engines are located where
the horizontal tail fin would be on an under-wing plane, the aircraft is designed with a so-called
T-tail, with the horizontal stabilizer located at the top of the tail. This places them out of
the way of the disturbed airflow behind the engines. It also allows for greater
control at lower speeds, again helpful for short landing and take-off capabilities.
The structure of the aircraft also needs to be altered to make sure that the wings still sit
at the center of gravity. This is why aircraft like the MD-80 and Boeing 717 have much longer
fuselages in front of the wings, as opposed to the more symmetrical A320s and the like. It also
requires a more rigid airframe, which adds weight, which, in turn, causes higher fuel consumption.
The simple answer to why the rear-mounted engine model fell out of vogue is cost and efficiency.
As twinjets have grown larger, so have their engines. Indeed, it would be impossible to mount a
General Electric GE9X at the rear of an aircraft! But even for smaller jetliners, the easy
maintenance access of the wing-mounted engine wins out over the harder-to-get-to fuselage-mounted
specimens. More time at MRO facilities means higher costs and more downtime for the aircraft,
making it unavailable to the airline for longer. Which was the last rear-engined aircraft you
flew on? Leave a comment below and share your experience!