We are all used to seeing two or four engines mounted in pods under the aircraft wing.
This has become the standard engine attachment for all large commercial aircraft. Their attachment,
though, is interesting and complex, and isn’t as simple as just bolting the engines onto the
wing as securely as possible. So let’s take a look at what’s involved in this process...
Placing engines in pods under the wing has several advantages. Firstly,
it provides wing bending relief. Weight in the wings (including fuel and engines) counteracts
the effect of lift bending the wingtips upwards. It also allows for easier access and maintenance.
One downside to this, however, is that it exposes the wings to more risk of foreign object damage.
Engines are usually mounted slightly forward of the wing to help prevent flutter of the wing-
this also allows the overall wing structure to be made lighter.
Some smaller aircraft have engines mounted on the fuselage tail, including, for example,
the Embraer ERJ family and the COMAC ARJ21 regional jet.
The podded engine is not attached directly to the wing but to a pylon.
Designs vary, of course, between aircraft types, but the principle remains similar.
The pylon is fixed into the wing structure, with a solid and very secure fitting.
The pylon creates some distance between the engine and the wing, which is vital in the event of an
engine fire to protect the wing (and fuel stored in it) until the engine fire is extinguished.
The engine pods are then connected to these pylons using bolts. Pods are usually connected at just
two points: The top of the fan frame and the top of the turbine frame.
These connections are designed to take the massive forces of the engine,
both the forward thrust force and the downward force of the engine’s weight.
The critical part of this design is to safely support the maximum forces experienced,
but not to take too much force. The attachment is, in effect, not as strong as it could be.
The bolts that attach to the pylon structure are incredibly strong, but this is a
carefully balanced and calculated construction. These bolts will support forces significantly in
excess of the maximum expected forces, even in the event of very hard landings or extreme turbulence.
But they would shear in the event of extreme forces.
If the engines come into contact with the ground when landing (for example, in a landing without
gear or a runway overrun), the forces experienced would break the bolts and cause the engines to
break off from the pylons. This is preferable over engines remaining attached, which is a significant
fire risk, or for the forces transferring to the wing and causing it to break off.
This also gives us the reason why modern aircraft engines are not
incorporated into the wing. This was the case with some early aircraft,
such as the de Havilland Comet. Such a design may seem stronger and more streamlined. However,
this design has significant safety drawbacks as highlighted in the benefits of podded engines.
Additionally, placing engines in the wing also uses space that is needed for fuel.
Finally, as turbofan engines have improved and increased in size,
it would just not be practical to place them into the wing structure.