How Engines Are Attached To Aircraft

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. It is not as simple as just bolting the engines onto the wing as securely as possible. There are important safety considerations that change this.

No matter how large the engine, the attachment method under the wings is similar. Photo: Airbus

Placing engines in pods

Engines on most commercial aircraft are placed in pods under the wing. This 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 but exposes the wings to more risk of foreign object damage.

The 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 737 MAX and its podded engines. Photo: Belavia

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Engines are attached to a pylon

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. This is vital in the event of an engine fire to protect the wing (and fuel stored in it) until the engine fire is extinguished.

You can see the pylon structure up close on this ANA Boeing 787. Photo: Getty Images

Connecting the engine pod to the pylon

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 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 (on the 737, at least, they are made of a superalloy, nickel alloy 718), but this is a carefully balanced and calculated construction. There is a good discussion of these strengths and figures on Stack Exchange.

This sketch shows the typical pylon and pod attachments (this is for a DC-10 aircraft, but the principle is the same for all jets). Image: Federal Aviation Administration via Wikimedia

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 to them remaining attached (a significant fire risk) or for the forces transferring to the wing and causing it to break off.

Why not incorporate the engine into the wing?

This also raises the question of why the engines are not incorporated into the wing. This was the case with some early aircraft (including the first jet aircraft, the de Havilland Comet). Such a design may seem stronger and more streamlined, but it has several problems and has been dropped in commercial jets.

The de Havilland Comet had engines mounted within the wings. You have to admit it looks great! Photo: Ian Dunster via Wikimedia

Firstly, there is the major issue of safety. An engine fire within the wing would potentially be more devasting than in a distant, podded engine. In a podded engine, the fire can hopefully be extinguished before it causes any wing damage. Placing the engines in the wing also uses space that is needed for fuel. Also, as turbofan engines have improved and increased in size, it would just not be practical to place them into the wing structure.

If you would like to read more about aircraft engines, take a look at our articles on how to start jet engines, how to wash them, or where they usually have spirals painted on them. Or for the largest engines in use today, see this article.

The GE9X on the new 777X is the largest engine currently. Photo: Dan Nevil via Wikimedia

Would you like to share any thoughts or more details on engines, housing, and attachment? It’s not a topic we discuss often, so let us know your thoughts in the comments. 

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