I've heard that the engines on a commercial air transports are typically mounted with 2 or 3 bolts. Is this true? If so, how big are these bolts and what are they made of?
I'm optimizing a bolted joint for my FEA class, and I'd like to check my results against reality.
PS: The Factor of Safety on such a bolt would be nice too!
On Sunday 4 October 1992, El Al Flight 1862 crashed into a suburb of Amsterdam after the right inboard engine separated from the wing and took part of the wing and the right outboard engine with it.
The separation was caused by fatigue fracture of the fuse-pins that connect the engine to the aircraft. The engines of the Boeing 747-200 are joined to the wing by four fuse-pins.
In the investigation report you can find detailed information about the construction that mounts the engine to the wing. Especially sections 1.6.3, 1.6.4 and 1.16 are of interest to you, although I recommend reading the entire report if you have the time.
Source: Raad voor de Luchtvaart, document AAR 92-11
Source: Raad voor de Luchtvaart, document AAR 92-11
You're correct that the engines are mounted to the aircraft using only a few bolts. Usually, the engines are mounted on the wing at three points (usually two forward and one aft) with shear (or fuse) pins, with one or two bolts at each point. The DC-10, for example 6 (later 8 pins), while the 747-200 has 6.
"DC-10 engine-pylon". Licensed under Public Domain via Wikimedia Commons.
The following figure shows the engine mounting in A350 XWB, in which the aft mounting point and the load struts can be clearly seen (the first box), in which the engine is attached with two bolts.
Image from airliners.net
The shear pins have a complex cross section and are designed in such a way that it shears off in case of an impact. For example, the following figure shows the design of (three different) shear pins used in Boeing 747.
Image from FAA overview of accident involving Israeli Airlines FLT 1862
The shear pin bolt size depends on the particular aircraft and the engine combination.
As for safety factor, they are designed to break off in case the (shear) loads on them cross a certain threshold. The engine-to-wing supporting structure is designed to release the engine when extreme forces are encountered, in order to prevent any structural damage to the wing that may impair the aircraft's ability to fly. As such, their design philosophy is different from safe load design and varies from aircraft to aircraft.
Incidents have happened where the engine has separated from the aircraft due to the large forces experienced.
