While reading through a bunch of old AARs recently, I found (in the context of a 1993 747 in-flight engine separation) this interesting tidbit:
[...] Boeing engineers report that the modifications will increase the pylon's vertical and longitudinal strength. However, the modification will provide a slight, if any, increase in the structure's lateral load-carrying strength. Additionally, it was provided by Boeing engineers that the greatest lateral loads on the pylons normally occur during taxiing. [Page 40 of the PDF of the aforelinked AAR, numbered as page 33; emphasis added.]
I'm having some trouble seeing how taxiing would be capable of generating any significant lateral loads on a 747's engine pylons; assuming that one is not attempting to drift one's 747, nor taking it excessively close behind another jet running its engines up for takeoff, taxiing it at a reasonable taxi speed should not produce any large lateral accelerations that would make the engines want to swing on their pylons, nor any significant lateral airloads that would tend to push the engines sideways relative to the wing.
Intuitively, one would expect that the situations placing the highest lateral loads on the engine pylons of a 747 (or, for that matter, any jet with engines mounted on vertical pylons1) to be those involving one or both of the following:
- Large, abrupt accelerations from side to side (such as during severe turbulence, as experienced by [for instance] the aforementioned 747 whose engine separated in midair).
- Flight at large sideslip angles (in which case the direction of the airflow experienced by the engines and pylons would be out of line with the longitudinal axis of said engines and pylons, generating considerable, though steady, lateral airloads on the engine-pylon assemblies).
So why would normal taxiing produce such large lateral loads on the 747's engine pylons?
1: For a jet with horizontal engine pylons (such as a DC-9 or CRJ-1000), one would instead expect the highest shear loads2 placed on the pylons to be experienced during abrupt pitch changes, high-angle-of-attack flight, large vertical gusts, or (in the mostly-hypothetical case of a jet with wingtip-mounted engines) high-roll-rate manoeuvring.
2: The loads that try to snap the pylon in two, rather than move the engine longitudinally or stretch/compress it out of/into the pylon.