Why does taxiing produce large lateral loads on the 747's engine pylons? 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.
The slip-skid ball indicates the ratio of "sideways" apparent acceleration to "upwards" apparent acceleration, in the aircraft's own reference frame.  In normal constant-airspeed wings-level flight (i.e. not looping or pulling out of a dive), or while taxiing, the "upwards" apparent acceleration is 1 G, so the sideways deflection of the slip-skid ball is a direct indication of the total lateral force acting on the aircraft and the total lateral acceleration experienced by the aircraft.

Turning at a high rate while taxiing will displace the slip-skid ball far to one side.  During high-speed taxi, and the portion of the takeoff and landing where the wheels are on the ground, steering corrections will typically involve low turn rates but may still cause a large displacement of the slip-skid ball due to the high speed involved.  In either case, the large displacement of the skip-skid ball indicates that a strong lateral force is being transmitted to the aircraft, including to the engines through the engine pylons. The fundamental origin of this lateral force is the sideload generated by the tires.

But we can't assume that in actual flight, a similar displacement of the slip-skid ball indicates that a similar sideload is being exerted on the engine pylons.

In actual flight, during a sideslip, the tires are no longer involved.  Instead, the sideways airflow interacts with the various surfaces of the aircraft to generate the aerodynamic sideforce that displaces the slip-slid ball.

Therefore, in flight, the lateral inertial loading created by the mass of the engine whenever the slip-skid ball is displaced, is at least partly offset by the aerodynamic sideforce generated by the engine nacelle.

Consider an aircraft banked to the left, with the slip-skid ball displaced to the left, showing that the airflow is blowing from left to right across the aircraft.  (A "yaw string" would blow toward the right.)  Which is generating more aerodynamic sideforce per unit mass-- the engine nacelles (including the engines contained within), or the rest of the aircraft?  

If the former, the nacelles will be forced toward the right relative to the rest of the aircraft.  If the latter, the nacelles will be forced toward the left relative to the rest of the aircraft.  If the nacelles are generating exactly the same aerodynamic sideforce per unit mass as the rest of the aircraft, then the nacelles will not be forced either to the left or the right relative to the rest of the aircraft, so the pylons will not experience any lateral force at all.

Regardless of which of these three scenarios actually apply, we can be sure that unless the total G-load is quite high, for a given displacement of the slip-slid ball the lateral load on the engine pylon is higher if the aircraft is taxiing than in actual flight.