Limits for latereal acceleration and Q about vert. accel

[TheSerb]

Hi,
Are planes certified with regards to lateral acceleration? If so, what are those limits?

I assume that standard G limits that are given for design limit are for vertical acceleration only.

Also are vertical accelerations for dynamic load or static load? i.e. is it designed to sustain cyclic loads with certain frequency when within those deign limits?

Thanks for any clarifications!

[Mad (Flt) Scientist]

There are structural design manoeuvres specified in the regs which generate loads in all axes, but only those in the vertical axis are predicated upon specific 'g' levels. The assumption is basically that crews will not induce sustained lateral 'g' on themselves, and so only the dynamic manoeuvres are considered.

Cyclical loads are considered in the context of fatigue life calcs, and are in terms of a best estimate of normal operating loads, whereas the design manoeuvres are intended to be extreme/worst case conditions. Which is why you can do a design manoeuvre "once", but fly normally for tens or hundreds of thousands of hours ... in theory.

If you get sophisticated about fatigue life, you end up considering different missions as imposing different levels of cyclic loads, and consuming different amounts of fatigue life per flight hour. The RAF "Fatigue Index" system assigns different weightings per flight hour to better manage fleet fatigue. So a air combat training flight counts for more fatigue than a navigation flight of the same duration, say.

[Brian Abraham]

As a pilot about the closest you get to what could possibly be a lateral acceleration consideration (Mad (Flt) Scientist is the man to comment) is when pulling "G" while at the same time rolling. One aircraft that comes to mind the "G" limit while rolling was reduced by one third.

[Mad (Flt) Scientist]

Indeed; I was mainly thinking "civil" in my reply, but yes, the rapid roll under 'g' manoeuvre will induce significant lateral loads and accelerations. I can also think of a type - it might be the same one - where that manoeuvre imposed quite significant limitations.

[alf5071h]

I participated in tests on a HS 748 which required a 0.25 lateral ‘g’ increment when turning off the runway at high speed. This was is support of accident investigation and fatigue measurements. Although the condition was easily achievable with nose wheel steering, the resultant feel in the flight deck was horrendous, and thus the manoeuvre tended to be self limiting. The tests also involved very rough strip operations. I was pleased to report that there was no resultant damage or concerns, except for a flat tire from a flint puncture on the gravel strip.

[Brian Abraham]

I can also think of a type - it might be the same one - where that manoeuvre imposed quite significant limitations

Mad (Flt) Scientist, Could you put some flesh on the bones, so to speak. Could imagine it be might be an issue of some magnitude on types with podded engines eg 747.