View Full Version : X-wind limitation

7th Jun 2019, 10:14
I am unable to figure out the reason behind the following X-wind limitation (for a twin jet):

When operating under MEL for 1 thrust reverser INOP, a maximum X-wind of 25 knots is defined for T/O and LDG on a wet RWY; this X-wind limit is more restrictive than the OEI LDG X-wind limit.

Does anybody have an idea what could be the reason?

Mach E Avelli
7th Jun 2019, 13:53
Not knowing which aircraft you refer to, the way you ask the question, the OEI limit does not specify wet runway?
So perhaps the higher limit could assume dry runway.
Also, for that type, is OEI landing flap less than normal, hence higher Vref, hence
less adverse effect of crosswind?

7th Jun 2019, 14:28
Perhaps ,
Firstly ,
" Wet " performance is based on use of Reverse Thrust... ( if it allows one u/s, perhaps EFB based calculator )
Also ,Twin Jets with wing underslung engines have a tendency to turn into the crosswind if any residual drift is left on , and High Reverse power is used... There used to be diagrams in the FCTM's that show the fancy Vector diagram of the forces involved.
Solution to the problem... cancel to Reverse Idle and revert to normal instinctive Rudder / Brake use.
Your 25 Knot Crosswind limit for Single Reverse use is possibly to reduce the chances of this forced Asymmetry
causing / increasing the Drifting tendency.... ( depending which side Reverser U/S verses crosswind direction )

7th Jun 2019, 14:48
Possibly because with a thrust reverser inop you still have thrust coming out the engine with the reverse inop. This adds to the yawing moment of the aircraft. With the engine inop there is no thrust.

Mad (Flt) Scientist
7th Jun 2019, 22:53
MEL cases must be "fully certifiable" and comply with everything in part 25. It's effectively a new version of the same aircraft that has a difference in equipment, and must meet all the rules.

Failures don't have to meet the certification rules, except in so far as they have to meet the ability to continue flight and safely land (or similar wording in different regulations)

Thus limitations for failures are rather different than MEL.

For example, you'd never get MEL for OEI dispatch :) Waaay too many rules not met. But you still have a procedure to fly if the engine fails.

edit: to say that the MEL'ed aircraft obviously has lost redundancy etc, and may not meet the safety rules for the next failure if left on the MEL for the life of the aircraft; the MEL time limits are determined to bring that consideration back into full compliance

Mach E Avelli
8th Jun 2019, 11:43
Certainly if we are comparing both situations on a wet runway when use of reverse is expected, Airmann’s reasoning seems the best so far.

8th Jun 2019, 21:01
BA46RJ Is there a specific limit for OEI?