Imagine an airport with 2 runways... On arrival, the crosswind is out of limits for the main, long runway and instead you are given a shorter, less used, lets say 1000m runway perpendicular to the main one... This is a possible scenario I would think, no? Well I'm not gonna land there with a 737-800 at 65 tons...
Now you are in-flight, making an in-flight decision, you can use the actual conditions. If the main runway is out of limits due crosswind, then the crossing runway will have a 40 knot headwind.
No problem for your 1000m landing, then.
Fair enough, but what's the abnormal landing factor for a 'low fuel' situation. There is none of course...
.. but with a low fuel situation, you aren't going to be 65 tons any more, are you?
You are, of course, now in the regime of using your command authority to ignore ANY rule prudent in an emergency.
as you start the flight/diversion to that (new) airfield, you can "throw away" your Dispatch distance as you only need QRH distance in flight...?? Which defeats the whole idea of having a Dispatch distance!
When you dispatch, in terms of fuel planning, you include a 5% contingency, no? This is a safety margin, and allows for change of winds, inflight weather diversions etc etc. If you use it, you don't divert - you continue and using this safety margin was "planned" for - that's why it's a
margin.
The 67% (or 92%)
margin for despatch on runway length is exactly the same. If is rains or you are a bit heavier - you aren't required to divert, as this was a despatch safety
margin. The fact that it has reduced a bit in flight is what it was there for.
Of course, if that margin has reduced to the point where the commander no longer considers it safe to land (given their inflight calculation of the minimum the aircraft requires, and safety factors the commander thinks prudent due crew experience or whatever) then you don't land. Welcome to the command decision process!