Runway Length

[mupepe]

Hello,I need answer to following point;
JAR-OPS define the MLW runway criteria as >>full stop must be possible from 50 feet TCH and within 60% of the landing available.
The runway length requirement at the planning stage should then be defined by that criteria.(not talking wet or contaminated runway)
I'm now in flight and small system malfunction is asking me to increase the unfactored landing distance by a factor defined in QRH.(X1.45)
Unfactored LDA in this case, a distance from TCH to full stop point.
Should that new distance calculation still being within the 6O% of the available runway ?
I'm not in the planning stage anymore and I'm not in emergency.
Thank you for your opinion :ok:

[Mad (Flt) Scientist]

Should that new distance calculation still being within the 6O% of the available runway ?

No.

The 60% margin covers many things, both technique variation and to some extent an allowance for minor failures, such that diversion will not be necessary for every minor event.

You're now in an "airmanship" situation.

You planned to have a 10,000ft runway, based on a predicted 6,000ft stopping distance. You now, accounting for the failure, will need 8700ft (nominal) to stop. Are you comfortable with 'only' 1300ft of margin? That's a decision you'd have to base on a variety of factors, not all of which are entirely objective. Even if your new predicted landing distance were 9,999ft, you'd be legal to attempt to land on that 10,000ft runway. But it probably wouldn't be a very smart thing to do. Unless other factors made staying aloft a worse choice.

All IMO. IIRC, when putting together the guidance for an RAF operated version, it transpired that they take the other approach - of applying the factor to the planning distance, not the actual distance - but from discussion it then transpired that their philosophy is that you can then use judgement to land on a "too short" runway. So, in the example given, you'd be at 1.45*10,000 = 14,500 but "knowing" that you have 60% margin built-in, you could potentially make the judgement that 10,000ft actual concreete was acceptable i.e. it's still an airmanship call in the end.

You wouldn't want a situation where an aircraft with an emergency stayed aloft unecessarily to maintain a 60% margin and encountered more risk in loitering in a failed condition or diverting than in simply landing with a bit less margin than normal.