Capot,
The internediate slope from the intersection, perhaps the lowest point of the runway. The max permitted intermediate slope can be well above the overall published net slope.
The true aircraft weight. A loadsheet ramp weight has an accuracy of, let's say, +/- 1 tonne, perhaps more depending on your aircraft type, if we are being brutally honest. The performance data provide for this, but is the allowance enough?
The runway surface. Has it got the desired friction? Was there a recent shower the crew don't know about?
The actual head-wind component during the roll, if it's a bit variable and gusty.
The actual state of the brakes, bearing in mind that the aircraft will probably be stopping with asymetric power. How close to the next check are they?
The actual distances available from the intersection. OK, ATC can usually provide, but didn't someone mention a ruler on the chart?
Pretty much all of these apply equally to the full length...
I notice this becomes a special problem when "margins are tight". In a modern airliner, if you use assumed temperature derates on your average <3,000m (10,000') runway, you will frequently end up with a balanced field, i.e. ASDR = TODA so a V1 reject will use it all,
no matter where you start from.
If you want to introduce more certainty into the stopping case, then you'll have to 'unbalance' the calculations by forcing a higher flap setting and/or using more power. If you want to be as "safe" as possible, use full power and all available high-lift devices; if you don't do that, then the situation often won't be substantially different from an intersection departure.
Some more thoughts: If it's a longer taxi to the full length (must be, really, or why bother with the intersection?), you're putting energy into the brakes - a significant amount, sometimes - which makes a brake/tyre fire/failure more likely with a high-speed RTO. Another thing is that performance software will often automatically add more flap when optimising a departure using a shorter run, so you might end up with a lower V1 than when using the full length: what everybody wants...
Again, for the third time, I imply no criticism of those who wish to partake of the maximum available tarmac at all times. However, in a
technical forum we should be able to discuss the actual physical ramifications of our decision making without taking any of it personally. A question has been asked (and it's turned out to be a goody), so we should try our best to answer it as objectively as possible. (Not that I'm always an innocent in that regard!). "You" in the above text is meant as a generic "you" rather than representing any of the other posters in this thread.
