I have to admit, it is the high speed RTO scenario that makes me post the original question. So from an RTO point of view, is the somewhat longer runway helping me with the derated thrust and different flap setting? What about when the runways are wet where reverse is in the calculations for an RTO?
Again, it depends.
On one of our more interesting runways we have a choice of intersection or full length.
Intersection:
TODA 2550m
VR 139
flex 48
stop margin 200m (+ a bit due to the assumed temperature)
Full length
TODA 3500
VR 155
flex 67
stop margin 700 (+ a bit due to the assumed temperature)
As a contrast, if you used TOGA to depart from full length with the highest flap the figures would look like:
TODA 3500m
VR 123
stop margin 2300m
So you ask if the longer runway is helping.
Again, the answer is: it depends what you are looking for.
If you take the longer runway for stop margin, then who are you kiddin' if you blindly accept the flex, reduced flap and the much much higher VR? The added potential problems overshadow the few extra meters of stop margin in my opinion.
Don't get me wrong. I'm not a maverick who likes to operate at the edge of the envelope. But what I dislike more is the mentality you see more and more on the line and on these forums where people are afraid to take a safe informed decision, just because that decision might be scrutinized in some imaginary court case should things go wrong. This kind of thinking causes more harm overall than the few meters of stop margin it gains.
I think FullWings wrote an excellent reply yesterday which sums it all up:
I think one of the key points is the use of the word “safer”. Like others, I’m paid to manage a safe *and* efficient operation - complete safety would be to lock the hangar doors and throw away the key, so there has to be a compromise somewhere.
Compared with all the other dangers out there which need to be avoided, shunning runways that are demonstrably acceptable performance-wise gives such a minuscule improvement in overall safety that it’s debatable you could even calculate it. It’s almost the equivalent of a lucky rabbit’s foot or four-leafed clover in that you may feel much safer but the underlying reality is little changed.
Incorrect responses to windshear, GPWS, TCAS, UAS; the wrong flap setting, the wrong thrust reduction, the wrong MSA, the wrong runway. Not de-icing properly, not being proficient in unusual attitude recovery and so on... These are some of the biggies that can really spoil your day. Just thinking about the possibility of any of the above before you set off could increase “safety” by far more.
To illustrate what this kind of thinking leads to I'll give the following example.
The above mentioned runway is prone to fog.
Nobody departs from the intersection when it's foggy, which is quite understandable for safety reasons.
But strangely enough, most are happy to accept the higher flex and speeds that the performance module calculates for the full length departure.
So they think they are taking the safe decision by accepting the longer TODA, but all they really achieve is a much longer, and much faster take off run in almost zero visibility. Who are they kidding?
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