Rob,
You may be sitting at the pointy end, but I'll probably be sitting right behind you, especially if its a Wednesday lunch time flight to London
Therefore I have an immense personal interest in keeping both of us safe and getting to drink some Guinness
That 25 foot margin is a red herring, I could have just as easily ended up with ZERO.
We are probably the least commercial airline that you will ever find, we do not use the digital AFM as an operational tool, we use the more conservative standard Boeing takeoff charts. These charts dont know what your actual temperature is, therefore the calculation is based on ACTUAL temperatures. They do not account for the excess performance available due to the difference in TAS and available thrust. I will try to show this conservatism with the following.
I'm guessing that some of you are used to this kind of chart, for those that arent, you enter it at the bottom, read up until the you find your actual weight in both columns. In our case, it happens at 48°C and its field length limited.
ELEVATION 0 FT , ,MUTT INTL
***FLAPS 05*** ,PACKS ON ,
B777-200 ,GE90-DER I , ,DATED 29-MAY-2001
OAT, CLIMB, MAXIMUM ZERO WIND WEIGHT-(100 KG) AND LIMIT CODE
DEG C, LIMIT, , 14L
(I'm trying to save space, so there is a very limited range of temps)
45 , 2336, 2262F
46 , 2308, 2245F
47 , 2280, 2228F
48 , 2253, 2211F
49 , 2225, 2194F
50 , 2198, 2176F
67A, 1805, 1884F
68A, 1784, 1868F
69A, 1763, 1852F
ADD KG/KT HEADWIND 380
SUB KG/KT TAILWIND 1770
MIN FLAP RET HT-FT 800
RUNWAY LENGTH-FT 9006
RUNWAY SLOPE-PCT 1.00
CLEARWAY-FT 0
Now if the actual temperature was 48°C with 220,000 kgs, the actual takeoff performance would be as follows:
Takeoff Distances
-----------------
All Engine Takeoff Distance = 8580 FEET
All Engine Takeoff Run = 7983 FEET
One Engine Inoperative Takeoff Distance = 8909 FEET
One Engine Inoperative Takeoff Run = 7877 FEET
Accelerate-Stop Distance = 8909 FEET
Critical Distance(s) : One Engine Inop TO, Accel-Stop
That margin of 91 feet will increase as the actual temperature gets colder. For a 15°C day it would be 210 feet.
Remember that these stopping distances do not include the use of one thrust reverser.
Would you consider this margin to be sufficient? If not, how much would you like it to be?
Now to move on a bit, what accelerate stop margins do you expect when you are taking off at the ACTUAL temperature?
Mutt