NWSRG,
The procedure for calculating takeoff weights and speeds varies from airline to airline
as well as aircraft to aircraft, my intention with this is to give you a
rather simplistic indication of what happens.
Crews/dispatchers are supplied with charts which provide limiting takeoff weights
for specific runways, this one is for a Boeing aircraft and doesn’t show V-speeds.
(Paste it into Excel and you will get better formatting)
DEG C LIMIT 14L 14R 32L 32R
-15 2874 2460# 2390# 2760F 2874#
-10 2874 2458# 2388# 2737F 2874#
ADD KG/KT HEADWIND 190 210 0 0
SUB KG/KT TAILWIND 870 910 1990 440
MIN FLAP RET HT-FT 1150 1130 800 800
RUNWAY LENGTH-FT 11483 7776 7776 11483
RUNWAY SLOPE-PCT 0.51 0.72 -0.72 -0.51
CLEARWAY-FT 0 0 984 0
If you look at the columns for 14L and 32R, these are the opposite ends of the same 11483 feet runway,
but have different limiting weights due to the obstacles. The actual weight must be equal to or less than
this limiting weight and is entered in the FMS to obtain the V-speeds and power settings.
Airbus (and some Boeings) use a slightly more complex method of calculating Vspeeds, so these are usually
shown on the takeoff chart with the limiting weight.
The aircraft knows exactly where it is for the purpose of navigation, it doesnt know where it is
for the purpose of performance. In order words, the aircraft wont stop the crew from lining up
on 14R, which is 4000 feet shorter than 14L.
AhhhVC813, thanks for that, I changed my previous to make it clearer.
Cheers
Mutt.