wrote in news:c3217254-afdf-40c0-b87a-
:

Out of curiosity I was wondering which civilian passenger airplanes
have the highest service ceilings? Wikipedia indicates that some
business jets have ceilings greater than 53000 ft or so while the 747
has only 43000 ft. Also why do large aircraft fly much lower than
their service ceilings? Usually I never see a large jet go beyond
37000 or so even on very long haul flights. I assume they would be
even more efficient if they flew close to service ceilings on long
haul flights.


AFAIK the new Bus has one of F430 as well. You can fly right up to the
service ceiling but the max allowable FL goes down with weight. THe buffet
margins become tighter when you are heavy and the max allowable altitude
goes down accordingly,. As you burn fuel you can go up in steps, so on a
long trip you might originally be limited to say, FL330 and then after an
hour or two your limit may rise enough that you can get to 350 and then
again to 370 and so on until you either get to max or its time to come
down. The performance computer (integrated into the FMS) gives you a
constant readout of the limit.
Also, it's not so clever to go up another 4,000 feet if you're going to
have another 100 knots on your nose! A typical rule of thumb tradeoff for
wind/altitude is about 7knots per 1,000 feet, though this isn't hard and
fast.We have tables for it or you can put some projected winds into the FMS
and ask the computer to do it for you.
Another factor is the distance travelled. It makes no sense to go to 410 on
a 200 mile trip. Having said that, the ideal fuel burn profile is close to
straight up and straight down on short trips. No level cruise. IOW, you
keep climbing until you intercept the descent profile and then come down.


Bertie