Max Service ceiling for commercial airplanes

wrote in
:


They're not constrained by revenue considerations and can have
fighter jet type wings and other shapes to allow good buffet margins
up there. You couldn't get a 747 up that high without buffeting it
out of the sky unless it were empty and out of gas!

Is this buffeting related to slow flight? I now remember reading
somewhere, maybe in stick and rudder that large jets do not want to
risk flying very high up at a low airspeed because of the possibility
of an inadvertent stall during turbulence (not sure how much
turbulence exists at FL600 though).


Well, actually it;s low and high. When the air going over a wing that's
traveling close to supersonic is accelerated futher by the action of the
wing, it goes supersonic over the wing. When you go too fast, max mach, you
get a loss of lift. When you go too slow, you accelerate the air over the
top of wing as well due to increased angle of attack. If you load the wing
up with weight or G (same thing as far as the wing is concerned) same thing
happens. The air buffeting from the shock wave is similar to a stall ( some
sources call it a stall) and loss of control and altitude results. Recovery
can be difficult, though not impossible. There are a number of other facors
at play here, but that's the meat of it.

Bertie

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.

I see.. that actually explains what I saw on a Cathay flight across
the Pacific last year. It appeared to level off at 35 but when I
checked 6-7 hrs or so later (on the moving map) we were at 40. At the
time I thought the climb rate beyond 35 must be very slow but I think
its to do with weight as you explained.

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

wrote in
:


They're not constrained by revenue considerations and can have
fighter jet type wings and other shapes to allow good buffet margins
up there. You couldn't get a 747 up that high without buffeting it
out of the sky unless it were empty and out of gas!

Is this buffeting related to slow flight? I now remember reading
somewhere, maybe in stick and rudder that large jets do not want to
risk flying very high up at a low airspeed because of the possibility
of an inadvertent stall during turbulence (not sure how much
turbulence exists at FL600 though).




You can google the Advisory Circular AC 61-107a and it has a pretty good
dscription of what's going on. I can't read it at the moment because my
adobe is fuxored.


Bertie

WingFlaps wrote in
:

On Jan 30, 11:44*pm, D Ramapriya wrote:
On Jan 30, 2:33 pm, Bertie the Bunyip wrote:



D Ramapriya wrote
innews:24252c28-895a-44da-bd25
:

On Jan 30, 2:08 pm, Bertie the Bunyip wrote:
D Ramapriya wrote in
news:2ff47125-cffd-4909-b028-
:

On Jan 30, 8:41 am, WingFlaps wrote:
On Jan 30, 2:03 pm, wrote:

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.

Could be due to weight? When you fly across the Pacific the
plane can only get higher as fuel is burned.

Get higher? I thought that's what the continual small
adjustments effected by the trim wheels do to preclude,
especially with the AutoPilot engaged?

Huh?

Bertie

I meant to say that the AP will ensure that you keep flying at
the programmed altitude (through trim and throttle changes)
instead of letting the plane go higher with diminishing weight.
Have I missed something?

We get cleared to an altitude and have to hold that altitude to
avoid running into other airplanes. The autopilot does not just go
where it pleases.

My point exactly, in response to WingFlaps' "When you fly across the
Pacific the plane can only get higher as fuel is burned".

The AP will anon go where it pleases too on the next-generation
'Buses, albeit in exceptional circumstances.

No, I think you are confused. Altitude is _not_ commanded, Only
thrust and aoa (CL) are commanded flight variables. The plane always
adopts an altitude where lift=weight. The lift is set by the point
where thrust equals drag, the latter set by airspeed, density and CL.
So once you have climbed to maximum altitude (for given fuel burn) you
will gradually drift up as fuel burns. If the max fuel burn is set by
range (e.g. across pacific) or engine performance I think it follows
that max (and/or best cruise) altitude is set by weight OK?

You might command an AP to go to max altitude but if the plane is too
heavy it won't get there and will fly along in a climb attitude (and
maybe even stall at high altitude) until enough fuel has burnt off to
allow it to get there.


No. Max altitude as defined here is not performance limited, but mach
buffet limited. Though what you are saying is correct if you ignore this
factor.
The airplane ( well, most modern jets) will happily climb way above it's
max altitude as defind by mach buffet. We're frequenty still doing aover
1,000 FPM when we get there.

Bertie


Bertie

wrote in
:



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.

I see.. that actually explains what I saw on a Cathay flight across
the Pacific last year. It appeared to level off at 35 but when I
checked 6-7 hrs or so later (on the moving map) we were at 40. At the
time I thought the climb rate beyond 35 must be very slow but I think
its to do with weight as you explained.


Yeah, that sounds like what they were doing. That'd be a typical sort of
step climb. They were probably at 370 for a while in between as well. The
rate was probaly not too bad. It's still worthwhile to go up for the sake
of an hour or two up there. A lot of fuel would be saved.


Bertie

On Jan 31, 2:38*am, Bob Moore wrote:
wrote

Also why do large aircraft fly much lower than
their service ceilings? I assume they would be
even more efficient if they flew close to service
ceilings on long haul flights.

A couple of reasons...........First, something known as "Optimum
Altitude". As an aircraft climbs, its true airspeed (Mach Number)
increases, and at high Mach Numbers, there is a rapid increase in
Drag. This results in a higher fuel burn at cruise altitudes above
Optimum Altitude. So as the aircraft climbs above optimum, there
is an increase in fuel burn and a decrease in buffet boundry margins.
At the aircraft's maximum altitude, there is literly NO buffet boundry
margin, ie. Mach buffet equals Stall buffet equals Coffin Corner.

Second, time for an Emergency Descent. FAR 25.841 (copied below)
limits the maximum useable altitude. The B-707 that I flew for 17
years was limited to FL410 just due to the time required for the idle
power,gear down, speed brakes extended, max gear extended speed descent,
while the Air Force flew them to FL420.

Below copied from the web:

Cruise
One cannot continue climbing for long because as the altitude increases
at a given speed the CL increases. Speeding up would reduce CL, but this
is limited by Mach number constraints or engine power.

Hi Bob,

I think this introduces compressibility. As I understand it, normally
CL is not considered a function of speed or altitude and it greatly
complicates the calculation of max theoretical altitude (ultimately
set by air separation and not simply weight). For example, during re-
entry the shuttle still has lift=weight but I think she's flying in a
fully stalled condition. Only later when air density is higher can she
actually fly "normally". Of course I doubt that I will ever have the
pleasure of flying so fast or high that compressibility becomes an
issue...

Cheers

"Bertie the Bunyip" wrote in message
.. .
wrote in news:c3217254-afdf-40c0-b87a-
:

Out of curiosity I was wondering which civilian passenger airplanes

snip

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.

In the Lear we sometimes found you could climb out of the headwind. With
the 100kts on the nose at FL350, we could go to FL430 and get out of half of
it.(Westbound Winter)


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.


The old Lears, did exactly that. They had enough thrust to go direct to
FL450 at gross, and burned so much fuel it was worth while to get high and
keep it there until you had to come down. Rule of thumb was trip distance in
tens of miles times 2 for the cruise altitude. 200nm=40,000, we would file
for 41.


Al G

On Jan 31, 7:25*am, "Morgans" wrote:
"Dan" wrote

Just guessing, but perhaps the smaller cabin size handles the larger
differential pressure reqiuired better than a big cabin. *The extra
expense (and weight) to reinforce a large cabin (ie. heavy jet size)
may not be worth it.

Absolutely! A narrow tube is better at resisting pressure according to
the law of Laplace.

Cheers

On Jan 31, 10:14*am, Bertie the Bunyip wrote:
WingFlaps wrote :



On Jan 30, 11:44*pm, D Ramapriya wrote:
On Jan 30, 2:33 pm, Bertie the Bunyip wrote:

D Ramapriya wrote
innews:24252c28-895a-44da-bd25
:

On Jan 30, 2:08 pm, Bertie the Bunyip wrote:
D Ramapriya wrote in
news:2ff47125-cffd-4909-b028-
:

On Jan 30, 8:41 am, WingFlaps wrote:
On Jan 30, 2:03 pm, wrote:

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.

Could be due to weight? When you fly across the Pacific the
plane can only get higher as fuel is burned.

Get higher? I thought that's what the continual small
adjustments effected by the trim wheels do to preclude,
especially with the AutoPilot engaged?

Huh?

Bertie

I meant to say that the AP will ensure that you keep flying at
the programmed altitude (through trim and throttle changes)
instead of letting the plane go higher with diminishing weight.
Have I missed something?

We get cleared to an altitude and have to hold that altitude to
avoid running into other airplanes. The autopilot does not just go
where it pleases.

My point exactly, in response to WingFlaps' "When you fly across the
Pacific the plane can only get higher as fuel is burned".

The AP will anon go where it pleases too on the next-generation
'Buses, albeit in exceptional circumstances.

No, I think you are confused. *Altitude is _not_ commanded, Only
thrust and aoa (CL) are commanded flight variables. The plane always
adopts an altitude where lift=weight. The lift is set by the point
where thrust equals drag, the latter set by airspeed, density and CL.
So once you have climbed to maximum altitude (for given fuel burn) you
will gradually drift up as fuel burns. If the max fuel burn is set by
range (e.g. across pacific) or engine performance I think it follows
that max (and/or best cruise) altitude is set by weight OK?

You might command an AP to go to max altitude but if the plane is too
heavy it won't get there and will fly along in a climb attitude (and
maybe even stall at high altitude) until enough fuel has burnt off to
allow it to get there.

No. Max altitude as defined here is not performance limited, but mach
buffet limited. Though what you are saying is correct if you ignore this
factor.
The airplane ( well, most modern jets) will happily climb way above it's
max altitude as defind by mach buffet. We're frequenty still doing aover
1,000 FPM when we get there.

Bertie

Yes. I was avoiding the compressibility problem for simplicity/
clarity. Damn you jet jockeys!

Cheers

"Al G" wrote in
:


"Bertie the Bunyip" wrote in message
.. .
wrote in news:c3217254-afdf-40c0-b87a-
:

Out of curiosity I was wondering which civilian passenger airplanes

snip

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.

In the Lear we sometimes found you could climb out of the
headwind. With
the 100kts on the nose at FL350, we could go to FL430 and get out of
half of it.(Westbound Winter)

Yeah, we hardly ever get on top of a jetstream unless it;s very low.
Alos, we'd be wary of even trying in case we hit the cold side CAt near
our margin.


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.


The old Lears, did exactly that. They had enough thrust to go
direct to
FL450 at gross, and burned so much fuel it was worth while to get high
and keep it there until you had to come down. Rule of thumb was trip
distance in tens of miles times 2 for the cruise altitude.
200nm=40,000, we would file for 41.


More importantly, you had the buffet margins. We could also go to that
altitude if power were the issue, but at max gross, we're limited to
about 350 initially.