Mark_D. From Canada, joined Aug 2001, 1447 posts, RR: 5
Reply 7, posted (11 years 11 months 3 weeks 5 days 4 hours ago) and read 32767 times:
Note that --depending upon whom you ask and what variant it is and payload it's carrying-- the fairly-standard-subsonic-wing-design B-52 bombers have advertised service ceilings of as much as 56 000 ft.
I think this is an interesting article, about airliners' environmental control systems:
Bellerophon From United Kingdom, joined May 2002, 586 posts, RR: 58
Reply 8, posted (11 years 11 months 3 weeks 5 days 3 hours ago) and read 32767 times:
The maximum altitude a civil airliner is certificated to fly at is determined during flight testing, and is imposed because the aircraft has come up against any one of a number of different limitations, depending on the design of the aircraft, its wing, its engines or its cabin environmental systems.
Just to give a few examples:
The aircraft engines may not be producing sufficient thrust to enable it to climb any higher.
The aircraft wings may not be capable of providing adequate lift at a higher altitude.
The Mach Number might become too high, causing undesirable aerodynamic effects.
The maximum and minimum speeds may be too close together at a higher altitude
If the Cabin pressure were to fail at high altitude, there may not be adequate Oxygen pressure available in the emergency drop down masks.
B777236ER raised an important and increasingly common limit, that of cabin air pressure. The air pressure in the passenger cabin must not be allowed to fall below a minimum value, equivalent to a maximum Cabin altitude of 8,000 ft (I think), under both JAA & FAA regulations
If the cabin altitude is already at the maximum permitted, and the aircraft then climbs, the differential pressure (the difference between the pressure inside the cabin and the air outside the aircraft) will rise as the outside air pressure decreases.
When you reach the maximum differential pressure, typically around 8 to 9 psi, you cannot climb any higher without breaking one of those two limits.
I hope these give you some idea of the varied reasons that can determine how high an airliner is allowed to fly, and why it is not a good idea to exceed the limit. There is even one airliner where the maximum altitude can be limited by the temperature of the nose!
Provided that the aircraft is certificated to fly above 40,000 ft - and many now are, with one being cleared to fly at 60,000 ft - it will be perfectly safe up there, and nothing untoward will happen.
However, deliberately flying any aircraft above its maximum certificated altitude, unless you are a qualified test pilot on a well planned test flight, is one of the dumbest things you can do whilst airborne.
Yikes! From Canada, joined Oct 2001, 284 posts, RR: 1
Reply 9, posted (11 years 11 months 3 weeks 5 days 2 hours ago) and read 32767 times:
A B52 is not going to fly at 56,000'.
It's not possible.
As altitude increases, indicated airspeed decreases.
As altitude increases, temperature decreases to a standard -56 degrees C.
It is not uncommon to see -65 degrees C or colder in a non-standard atmosphere.
Consequently, to maintain an indicated airpspeed at higher altitudes, true airspeed (or equivalent Mach number) must increase. If it doesn't, the IAS decreases to the point the aircraft stalls. The B52 is a 1950's technology airframe. It is not capable of high Mach capability. Even with ISA -20, the Mach number required to maintain an IAS > Vs will exceed the B52's Mcrit.
Especially given the ancient technology of their 8 pure turbojets. The engines might be willing (flat out) but the airframe is wanting...
Original Question: The B767 is certified to fly at FL420. Concorde is certified to (I think) FL 650. Rembember though that Concorde cruise climbs at M2.0
Redngold From United States of America, joined Mar 2000, 6907 posts, RR: 42
Reply 10, posted (11 years 11 months 3 weeks 5 days 2 hours ago) and read 32767 times:
But... 45,000 ft. is approaching that coffin corner for a lot of aircraft. They MUST slow down or change AOA to remain stable in flight. Otherwise you get an incident like that Northwest Airlines 727 that took a dive out of the sky over Michigan.
Mr.BA From Singapore, joined Sep 2000, 3423 posts, RR: 21
Reply 11, posted (11 years 11 months 3 weeks 5 days 1 hour ago) and read 32767 times:
It's because the higher you go, the speed margin is minimal. You must be very careful to ensure airspeed does not drop too much or you might not have enough thrust to increase the speed back cruising speed for example. And the higher one goes, the KIAS drops, so it's closer to stalling.
Hmmmm... From Canada, joined May 1999, 2114 posts, RR: 5
Reply 13, posted (11 years 11 months 3 weeks 5 days 1 hour ago) and read 32767 times:
At more rarefied altitudes, an airliner is in an environment that is very unforgiving of a sudden change in AOA. The aircraft could lose stability and control very easily and go into a nose dive. It's happened. The higher you go the less air to ride on, the easier it is to lose control and the harder it is to recover immediately.
If you have MS Flight simulator, program your aircraft for a very rarefied altitude and watch what happens when it loads. The plane will tumble and drop like a rock and you will not be able to recover until you get down to lower altitudes. I once did this with the soar plane, I loaded it in at 80,000 ft, and the bloodly thing tumbled wildly until I got down to 45,000 ft. simply because there was not enough air to ride on. If you were to magically insert an airliner at, say, 80,000 ft, it would plummet like a rock. The dynamic forces would probably destroy it before it could recover at a lower altitude.
An optimist robs himself of the joy of being pleasantly surprised
Derekf From United Kingdom, joined Feb 2001, 914 posts, RR: 0
Reply 18, posted (11 years 11 months 3 weeks 4 days 13 hours ago) and read 32767 times:
You are wrong. A DC-8 exceeded Mach 1 in a dive during a test flight. There are also rumours about the VC-10.
Mach 1 is not a barrier - thats what everyone thought in the 1940s. BTW I've been to 45000ft in a 125 where we had only 210kts and 0.8M.
B747skipper From , joined Dec 1969, posts, RR:
Reply 20, posted (11 years 11 months 3 weeks 4 days 13 hours ago) and read 32767 times:
Surprised to see this question here - why not Tech-Ops...?
The "ceiling" (maximum operating altitude-level) is a certification number...
For transport airplanes (airlines) it is generally an altitude limit for pressurization, that is, maintaining at least 8,000 feet (2,500 meters) equivalent pressure in the cabin...
Many transport airplanes are certificated to 35,000 feet (old 737s)
Other older airplanes (707, 727, DC8) are certificated to 42,000 feet...
The 747 is certificated to 45,100 feet...
Some Learjet types are certificated to 51,000 feet...
The Concorde is certificated to 60,000 feet...
For all above airplanes, this is the level at which their cabin will reach 8,000 feet OR limited by the time it would take them to reach 14,000 feet in case of an emergency descent for failure of the pressurization system...
The 747 I fly is legally limited to 45,100 feet, yet, when I was in the Air Force, the E-4 (747 Command Post) was able, in emergency (and when very light in weight) to reach nearly 48,000 to 52,000 feet... at the verge of a stall...
When airplanes are heavy (fuel for very long trip) they are far from able to reach high levels... With my 747, we can barely reach 29,000 feet when we are fully loaded for a trip from Argentina to Europe, yet, at the end of the cruise flights, we occasionally are at 39,000 feet... I hardly ever fly at the maximum certificated of 45,000 feet...
Such a question really belongs to Tech.Ops -
JmhLUV2fly From United States of America, joined Dec 1999, 559 posts, RR: 0
Reply 23, posted (11 years 11 months 3 weeks 4 days 11 hours ago) and read 32767 times:
From what a AirTran 717 first officer said to me once, my understanding is that for the 717 at least the max altitude that airplane can really go is 35.0 any higher then that and from what he said the wings really are not designed to support the aircraft much above that altitude...
now most definitly for your 747's and 777's , A340s etc
they are designed to fly really high 40.0 or so altitude if need be... for max fuel burn etc, I mean keck the trips they commonly serve are 12+ hour flights so it would make sence for them....but not all airplanes can fly that high.