View Full Version : Highest Altitude a 747-400 can accept?

23rd Mar 2002, 04:17
What determines the highest altitude an aircraft can accept? I have seen pilots decline flight level 410 but accept 390. What determines this?

23rd Mar 2002, 04:31
The thrust produced by a jet engine falls dramatically with height. In the early stages of a flight, a heavily-loaded 747 may not have enough power to get to 41000 feet.

23rd Mar 2002, 08:39
Technically, it is the maximum (certified) operating altitude which is essentially limited by the maximum differential pressure (pressure difference between cabin and outside air pressure) plus a good buffer to avoid "sleepless" nights for it's manufacturer BOEING.. .. .The maximum operating altitude for the 747-400 is FL450/45100 ft. That means if the airplane is excetionally light such as on a ferry flight with no passengers and cargo, it can fly up to FL450 (which happens sometimes, but rather rarely).. . . .With Payload (Passengers and Cargo) and the fuel needed to fly from A to B the aircraft is too heavy to climb straight to FL390 or above.. .. .Instead it climbs in steps to it's final cruising altitude as it burns fuel and gets lighter.. .. .As far as the question goes, "the highest altitude a 747-400 can accept ??" The answer would be: It depends on the circumstances. But the factors involved would be Aircraft Weight, Air Temperature (which are rather inflexible) and Economy (which is the most flexible and complex of the three).. .. .Therefore, in most cases 747-400 flight-crews are able to make their decisions based on the economy factor and decide which levels they "can" accept or not. Sometimes even on health issues (i.e. flying in the arctic area in FL450 is not really healthy, due to cosmic radiation...). . . . <small>[ 23 March 2002, 12:27: Message edited by: Avius ]</small>

Ford Airlane
23rd Mar 2002, 09:57
Max Alt is FL451. In practice though that altitude will never be reached except maybe on a l-o-n-g ferry (i.e. no pax/freight) flight. Highest I have seen one go in revenue is about FL410.. .. .Factors are weight and temperature. After a MTOW (397 ton) departure, FL330 is almost always reachable. Another handy rule I remember is FL350 is usually acceptable at about 350 ton.. .. .The Max Alt figure that the FMC displays is variable from company to company depending on the buffet margin (1.2 or 1.3 I believe) they specify.. .. .Optimum (from a fuel perspective) altitude though will always be lower.

Al Weaver
23rd Mar 2002, 10:09
I was under the impression that one of the limitations was the passengers in the event of a decompression. That is, given that a decompression *may* occur the time to descend must be weighed against the supply of O2 for the passengers in that it is not acceptable to kill them outright, unconsious is OK. I believe that this is the certification pinch point rather than performance or structural

23rd Mar 2002, 11:36
Lomapaseo. .. .Sorry, but you are wrong.. .. .Obviously one tries top keep the pax alive, but decompression does not affect the max altitude. There is always sufficient O2 on-board to deal with pax oxygen requirements for an amount of time. The amount of time varies from operator to operator as a function of the routes they fly and the amount of time they may need to operate above 10,000ft (at which point pax usually come off oxygen following a cabin altitude excedance.. .. .The operator I fly for has extra O2 on board to allow operation over routes with extended operation over the Hindu Kush, for example.. .. .The factors determining max alt capability are the ones mentioned in previous posts.. .. .TB

23rd Mar 2002, 21:19
Maximum aerodynamic ceiling is surely that at which there is an acceptable spread between the low and high speed certificated buffet boundaries, although the high speed may be in excess of Mno. These values will vary with aircraft mass; other factors may also influence the maximum attainable altitude such as thrust available at high ISA deviation.. .. .I believe that it was the B-47 Stratojet which first encountered the problem of flying at such an altitude that the +1.0 Gz high- and low-speed boundaries were co-incident. In other words if it accelerated, it would 'Mach stall', if it decelerated it would stall. I was also told some years ago that the U-2 would fly on autopilot with a 2 kt spread between low and high speed buffet boundaries; it took an autopilot to achieve this with acceptable safety.

Final 3 Greens
23rd Mar 2002, 23:44
I was once lucky enough to be a pax on a Big Airlines 744 that managed FL420 - although it was in the last couple of 90mins of an 11 hour flight.. .. .Never been that high before or since!

25th Mar 2002, 15:43
I was told the U-2 situation was so extreme that a turn would put the outside tip in the mach buffet and the inside tip in the low speed buffet.

25th Mar 2002, 15:48
BIK. .. .What's this mean? </font><blockquote><font size="1" face="Verdana, Arial, Helvetica">quote:</font><hr /><font size="2" face="Verdana, Arial, Helvetica"> Or it may have the power to get it there, but not the LIFT to keep it there. </font><hr /></blockquote><font size="2" face="Verdana, Arial, Helvetica">Actually Shon I need to correct myself. The power of a jet engine falls with altitude so that a point can be reached where the engine is working too hard in order to maintain sufficient thrust to climb any more. Unlike Concorde, 747 engines are kept within limits to extend time between overhauls. Going higher would become un-cost effective.

26th Mar 2002, 15:44
OK. Just for casual readers, I think it is fair to say that unlike engine power, lift remains the same at 41000 ft as at 410 ft. The problem is you need to increase true airspeed in order to maintain indicated airspeed and at high altitudes this can mean the Mach Meter needle reaches the barber's pole before you have reached the altitude offerred by ATC.

Right Way Up
26th Mar 2002, 16:44
Surely for a given TAS, lift is less at 41,000 ft than 410 ft. . .From memory lift=1/2 x air density x TAS squared. (or something like that)

26th Mar 2002, 18:40
Right Way Up...it is IAS that governs this, the temperature/density part of the correction for TAS has already been done in the original calculation with the use of density already...LifT =CL 1/2 roe (density) V squared S. Where CL = Coefficient of Lift for the wing concerned. 1/2 = 1/2 roe (greek letter for density) V = indicated Airspeed, S = wing area.

Right Way Up
26th Mar 2002, 20:41
Siddique,. .Thanks for filling in the rest of the equation, I have mislaid my old manuals. I still think the velocity part is TAS?!

27th Mar 2002, 09:22
Right Way Up, yes the velocity in the lift equation is TAS (or, more correctly still, EAS). The lift at 41,000'and at 410' being produced by a wing in Straight and Level flight would be the same (regardless of speed) as it would be equal and opposite to the weight of the aircraft. At the same TAS the angle of attack (and hence Cl) would be different.. .. .What BIK is talking about is the ability of the wing as it approaches the speed of sound - the higher you go the higher the Mach number for the same TAS, and as the wing approaches the speed of sound a shock wave forms, and ariflow separation roughly similar to a stall occurs.

29th Mar 2002, 14:02
Certified to 45100ft. Been up to FL450 many times on ferry flights.

29th Mar 2002, 21:04
Rat. .. .If you regularly go up to 450 and thus you are not constrained by coffin corner one wonders how Tricky Bikky can be right.. .. .If it's not power and it's not buffet, that leaves nothing ie 747-400s on 58000lb engines can always take any level???

30th Mar 2002, 00:47