747-200

[deefer dog]

An old mate of mine who used to fly the 747-200 once told me that at high altitudes the difference between over speed and stall was barely 6kts - certainly well in coffin corner.

At max likely levels in what I fly the margin is much greater, but if one is flying with such a small margin of only 6kts surely it is necessary to be sure that you are not likely to meet turbulence en route.

Are the margins as small as perhaps 6kts in commercial pax jet aircraft used today? And if so what then does one do to mitigate encountering turbulence when flying, for example the NAT tracks?

[fire wall]

Deefer,
Please excuse me as it has been 8 years since I moved off the classic to the electric variant .... so, for the alzheimer's version:
The speed spread between cruise and MERS was small at altitude, perhaps 6 kts....not stall. MERS (not a term used as much anymore so excuse me if I am telling you how to suck eggs) is a high (er ) drag condition whereby you needed to descend to get the speed back.
The old girl was temperature sensitive. With MTOW 377.8 and ISA + 16 or above you would struggle to get to Fl280 before most FIR boundaries. As such you were always doing calcs in your head on the climb working out whether the flt plan ISA dev was accurate.
Pushing alt at a heavy wt is dumb in any a/c, but particularly dumb in a classic and it is this condition that would get you close to what you are describing.
6 kts however does come to mind on a hvy wt departure as the diff between the flaps up speed and the over speed (275 rings a bell) as the leading edges retracted (groups 1 and 4 from memory) and it took a deft hand to keep it finely balanced.
Great a/c, truely loved my time on her.

[Captain Kaboom]

The Learjet 60 is supposed to be able to climb up to FL510.
Once I have been at FL470, scary, a margin of about 6 kts and the 60 is very speed unstable above Fl400, starts to "rock a little" bit, doesn't seem to find her balance.

High enough Fl400...

[bigduke6]

Not very likely to have that low of a difference on a commercial aircraft.

U-2 was something close to that, but back of my memory recalls reading 10 kts, but then that is a military aircraft try to get absolute max altitude.

Big difference than a commercial aircraft using a 1.2 or 1.3 g stall margin so you can actually bank to turn on an airway.

I guess I didn't pay that much attention, and I don't do it too often, but at FMC calculated max altitude in B744, you have well over 20kts between overspeed and stick shaker (probably closer to 30), so I image in the B742 is similar.

[deefer dog]

Thanks, above answers my question. The old mate of mine did tend to exaggerate a bit!

[TopBunk]

I once took a B747-400 up to FL430 when we were light, just to see what it was like - all within the limits. I would imagine mass was about 220 tonnes on a flight back from JFK to LHR.

I took a picture of the PFD and it showed (can't be bothered to post image):

Mmo/Vmo at 259kts
max maneuvring at 255 kts
IAS of 240 kts (M0.847 just after NAT exit over Ireland)/FL430
Min maneuvring at 231 kts
Min speed at 199 kts.

So quite a big margin. Have however seen much smaller margins at lower levels of probably about 10 kts between max/min maneuvring.

[eckhard]

And I took one up to FL450 on a flight from Lagos to LHR, again, just coz we could! Only 80 pax, so very light. Don't remember any problems with speed margins. When the thrust reduced for the descent, there was quite a 'whoosh' from the air con as the bleed valves did their thing.

[Fullblast]

Deefer Dog, are you sure mate was talking about maneuvering margins instead that overspeed and stall? That would sound more reasonable.

[DozyWannabe]

Right, but isn't the difference with the 744 that it was provided with far more powerful high-bypass engines than the Classics were?

[tdracer]

but isn't the difference with the 744 that it was provided with far more powerful high-bypass engines than the Classics were?


It depends, there were a wide range of engines available on the 747-200 - everything from 47k to 54.5k ratings. Baseline engines for the 747-400 were about 56k, but it was also higher MTOW than the -200.


I've been on a 747-8 flight test at 43k, Mach 0.78 (VMO Mach 0.9) so there was a considerable speed margin. However we also quite light, roughly half MTOW :E.


BTW, the -8 is only certified to 43.1k - while the airplane is fully capable of going higher, the FARs have changed with regard to depressurization. As a result, I doubt you'll ever see another Part 25 airplane certified to above 43.1k :rolleyes:

[galaxy flyer]

BTW, the -8 is only certified to 43.1k - while the airplane is fully capable of going higher, the FARs have changed with regard to depressurization. As a result, I doubt you'll ever see another Part 25 airplane certified to above 43.1k

I fly several cert'd to F450 and F510. Several recently were cert'd to F510 and several more in the works. About 30 knots either side of M.84 in the Global at F470.

[tdracer]

Galaxy, you're talking business jets - they are not (typically) certified Part 25 - they are certified Part 23. That's why biz jets can go higher than "transport" Part 25 aircraft.
The current Part 25 rules for decompression are such that I don't think it's possible to meet above 43k (at least with current technology).

[grounded27]

I know it is an SP but SOFIA is certified up to 45k. Highest a pig can fly to my knowledge (certified).

[main_dog]

Our 744s are certified to FL451. Probably old regulations though, as you say the -8s are only certified to 431.

[Double Back]

Firewall, indeed MERS I have forgotten or our CY never used that term, anyhow, I had my share as well.
Like around Delhi You were always required to climb to an altitude, where the old lady was on, or a tad over max. But we had no FMS so the F/E (Flight Engineer for You magenta generation, the one in the 2nd row who dared to make jokes about captains) had to look at the charts. When it took long You knew the answer was biased because otherwise he might had to start being anxious about fuel levels, hours later...
So You crept to the assigned altitude and deliberately overshot it by some 300 feet (no mode C then...), then V/Sed it back to the required alt, keeping climb power, and getting it "on the step" as those old warriors called it then. No big deal.
Cruising was all w/o ATS then, speed monitoring during cruise was an issue not to forget. But at high altitudes the throttles were close to max CRZ anyhow.
It got dangerous however when we started mixed flying with the -300 who did have an ATS. After a few legs with the -300 You completely forgot to watch the speed in CRZ if an occasional 200 was in one of the legs...

Yes, years later and in CMD myself on the -400 doing GUA-MEX at a very light load I also tried FL451. Also for the heck of it. We left a clear and smooth 410 or so and levelled 451, and then all hell broke loose with unexpected heavy turbulence, and we went down in hurry again!. So my FL451 experience has been limited tot less than one minute!

[safelife]

tdracer, all larger business jets are FAR25. Gulfstream, Global, Falcon, Citation X. And all are certified up to FL510.

[galaxy flyer]

tdracer,

Since the Lear 24 in 1965, all business jets, save the latest single pilot ones, are FAR 25 except the early ones like the Sabreliner which were CAR 4b. Transport category isn't just the preserve of Boeing/Airbus. The F510 test is quite severe, basically removes a window and frame. The Global cabin rises at about 800-1000fpm on the test. The descent is the same 4 minutes to the bottom. With HUGE speedbrakes and M.88, it does quite nicely, if a bit nerve-racking.

BTW, the latest ones have to have fuel inerting, too.

[tdracer]

Ok safelife and galaxy - I stand corrected. I'm not an ECS guy (but ask me questions about engines :E). I was just reflecting a comment from an ECS guy during the 747-8 development that we'd never be able to certify to 45k again because of the rule changes (and my understanding was that we had to be pretty creative to get 43.1k on the -8). Perhaps it's related to the hole size that we have to use on the bigger airplanes.:confused:

[galaxy flyer]

That might be, the Global's ACMs are supposedly the same as an A320's. Lots of air!

[main_dog]

I was told the -8 is only certified to 431 (instead of 451 like the -400) because the wing is so much more efficient that in case of depressurization it is impossible to get the thing down within the required time?

[deefer dog]

The F510 test is quite severe, basically removes a window and frame. The Global cabin rises at about 800-1000fpm on the test. Are you stating that with a window blown out at FL510, the Global cabin only flies up at 800-1,000/min?

I'm not stating that you are wrong, but if it is true it is very difficult for me to comprehend.

Also, if it's correct, what is the big deal about a decompression at 510 in a Global? Jeez, if that's the case one would have plenty of time to go to the lav to sh1t oneself in comfort before trying to get a mask on!

[galaxy flyer]

That what I understand is the cabin's ROC is on the test--essentially another outflow value that simulates a window failure. There is a huge inflow of air from the ACMs. I can tell you, turn off a pack at F450, and you can't feel the change in pressure.

I'll check my info this week.

[tdracer]

Galaxy, is it safe to assume that's done by analysis? It'd hate to think the Feds would make you actually test the loss of a window at 51k :eek:

[galaxy flyer]

tracer,

Let me get some details and confirm the test and it's results.

[galaxy flyer]

tdracer,

Spoke witha pilot familiar with the flight test protocol; yes, it's actually done at F510, but the cabin ROC is closer to 1,800-2,200 fpm when the simulated window is opened. I was off by an order of magnitude, I think.

[David Charlwood]

Did five years on the B747-200 what a beautiful aeroplane!

[imriozer]

After hearing the information gf shared with us, we need to know what gulfstream has to say about it.

They (bombardier( can use this information for marketing...