Another maybe stoopid question from the boys at the bar, cruise speed.

[John Hill]

Last nights' bar room discussion, not a pilot among us, gave rise to the comment that long distance heavy aircraft (the example was B747s across the Pacific) are normally cruised at a speed not much above what would be the stalling speed at that altitude! As you would expect some roundly decried this statement but it did get me thinking especially the comment that if the aircraft had to slow for any reason it would have to descend to avoid stalling.

Any comment from those who might know? Thanks.

[Intruder]

Generally, even at heavy weight and high altitude, there is at least a 15 KIAS margin between cruise and low-speed buffet, plus whatever the margin is built into the buffet warning speed tables. For most of the flight the margin is much more.

[John Hill]

Thanks! Now I am off to try an learn what 'low speed buffet" is!

[Mad (Flt) Scientist]

Very approximately, your speed for minimum drag will be about 30% above your stall speed (it will vary with type and configuration, but that's a decent enough rule of thumb).

So there would be no benefit in flying slower than that "minimum drag speed" - you get there slower, and use more fuel in the process.

I'd be astonished if any modern airliner cruises, even at "long range cruise" speeds, below that speed.


Low speed buffet is the vibrations set up by the very start of the stall, as small areas of the wing start to suffer small flow separations. It isn't the stall, it's the first natural warning that you are approaching the stall.

[BOAC]

John - a climb to higher altitude is not normally made until there is sufficient 'fat' in the margin between the critical speeds, both high and low.

[John Hill]

Thank you gentlemen for your explanations.

[411A]

Last nights' bar room discussion, not a pilot among us, gave rise to the comment that long distance heavy aircraft (the example was B747s across the Pacific) are normally cruised at a speed not much above what would be the stalling speed at that altitude! As you would expect some roundly decried this ....

Cruising just above stall speed.
Bad results, if tried.

And, I expect a few have tried...:hmm: