Speed Brake - High Altitude vs. Low Altitude

[Airmann]

Does anyone have any information regarding speed brake usage at low vs high Altitude. Specifically are the speed brakes more effective at producing drag at low altitudes due to the denser air? In addition do some aircraft limit speed Brake extension angle at higher speeds?

Thanks

[ImbracableCrunk]

B737 - there's no max speed for Speed Brake because it's used to correct and overspeed.

[Intruder]

Since you are generally closer to the stall margin at altitude, you must be VERY careful if you use them to reduce speed without descending.

[Fursty Ferret]

I'm not aware of any aircraft limiting extension angle at high speed. I suspect that on fly-by-wire aircraft the initial extension to a pre-defined limit is swift (to prevent overspeed) and then a much reduced rate after that. Certainly on the A321 at high altitude / weights speed brake use would see VLS rocketing up the speed tape.

At high speed the effect of speed brake is pretty much pure aerodynamic drag. At lower speeds they tend to just disrupt lift, forcing an increase in angle-of-attack and consequently increased induced drag. They can still be effective but the effect takes longer and results in unpleasant wallowing around on the glide path.

[FlyingStone]

Some B737s with winglets feature speed brake load alleviation, which retracts the speedbrake to 50% at high gross weight / high airspeed conditions. On the MAX this is called Manuever Load Alleviation, which works in a similar way, but only at high loads and the speedbrake reduction will vary, as it's FBW.

[blind pew]

Fokker 100 had travel reduced by 50% at high speed…split tail design. Can’t remember the criteria but will ask someone.

[FullWings]

Originally Posted by Airmann

Does anyone have any information regarding speed brake usage at low vs high Altitude. Specifically are the speed brakes more effective at producing drag at low altitudes due to the denser air? In addition do some aircraft limit speed Brake extension angle at higher speeds?

As form drag is mostly a function of IAS, then there wouldn’t be much difference between deploying them at a given airspeed at sea level or 40,000’. What is different in passenger aircraft is that the achievable IAS without exceeding Mmo is greater at lower level, therefore more drag is available by flying at higher speed.

It seems like there are aircraft that limit deployment at high speeds under certain conditions for structural reasons, but that is the opposite of what you would prefer for an emergency descent or trying to correct an overspeed - I presume that even at a reduced level, the drag is deemed sufficient for these circumstances.

[Alpine Flyer]

Originally Posted by blind pew

Fokker 100 had travel reduced by 50% at high speed…split tail design. Can’t remember the criteria but will ask someone.

That was purely a matter of airload vs. hydraulic pressure. At higher speeds the air pressure on the barn doors would match the hydraulic pressure pushing them out at less than full extension. Can't remember the speed either, but I dimly recall that they would not open fully until below 200ish. Full opening at Vmo would probably have resulted in a descent angle making the Space Shuttle look like a performance glider....

Being a true speed brake rather than spoilers it would result in drag only while spoilers will reduce your AOAcrit in a hurry. Effectiveness as a speedbrake should be the same for any given indicated speed regardless of altitude.

[blind pew]

Alpine flyer
i seem to remember full air brake came in around 250 knots.
I can confirm your estimation of descent rate as tried an emergency descent going into FCO, brought the speed back a tad and then went for it. Got 16,700 fpm before I chickened out due to flight deck angle which put similar on a Trident one but with 1 and 3 in reverse 10,500 rpm, full air brake and 11,000fpm to shame.