Mountain Wave Question

[labbats]

Today around Denver I heard reports from several aircraft of +/- 30kts at FL370. It got me thinking... is there really that much margin from max down to initial buffet in cruise for most airliners?

I remember the Embraer went as low as 175kts at FL370 but I don't recall the top speed. And I know the MD80 doesn't have much wiggle room at FL370... heck you can only get there if you're empty. Just curious how much room Airbus and Boeings have from initial buffet to max forward. Thanks.

 

[GuppyWN]

Barely. A heavy 737 will have 40 knots max margin. Heavy and in mountain wave is less than comfortable.

Gup

 

[Green Banana]

There have been some high altitude stalls and over speeds due to this. When approaching the limits in a 737, my airline endorses to disengage the autopilot, pitch for airspeed, and RIDE THE WAVE!

 

[Ralph Cramden]

Taking off heavy from the west coast on a transcon the A320 most likely will not be able to do 370 by the time you cross the rocks. If the box says you can, it won't be with much margin. Going the other way you are usually light enough to get 380 with a good margin. Was over DEN yesterday around 1 PM local going east at 350 and got into several smooth but strong +/- 15 kt rides. It was setting up a big wave with 100 kt winds directly from the west.

Fifi's lazy airspeed control (alt cruise) doesn't respond well to the wave.

 

[Ty Webb]

I've had it happen over ABQ at 380 (FMC said we could do FL400 but remained at 380 due to concern about mountain wave). Glad we weren't at FL400. We, too, had +/- 30 Kts (at least), setting off alternating high and low speed alarms/shaker . . . . No choice but to descend, obviously.

 

[Mookie]

Barely. A heavy 737 will have 40 knots max margin. Heavy and in mountain wave is less than comfortable.

Gup

Maybe in a 700..the 8's and 9's are waaaaay tighter at altitude. And just bc the box says you can't do it, doesn't mean ya should

Mookie

 

[GuppyWN]

And just bc the box says you can't do it, doesn't mean ya should

Mookie

Can I get an AMEN!?

Gup

 

[RJ Bum]

Maybe in a 700..the 8's and 9's are waaaaay tighter at altitude. And just bc the box says you can't do it, doesn't mean ya should

Mookie

+1

Can I get an AMEN!?

Gup

AMEN!!!!! three words for ya, Auto throttle OFF.... learned that the hard way in a 737-800

 

[jetpig32]

+1



AMEN!!!!! three words for ya, Auto throttle OFF.... learned that the hard way in a 737-800

Exactly. Set them n1 carrots to CON or CLB. Most pilots have a better feedback loop then the autothrottles in mountain wave. You can get a good rhythm timing the peaks and troughs.