Maneuver margin

[Jack C 8]

A friend of mine and I are doing a fllight. Hes in the -300 I'm in the 200. We are both at FL400. His current GW is 478,000. So no where near max gross weight. He has no maneuver buffer of coffin corner. Like 3 or 5 knots. SAT -51. Same as mine. I have about  75 knot maneuver margin. His MAX FL on CRZ page said 39,400 mine is 42800 feet. He decided to drop only 2000 feet to FL380 and the maneuver margin split to about 50 kts. I dunno but from my RW experience this doesn't make sense. Any ideas?

[downscc 1,327]

Full names in on all posts in the PMDG forums please.

 

Different airplanes, different wingspans, etc.  The longer wing on the 200LR/F give it better high altitude performance.  Getting to FL400 or 410 is pretty rare in these airplanes, they are carrying at least 85% of max payload and long trips fill the tanks. Regardless, I've been squeezed at altitude on several occasions. It happens.

[Jack C 8]

Hey Dan. After I viewed him from the outside I noticed he was a 200 and I guess he was a 200. Regardless, out wouldn't have mattered if he was a 200 or 300. Like I mentioned he was 300,000 pounds under gross weight. And another thing, dropping 2000 feet from 400 to 380 shouldn't give you an extra 50 kt maneuver margin. IRL I fly the 37 600/700/800 and the 767-300 and none of them would do that. Mind you I don't fly the 777 but can't imagine it doing that.

Jack Colwill

[downscc 1,327]

Jack:  I agree, the observation is odd but also I'm a little skeptical of the way the information was collected.

 

The PMDG 777 will usually have a more than adequate buffet margin at any level below maximum operating as indicated by the FMS.  I've squeezed myself once when I locked at 0.84 for a flight level higher than optimum for a NAT segment but that was mach buffet region only not the stall buffet region.

 

I usually set my loads to either full or 85% and rarely get above FL380, and I've never seen what you've reported.

[Jack C 8]

I know Dan that why I'm reporting it. He showed me his screen through Skype so it wasn't misinformation he was giving me.

Jack Colwill

[downscc 1,327]

I took a look Jack, I had a B77L saved scenario at FL370 so I added fuel to bring the GW up to 480 klbs and started a FLCH climb to FL400.  The stall buffet slowly increased from 228 KIAS at FL370 to 230 at FL385 and 242 at FL400, the unexpected was the mach buffet that stayed on the dial at the same spot just below the red bricks then above the FMS MAX CRUISE of FL391 it began a rapid decrease to do the coffin corner squeeze.  I end up at 0.812M/426 KIAS with barely 5 kts of margin.

 

Even if I empty the aircraft to only 362 klbs the FMS MAX CRUISE is FL395 with a slightly larger margin but still less than 10 kts.  Clearly this aircraft does not like FL400.  Drop down to FL390 and the mach buffet increases almost 20 kts with the stall buffet decreasing about 10 kts, to give a comfortable margin of almost 40 kts.

 

What I take away is that this aircraft should not be operated above FMS MAX CRUISE, and like most it probably is operated at least a 1000 below.  The technical reason behind this behavior I would guess to be related to the super critical performance of the wing, but I'm the wrong kind of engineer for that discussion.