FMC Max cruise altitude.

[Rocky Rookie]

Had a route check ride with a long time airline captain in his late forties and an old check pilot on the jump seat. It was a transoceanic flight at FL 350 as we encounter some high clouds giving rise to some light to moderate chops. The skipper checked the route forecasts for any turbulence and tropopause, etc and checking the FMC/QRH ( it was his check flight as well ), the maximum FMC cruising altituse was FL375. Checking the weather radar, there wasn't any CBs and the route legs was a straight great circle route, he then requested for FL370.

The check airman on the jumpseat questioned the decision to climb to FL370 as he reckoned that it was too close to maximum and there was turbulence. The skipper maintained that the aircraft's ( a B777-300 ) in this instance was limited by max cruise thrust available and not by manoeuvre margin. He has no qualms about climbing to the maximum altitude as he has already check on the QRH ISA deviation, manoeuvre margins and max. cruise limits. He was comfortable with the climb thrust available to ride out any speed shortfalls. The check airman was not impressed and went on and on with the subject of " unsafe " operations and even showed us a performance book on buffet onset and manoeuvre margins.
It turn out that the flight at FL370 was smooth and uneventful; however checker was still not happy. Skipper maintained that he had checked ISA deviation the tropopause level and in his judgement there was no cause for fear that the aircraft could not maintain speed.

Skipper, though relatively younger was an airline veteran with lots of long haul experience. Check airman rose from bush flying, GA, and only got his break into the airline relatively late but mighty proud of his " checking and aircraft handling " prowess. At debriefing after flight, a big argument ensued with each guy maintaining that each was right.

Poor rookie me, after analysis my gut feeling was the skipper was right but the checker held the trump card. What do you professional out there think?

[Cardinal]

I can't speak for the Boeing, but the Airbus' computed "Rec Max FL" displayed in the FMC includes a maneuver margin (I want to say 1.3g, but don't have a manual handy). So sure, FL370 seems perfectly workable as long as skipper was reasonalby assured they would escape any turbulence.

[411A]

Some of the older guys (and I can say this with some authority, as I'm one of those much older guys, although highly informed) seem to think that you absolutely need 1.5G buffet margin, even when it's gin smooth.

I say...nonsense.
Your skipper was right.

[Beg Tibs]

Agreed....just dont go too close to max and took close to thunderstorms, That temperature rise can hurt ..!

The max altitude is a thrust issue and should encompass the 1.3g initial buffet margin. Ie, at MCRT, there will be a pad of 1.3g on the buffet boundary.



Your skipper was correct.

[skiesfull]

Correct in his assessment, but probably unwise if the Checker argued against it. However, what did you say as the operating co-pilot? You have an important input to any and every part of the operation - never forget that.

[mrjet]

The max altitude in the FMC is the lower of the following
1) Max certified ceiling
2) 1.3 g altitude (JAA is .3g, believe FAA allows a little less margin of .2g)
3) Thrust limited altitude

Can't speak for the 777, but a 737-800 with winglets would be limited by thrust when ISA goes above approx +18C. Below that it will be limited by maneuvering margin, or max certified ceiling if at light weight.

My opinion is that you only climb close to max alt if you know it will be smooth at the higher level. However, if it's not smooth you could always descend back down to increase your margins again.

The .3 g margin is there to keep you sufficienly away from the coffin corner for obvious reasons.

Being thrust limited is not necessarily any better than being maneuver margin limited since the aircraft may end up stalling. If in HDG SEL on the 737NG you can in fact stall the aircraft during a turn since the thrust required may exceed thrust available.

[stiffwing]

Another area of concern when flying so close to max altitude is if you get a TCAS RA "climb" warning.. If you subsequently went straight into coffin corner you may then have the added drama of a stall warning, which overrides the TCAS climb warning. Not a comfortable situation. I was always taught to leave at least 600' spare.
I realise many of these routes are one-way and that you shouldn't get such a warning, but with fuel such an issue I note many aircraft at non-standard levels these days in an attempt to fly at or near optimum.

[Capt Fathom]

The FMC Max Alt is only as good as the ZFW you have typed in!

A lot is riding on the accuracy of that ZFW!

OR

You can ascertain your real weight by climbing until you get buffet..then just go through the chart in reverse to find your real AUW.

Just kidding...:uhoh: :E

[mabrodb]

Looks like @ 530,000lbs (or 239t) you have 1.33 buffet, from there it goes to 1.39g @ 500,000lbs (or 226t)

What was your GW @ the time?



777-300ER GE90-115BL CONFIG 01 REPORT VERSION = 2.6

UNITS FOR INPUT VARIABLES: UNITS FOR ALTITUDE ARE: FT UNITS FOR RATE OF CLIMB ARE: FPM
------------------------- UNITS FOR WEIGHT ARE: LB UNITS FOR TEMPERATURE ARE: C

ALTITUDE = 37000 FT WIND = 0.0 KT DELTA ISA = +0.0 DEG C OAT = -56.50 DEG C
SPEED OF SOUND = 573.569 KTS DELTA = 0.21379 THETA = 0.75187

INPUT SPEED TYPE = TABLE CRUISE LIMIT POWER SETTING = CRS CRUISE INPUT TEMPERATURE TYPE = DELTA ISA

SPEED SELECTED = LRC VREF INCREMENT VALUE = 0.000 RATE OF CLIMB LIMITS BUFFET LIMIT IS: 1.30
R/C POWER
VMO/MMO CUTOFF OPTION: YES INCLUDE APU FUEL FLOW: NO 300.00 CLB
0.00 NONE
------------------------------------------------------------------------------------------------------------------------------------

FUEL MACH IAS EAS GRND AIR FM CL D FN BODY R/C N1-1 N2-1 N3-1 G'S
WEIGHT FLOW TAT CAS TAS SPD GRND FM W/DELTA CD D/DELTA FN/DELTA ANGLE AVAIL N1-2 N2-2 EGT1 TO
LB LB/HR DEG C KTS KTS KTS NAMS/LB LB LBS LBS DEG FPM DEG C BUFF
------------------------------------------------------------------------------------------------------------------------------------
560000 * NOT POSSIBLE TO GENERATE VALID DATA FOR THIS POINT BECAUSE BUFFET LIMIT WAS EXCEEDED
 FUEL MACH IAS EAS GRND AIR FM CL D FN BODY R/C N1-1 N2-1 N3-1 G'S
WEIGHT FLOW TAT CAS TAS SPD GRND FM W/DELTA CD D/DELTA FN/DELTA ANGLE AVAIL N1-2 N2-2 EGT1 TO
LB LB/HR DEG C KTS KTS KTS NAMS/LB LB LBS LBS DEG FPM DEG C BUFF
------------------------------------------------------------------------------------------------------------------------------------

545000 * NOT POSSIBLE TO GENERATE VALID DATA FOR THIS POINT BECAUSE BUFFET LIMIT WAS EXCEEDED

530000 15201 .8396 274.1 256.8 481.6 0.03168 2479019 0.5155 26529 37403 2.48 1015.9 84.7 1.33
-25.95 274.1 481.6 0.03168 .02580 124085 174949

515000 14799 .8400 274.2 256.9 481.8 0.03255 2408858 0.5005 25818 37403 2.37 1111.0 84.1 1.36
-25.93 274.2 481.8 0.03255 .02509 120759 174948

500000 14452 .8399 274.2 256.9 481.8 0.03334 2338697 0.4859 25193 37403 2.27 1182.7 83.5 1.39
-25.93 274.2 481.8 0.03334 .02448 117838 174948

485000 14128 .8394 274.0 256.7 481.5 0.03408 2268536 0.4719 24613 37403 2.17 1238.4 83.0 1.43
-25.97 274.0 481.5 0.03408 .02395 115125 174949

470000 13802 .8381 273.5 256.3 480.7 0.03483 2198375 0.4588 24035 37404 2.09 1289.0 82.5 1.47
-26.07 273.5 480.7 0.03483 .02346 112422 174951

455000 13470 .8353 272.5 255.5 479.1 0.03557 2128214 0.4471 23452 37405 2.03 1326.5 81.9 1.51
-26.27 272.5 479.1 0.03557 .02305 109697 174956

440000 13130 .8309 270.9 254.1 476.6 0.03629 2058053 0.4370 22862 37406 1.99 1343.9 81.2 1.56
-26.59 270.9 476.6 0.03629 .02271 106936 174963

425000 12759 .8245 268.5 252.2 472.9 0.03706 1987892 0.4287 22238 37408 1.98 1377.9 80.5 1.62
-27.05 268.5 472.9 0.03706 .02243 104017 174974

410000 12349 .8162 265.5 249.6 468.1 0.03791 1917731 0.4220 21569 37411 1.99 1422.0 79.8 1.68
-27.64 265.5 468.1 0.03791 .02220 100888 174987

400000 12056 .8098 263.2 247.7 464.5 0.03853 1870957 0.4182 21095 37414 2.00 1471.6 79.3 1.72
-28.09 263.2 464.5 0.03853 .02206 98670 174998
 CRUISE PERFORMANCE TAB OUTPUT