767 too high cruise

[AirbusFan]

From an armchair aviator, what would be the expected responses if an excessive (i.e. unachievable) cruise altitude is entered on the ECON CRZ page and then executed while VNAV is active.

many thanks

[Omark44]

A message on the FMC - "Unable CRZ ALT"

[PAXboy]

You then look up: "Unable CRZ ALT" in the manual and it states: Unable to achieve theat crazy altitude you plonker.

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"I tell you, we are here on Earth to fart around, and don't let anybody tell you any different." Kurt Vonnegut, Jr.

[Charles Darwin]

But wouldn´t she try to climb? I think so, until she just couldn´t any more.
Just as brilliant logic as the v/s protection on the 757/767. :{

[PAXboy]

Amateur answer coming up: No. Because the computers know the weight of the a/c, it's power and design limits, as well as the climatic conditions. I sit to be corrected.

[Charles Darwin]

I´m afraid the onboard Sinclair Spectrum isn´t even that intelligent.

[FLCH]

We have software on our Boeings that start yelling " Captain ! she canna' take any more...she'll fly apart!!!".....

[LastCall]

AirbusFan

On the 767, if you are at cruise in VNAV and you want to find out how high you can go, you first select ALT HOLD to get out of VNAV, then enter the service ceiling of the airplane on line 1L of the ACT ECON CRUISE page, FL431, then execute it. You will get a message in the scratchpad area, MAX ALT FLXXX. (Rule-of-thumb is MAX ALT = OPT ALT + 3500').

Not sure if the airplane will try to climb if a higher than max alt is entered, then VNAV re-engaged. Never tried it, but I suspect the airplane will try to climb to the altitude that was executed. It won't get there of course.

Learned a valuable lesson about 10 years ago in an A320.

We were departing on a 5-hour trans-con flight. Just prior to take-off the load figures were received. The F/O entered an incorrect zero fuel weight on the PERF page. He entered a figure that was a several tons below our actual zero fuel weight which gave a corresponding less than actual gross weight readout. Being relatively new on the airplane at the time, I didn't catch it.

We took off and an hour later were cruising at FL350 and approaching a line of thunderstorms. We checked the FMC to see if we could make FL390 to get over them and the box said we could. Got our clearance to climb, and up we went.

Initial rate of climb was about 1000 fpm, then decreased to about 500 fpm out of FL365, then was zero around FL375 with max continuous climb power on. Wouldn't climb any higher, even though the FMC said we were OK for FL390.

Started checking things and discovered the zero fuel weight entry error, which got corrected. Found out we were too heavy for the altitude we were at. Back down to FL350 we went and all was normal from there.

Lesson learned: the box will do all the calculations and give results based on the data that was input. Bad data in = bad info out. All data entries are double checked by both guys now!

[AirbusFan]

Many thanks for the answers so far - plenty of laughs....

No VNAV 'disconnect', bells, whistles, T/C after T/D, deleted predictions etc etc?

[Charles Darwin]

The F/O entered an incorrect zero fuel weight


Bl**dy first officers! :E

[FLCH]

I know VNAV DISCONNECT shows on the 737, which I've seen plenty of times...I don't believe you will see it on the 7576, maybe I haven't screwed the FMC enough.....

[Pub User]

The FMC knows the weight you have entered, the climatic conditions you have entered and it knows the aircraft performance envelope, hence it can predict its max Alt. However, the 'VNAV engaged' envelope is slightly more restrictive than the aircraft envelope.

If you ignored the message 'Unable CRZ ALT' I suspect it would climb to the top of the VNAV engaged envelope, then drop into ALT HOLD, although like others here I've never tried it, and it's not in the manual.