757 uncommanded descent
 

In the climb for FL340. VNAV engaged. asked to pass FL310 10 miles before....... selected full climb, speed intervene.

Made the level, no problem.

closed the speed window, selected climb 1.

Next thing I know aircraft was in 1500fpm descent. Took out automatics, eventually fd's caught up. re-engaged c cmd, flight continued normally.

Anyone seen this before?

If so, know why?

The obvious question - no-one had put '310 - 10 before' in the legs page?:)

What were the the constraints for the next fix when you selected VNAV?

Sounds like there was a lower height constraint left somewhere.

What altitude was set on the MCP?
"Closed the speed window and selected CLB1" - at what speed was this done? Perhaps the a/c was below zero RoC speed and needed to descend to accelerate to a safe speed.

FL340 in the MCP

FL380 was in the CLB page

M0.76 when CLB 1 selected

Next hard constraint was FL150 at Bexil

(didn't use wpt-10 FL310)

Sounds to me like mustafagander has probably got it right. If you opened the speed window, presumably that was to bring the speed back a bit and help out with a bit of a 'zoom climb' - bringing the speed back to M0.76. When you closed the speed window the commanded speed presumably increased to something like M0.79 but at the same time you reduced thrust by going from full climb to climb 1.

FL Change - Heading Select - its the only true way !

Half agree with beamer. Ive seen too many "strange" things happen in VNAV (all finger trouble). Always FL change to TOC. Not been caught out with LNAV yet.

The FMC may in some circumstamces descend you if it feels that it needs to sacrifice height for speed...Although at M.76 the deficit to M.79 is pretty small and doesn't sound like it should be any where near the need for such speed protection - even if you reduce thrust from CLB to CLB1.

F340 in clmb1 sounds a bit of an ask. Were you empty?

Embeded in the software but not explained in the fcom regarding vnav mode.
At a frequency of 12.3 x 3(as a derivative of tas) the vnav function of the afcs shall command a random act of utter bollox.
The following actions are required ; Pf- put down crossword and utter "wtf is it doing that for? "
Pnf- call "hmm,, on the scarebus we used to etc etc,,"
Pf- "Flch!"
pnf-"Check"
Pf-(Capt) "Three letters, the cat sat on this, middle letter A ??"

H S LOVECHILD- You must have flown the TriStar too:} Although it would have been the Eng who would have put down his porn, the FO wasn't allowed to touch anything!

When I converted on to the 757 in 1983 LNAV had not been installed/certified, but we still suffered from numerous uncommanded descents, particularly on LGW-Palma-LGW sectors. Outbound the FMC would decide it was time to start the descent just north of the Pyrenese, inbound it tended to happen around Nantes. Boeing reckoned it was caused by a bug in the FMC program that was triggered by certain DME/VOR combinations. It got so bad at one stage that I used to disengage the autothrottle (which overcontrolled anyway) when in the suspect areas, then selected FL hold when the premature descent was commanded.