777 VNAV and Altitude Constraints

[Ztr01]

I am a new cruise relief pilot on the 777 and have a question regarding VNAV:

If MCP Speed Intervention is used (speed window open) in the descent, the ptich mode will revert from VNAV PTH to VNAV SPD to maintain the MCP Speed. According to FCTM 1.36, "the selection of pitch mode other than VNAV PTH or VNAV SPD will result in risk of violating altitude constraints". This basically says VNAV SPD will respect altitude constraints. However, I dont quite understand how a "pitch for speed mode" (VNAV SPD) will resepct an altitude constraint?

I have seen some threads that say VNAV SPD will respect an "AT OR ABOVE" altitude constraint in the descent (and conversely in the climb) but I cannot find any references in the book. Any guidance will be appreciated thanks.

[rudestuff]

I can tell you how it works on the 767: Altitude constraints determine the descent path, whether its idle (Idle/Path) or geometric (Spd/path). You're right to question VNAV speed respecting altitude constraints.

Speed intervention (other than App logic) will prioritise speed over path and you'll 'decouple' from the path. If you slow up you'll go above the path, speed up and you'll go below the path. You could also be in VNAV speed (window closed) following a late/early descent, high/low on profile and still bust a constraint. Basically it's telling you that the constraints can only be guaranteed if you're following the magenta diamond. The best way to do that is vnav path.

[FullWings]

If you have the speed window open in the descent, it is possible to be above an FMC altitude constraint (as you have fixed the speed and it may not be appropriate to meet the restriction) and the VNAV pointer will show this, but VNAV should not allow you to go below it as it uses the higher of the MCP altitude and constraints in the FMC as a level-off altitude. All VNAV modes respect FMC restrictions as far as they are able to (i.e. you won’t go through a FMC at or below in the climb or an at or above in the descent) but basic modes like V/S, FPA and LVL CHG ignore them and only go by what is set in the MCP.

[PPRuNeUser481044]

Are you sure that simply having the window open in VNAV causes the aircraft to go into VNAV SPD? On the 737, I am nearly sure I have had the speed window open, VNAV engaged and above profile, bugged a higher speed, and as the aircraft has re-intercepted the original path .. it goes into VNAV PTH - with the speed window still open (at which point I close it). I am also sure I have opened the window (to bug a higher speed on ATC request) whilst on/below path, and the aircraft has kept VNAV PTH, it just adds thrust. Then again, it is nearly 2AM ... recollections may vary. It could also be a difference among Boeings.

[jetpig32]

Originally Posted by tecnamflyer

Are you sure that simply having the window open in VNAV causes the aircraft to go into VNAV SPD? On the 737, I am nearly sure I have had the speed window open, VNAV engaged and above profile, bugged a higher speed, and as the aircraft has re-intercepted the original path .. it goes into VNAV PTH - with the speed window still open (at which point I close it). I am also sure I have opened the window (to bug a higher speed on ATC request) whilst on/below path, and the aircraft has kept VNAV PTH, it just adds thrust. Then again, it is nearly 2AM ... recollections may vary. It could also be a difference among Boeings.

Depend. Does your company have Geometric Path Full or App only enabled?

[skkm]

As FullWings says, if you have departed below the path in VNAV SPD with the window open, the aircraft will go to PTH and level off to comply with an "above" constraint that's above the MCP altitude. It won't comply with a "below" constraint if you have departed above the path as there is no way for it to achieve that.

Tecnam, on the 777, aside from the case above where it's temporarily levelled for a constraint, the only time you can have the speed window open in PTH is once you are in "on approach" logic. Prior to that, the only way to get back to PTH is to have the window closed and be within 150' of the path.

[RudderTrimZero]

...will result in risk of violating altitude constraints

Am I the only one who finds that statement from Boeing to be an insult to my intelligence?

You won't find that kind of suggestion from Airbus where everything "managed" is king.

[hawjl]

When there is a altitude restriction, and the aircraft is close to that altitude, VNAV SPD will turn to VNAV ALT unless you press the alt selector to delete the restraint.