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Problem with disconnecting VNAV during midflight

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I just noticed recently that the VNAV won't engage after disconnecting it in midflight(Cruise) thus the autopilot only engages speed and altitude hold even if I press VNAV several times. I can only engage VNAV until the FMC passes top of decent. Does anyone know whats going on?PS.This also happens(VNAV disconnects) if I use the Speed Restriction function in the AT.Thanxpluto1168(kelvin)

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Guest Ian_Riddell

"I just noticed recently that the VNAV won't engage after disconnecting it in midflight(Cruise) thus the autopilot only engages speed and altitude hold even if I press VNAV several times. I can only engage VNAV until the FMC passes top of decent. Does anyone know whats going on?"Just went for a test flight to see if I could reproduce this problem, Kelvin. I couldn't, but found a problem of my own (D'oh)... I couldn't engage V/S from VNAV. I can't find anything in the Boeing Maintenance Manual to suggest that I can't select V/S when cruising in VNAV. I did however come up with other useless V/S trivia such as:With V/S engaged, the airplane holds the existing V/S if within +/-30 degrees pitch angle. If greater than 30 degrees, the aprlane attitude is reduced to 30 degrees and the corresponding vertical speed is used as a reference.Can anyone reproduce my VNAV PATH to V/S transition anomaly? (or is this an AA option?)Thanks.Cheers.Ian.

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Ian,I _know_ you did this, but just to make sure: Did you dial an altitude, other than your present one, to the MCP alt before trying the V/S button ? sorry... just checking, there have been dumber mistakes ;)br,Tero


PPL(A)

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Guest Ian_Riddell

"sorry... just checking, there have been dumber mistakes"Ooops! (I think you might have hit the nail on the head here, Tero) :-)This is interesting though(Dumb mistakes aside). What I'm getting in PIC is no response when I hit the V/S button when flying straight and level in VNAV PATH (altitude selector at current altitude). Should the aircraft transition to ALT HOLD in this situation or remain in VNAV? My training notes' transition tables show that VNAV to V/S to ALT HOLD is possible (but they may be oversimplified)."Did you dial an altitude, other than your present one, to the MCP alt before trying the V/S button ?"I'm just wondering what sort of altitude and V/S tolerances are involved here. If, for example, you are flying in VNAV and you hit the V/S button just when a gust of wind hits the airplane from beneath, this would give the airplane both a minor altitude difference and a minor positive vertical speed. In V/S, the aircraft is allowed to fly away from the selected altitude (ref: PIC manual). This seems to be the case in Aerowinx's PS1 with minor turbulence, but I'm wondering what tolerances apply to the real aircraft (My notes simply state that Zero V/S + Zero actual vertical speed will result in ALT HOLD engage. No tolerances are given. The Boeing Maintenance Manual does have a lengthy discourse on the requirements for altitude capture, but doesn't mention the values we would need to solve this problem).Expert input required, methinks ;-)Cheers.Ian.P.S. BTW, I remember a while ago, Eric (I think) was saying that on Pegasus-equipped aircraft, there is some wierdness regarding the actual height flown when in VNAV... It differs from both the Captain's and F/O's altimeters. On older aircraft, the altitude which the aircraft flys at depends on which A/P is engaged (e.g. If the Left A/P is engaged, the airplane flys at the altitude shown on the Captain's altimeter). V/S mode on a Pegasus aircraft might follow old aircraft logic. If it sees any discrepancy at the point V/S is engaged (from VNAV), the aircraft might depart the altitude shown in the MCP window.P.P.S. I apologize for hijacking this message thread :-abduct

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Thanx for your reply its getting kinda funny that some people get the opposite problem to what I'm currently having (well I think I had that problem before and still having it) if anyone out there can explain whats going on with VNAV please help! :-zhelpThanx againPluto1168(kelvin)

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