VNAV reversion mode from PTH to SPD in late B737NG.

[thiagomoltocaro]

Does anyone know the logic that the new FMC soft installed on B737 uses to revert from VNAV PATH to VNAV SPD automatically? In previous version, this reversion was made through the prompt in the Descent Page, nevertheless, recently, some AFDS/FMC have been reverting automatically,without any pilot action.

[caber]

One of my least favorite changes. The system will revert to speed now if you get high on the path or go into heading and get far enough off the lnav track. An annunciator light that this has happened would have been appreciated.

[Denti]

Could it be another customer option? Our new delieveries do not exhibit this behavior. You might have to talk to your fleet management.

[caber]

Possibly an option, my experience with the 737 comes solely from a single U.S. legacy carrier. It was introduced to us some time ago. Not an issue if you are paying attention but the reversion to speed from being high on making a crossing restriction is not an intuitive change.

[RAT 5]

The system will revert to speed now if you get high on the path or go into heading and get far enough off the lnav track.

It's been a while, but I seem to remember on the B733CL that if you selected HDG SEL while in VNAV PTH it tripped to VNAV SPD immediately. I can't remember about being high. (if I could remember it must have been lousy).
In my last NG if you were kept hi on path the VANV stayed in path, but the speed bug reduced to UP and the a/c dived for the path. The gotcha with teaching the newbies was they were fixated on the FMA which stayed in VNAV PTH, which was not totally true. It had the VNAV PTH captured and was trying to re-establish, similar to LOC CAP. Perhaps this trip to VNAV SPD and giving you direct control of the MCP and ROD is a thought to make the FMA more honest as the B733 CL was.

[AlexEspana]

I am currently flying the 733CL and as RAT said if you select HDG SEL it reverts automatically to VNAV SPD.

It depends on the FMC version, on the older versions it reverts to CWS Pitch :rolleyes:

[Al Murdoch]

Caber - there is a warning light isn't there? The FMAs?

[Technocraft]

On the NG with FMC version 10.1 if you get too high...500ft above your path on descend the pitch mode goes to VNAV SPD from VNAV PATH and the throttle mode changes from ARM to N1. So on the FMA the display will be N1-LNAV-VNAV SPD not the normal ARM-LNAV-VNAV PATH.

[Mikehotel152]

500' ? That's not particularly high.

[172_driver]

N1.. really?

.. what have I missed?

[framer]

Does anyone know the logic that the new FMC soft installed on B737 uses to revert from VNAV PATH to VNAV SPD automatically?
If your aircraft is operating on update 10.8 like ours is then this from our FCOM should apply:
VNAV reverts to VNAV SPD if a limit speed will be exceeded.
also this;
A path descent must be initiated while within the allowable cross–track error for LNAV , however LNAV may be disengaged during descent while remaining in the path mode. VNAV will remain in path regardless of cross-track.
Hope that helps.

[thiagomoltocaro]

Caber, you bring me some light when you talked that reversion occurs when HDG SEL / VNAV is selected. Do you know how offset from the track it must be to happen this reversion?

Regarding the verticel offset, the question is the same: how higher about the path the plane must be to revert?

And the last question: have you seen any information about this matter in any official boeing publication?

Once again, thanks a lot.

[framer]

I think the above quote from the Boeing FCOM regarding limit speeds would govern how higher about the path the plane must be to revert?
For example, if the aircraft was ten feet higher than the calculated vertical profile, it is unlikely that regaining it would exceed any limit speed and it would therefore remain in VNAV PTH. If however the aircraft was ten thousand feet higher than the calculated vertical path, then regaining the path would exceed VMO/MMO and VNAV SPD would result.
I stand to be corrected by folk more knowledgable than myself.

[plain-plane]

VNAV is full of costumer selectable options...

And this one is one of the more ****e ones... and apparently one that is getting more and more popular, as airlines apparently are less concerned about level busts than flying a few knots faster than planned or in extreme and remote cases a slight overspeeds :ugh::ugh: