737NG: Flying away of the selected altitude while in VS mode

[alistomalibu]

Hi

I would like to ask you about the VS mode in the 737NG.
I know that in the MD-80 family it is possible to do the following:
Fly at 3000 feet in ALT HOLD and SPD mode. After that you can change the altitude selector from 3000 to 4000 and you can engage VS mode. After VS mode is engaged you can now fly away of the selected altitude (4000). I mean you can start a descent from 3000 feet while in VS mode. This is a good option for visual approaches, where you want to fly with AP on, but you don’t want to reach any special altitude.
I read that it’s an option of the 737NG as well.
Could someone tell me if this is a real option in the real aircraft?
If so, does this option have an altitude limit? I mean, could be possible to fly away of the selected altitude while in VS mode in high levels, for example above FL300?

Let me know if you need more details
Thanks
Alejandro

[FCS Explorer]

YES, descending in V/S after setting a higher ALT in the ALT-window is possible on the NG. we actually used that for non-prec approaches before we became addicted to VNAV. :)
and there's no altitude protection/restriction - gotta stay sharp!

[alistomalibu]

Thanks FCS Explorer
Could you tell me what happen in this other scenario?
MCP alt window = 3000, ALT HOLD mode is engage
MCP alt window changed to 1000 feet, VS mode engaged (we are descending to the new altitude selected on the MCP)
Crossing 2000 feet we changed the MCP alt window to 3000 feet again.
What will happen?
Will the FMA go to ALT HOLD at 2000 feet?
or
Will it continue the descent in VS mode without altitude restriction?

Thanks
Alejandro

[FCS Explorer]

VS will keep descending.
unless you turn the alt-wheel reaaaally unusually slow and happen to have '2000' in the window when you're just descending thru about 2030 feet - in this rare case the AP might engage in "ALT ACQ" and then maybe "ALT HOLD".

[Rainboe]

Won't it capture as soon as aeroplane alt matches selected alt in the alt window? It will be in Alt Acquire, then as you have wound the selected alt more than a certain limit away from actual alt, won't it go into Alt Hold mode? Interesting question- not a normal procedure. I shall try and carry it out without the copilot noticing (I won't get away with it- they have eyes like hawks!).

[Denti]

Only if you turn the altitude reaallyy slow. Otherwise it won't capture and just continue descending. It is still our SOP for the least used method of doing non precision approaches (using V/S instead of VNAV or IAN) to dial the altitude window down to MDA and 500' above MDA turn it back up to the missed approach altitude.

[BOAC]

Yes - that works as Denti says

[Pilot Pete]

Won't it capture as soon as aeroplane alt matches selected alt in the alt window? No. Caveat explained by FCS Explorer. Interesting question- not a normal procedure. For your operation maybe. For some of us it IS our normal procedure for a NPA - we are not permitted to do VNAV approaches (yet).

PP

[alistomalibu]

Thanks for your answers

Denti said:
"It is still our SOP for the least used method of doing non precision approaches (using V/S instead of VNAV or IAN) to dial the altitude window down to MDA and 500' above MDA turn it back up to the missed approach altitude"

It's not better to select the missed approach altitude when you are reaching the MDA and the FMA change to ALT ACQ? (I think that happens 200' feet above the selected altitude, although you are flying a 1200 f/m descent - dive and drive...)
At 500' above MDA you probably are flying still in VS mode. I think that if you change the Alt window at this moment and you put a higher altitude like a missed approach altitude, you won't have an altitude restriction and you will keep descending as FCS Explorer said (you will need to level off using VS mode)

Thanks
Alejandro

[Denti]

There is no dive and drive or level off at MDA in our operation (dive and drive is considered an unsafe operation and will prevent passing an IOSA audit). All non precision approaches are done on a relative glidepath that respects all minimum altitudes with a mandatory go-around 50 ft above MDA if no visual contact. So the usual VS during a non precision approach (standard is to fly it fully configured) is somewhere between 550fpm and 800fpm, not 1200 which would of course violate our SOPs.

And yes, of course if you do nothing the autopilot will happily descent you into the ground operated like this, that is why we have mandatory approaching minimum, minimum and 500' callouts as gates where decisions can and should be made.

To switch back at 500' above MDA is actually something that originally stems from a boeing recommendation for VNAV flown approaches and the current drive with boeing is to have the same procedure for every kind of approach, which is why all our new delivered planes since 2006 have IAN capability which assures ILS like procedures and safety nets for every kind of approach, not VNAV/LNAV or VS for non precision approaches anymore.

[alistomalibu]

Hi Denti

Yes, I was thinking only in dive and drive approaches.
With a constant descent it sounds better your method.

Thanks
Alejandro

[IndAir967]

Thanks a lot Allistomalibu .. was about to ask the same question..
and thanks a lot guys.. its been one helluva heads up !

[Jinkster]

In alt hold, if you wind up the alt from say 2000ft to 3000ft it will arm vs.

If you wind the vs down to descend, it will descend. If you wind it up, it will climb and alt acquire and alt hold at 3000ft.

If it cannot maintain the speed (not sure limits!) it will revert to level change!

[Capt Ted Crilly]

hi guys,

i think if one changes the alt rrreeeeaaallllyyyyy slowly as some are saying the best you could get is alt acq. but once in alt acq and a new alt is set in the alt window the vertical mode will revert to the current rate of v/s again. so the fma in the pitch mode would go something like this 1) v/s 2) alt acq depending on how slowly one is moving the alt selector 3) v/s again at the current rate.

i stand to be corrected but v/s only has spd reversion protection on the 737ng i flew and now fly again ie spd gets too slow for the given rate of v/s commanded and the pitch mode changes to lvl chg( for selected spd) upon reaching alpha floor and vice versa for high rates of v/s descent.

rgds