B737NG VNAV App Question

[FCS Explorer]

ok, let's assume we're in 3.000ft and ALT HOLD. in the FMC an approach is programmed down to the rwy. now i set 4.000 in the ALT window and push VNAV. will it descend according to the FMC? or do i have to set an altitude lower than the actual to even get VNAV engaged at all?

[Denti]

Dunno actually what happens if you put in a higher altitude in the MCP panel, SOP is to put in MDA and then use VNAV, 500' above MDA put in the missed approach altitude.

Best way however is not to use VNAV at all and just use the normal approach mode for non-ILS approaches, boeing made it available for a reason with VNAV just being the inferior backup mechanic.

[de facto]

FCSExplorer,

You owe some food to IThoughtTheredBeFood :E BUT

If you set an MCP altitude higher than your your planned descent path and engage VNAV (Vnav descent now),you aircraft will go into an Energy Compensation ,reducing your airspeed .If the speed reaches a too low value,vnav will disengage as it wont be able to recapture your vnav profile.

You must set your Missed approach altitude when it is MINIMUM 300 feet ABOVE your current aircraft altitude to allow Vnav to continue its descent.
I believe you mentioned 4000 ft as being your Miss App alt, therefore you should set it not at 4000ft,nor at 3900 or 3800 but when you pass below 3700Feet....AND if you are within 2nm of the first approach waypoint(when your FMC transitions to "ON APPROACH" logic.

If you fly VNAV approaches I suggest your read Flight Navigation Systems, appropriate limitations, the FCTM and follow your Airline SOPs:ok:

In any case don't forget temp corrections next winter:cool:

[Callsign Kilo]

I believe Boeing advise you to set the published missed approach altitude when at least 300' below the missed approach altitude during the PTH segment of the approach - as defacto states. Our policy is for the PM to either set the missed approach altitude when the aircraft reaches Alt Hold at the published MDA (we only use VNAV for NPAs - MDA = published rounded up to the nearest 100') and then carry out a GA or if visual and before ALT Aquire, disconnect the A/P and A/T requesting the PM to recycle the FDs and then set the missed approach alt in the event of a subsequent GA.

I also cannot reiterate the importance of allowing for cold weather corrections and flying the correct barometric setting (especially if the QNH is low). The GP angle displayed on your LEGS page is based on a standard atmosphere and the path is flown utilising barometric reference. Be careful and avoid a nasty shock. It has happened.

[Checkboard]

If you set an MCP altitude higher than your your planned descent path and engage VNAV (Vnav descent now),you aircraft will go into an Energy Compensation ,reducing your airspeed .If the speed reaches a too low value,vnav will disengage as it wont be able to recapture your vnav profile.

Energy compensation only occurs when the aircraft has captured an altitude (set in the MCP) and consequently drifts above the VNAV profile - nothing to do with this situation. (Yes, I know what it says in the manual - very poorly written.)

The recommendation to set alt alert when 300' below the missed approach altitude is simply to prevent an inadvertent altitude capture.

In the circumstances supplied - in ALT HOLD, set the MCP to a higher level, push the VNAV button - if the FMC is between the FMC target altitude (depicted on the LEGS page) and the MCP target altitude (as in this case), VNAV will not engage. (FCOM 4.10.5)

[Whatsit Doingnow]

Best way however is not to use VNAV at all and just use the normal approach mode for non-ILS approaches, boeing made it available for a reason with VNAV just being the inferior backup mechanic.
Denti, I am curious as to what you mean by "normal approach mode". Are you talking about HDG and V/S? If I am conducting a NPA such as VOR or NDB approach, I always prefer to use VNAV/LNAV because I believe when used properly they are the best option and they reduce the workload considerably.

[Denti]

No, simply press the approach mode button and fly it like an ILS. Doesn't really matter what kind of approach, it all behaves similar to a normal precision approach (ILS/GLS/MLS). Only thing is you cannot fly NPAs dual channel (SINGLE CHANNEL in the PFD will be green instead of amber) and you will get an auto-callout at 100ft RA if the autopilot is still engaged. It is a standard 737NG method called IAN (http://www.boeing.com/commercial/aeromagazine/aero_22/737approach_story.html#3).

[Whatsit Doingnow]

Thanks for the info Denti, it sounds like an easier way of doing it.
Unfortunately our company has chosen not to avail iself of this IAN option, so VNAV/LNAV remains my preferred mode.