Cold Temperature and VNAV approaches

[Walter Sobchak]

Whilst I was reading the OM check thread I started wondering about the temperature corrections during a VNAV approach.

How do you correct for it during a VNAV approach(B737NG)? According to Boeing you shouldn´t alter anything from FAF onwards because you are messing up the programmed approach logic, except as stated in FCTM 5.36 “... crews should make cold temperature altitude corrections by applying a correction from an approved table to the waypoint altitude constraints.”

So what should we do?
Use the descent forcast page?
Change values from FAF onwards?

I am quite sure the isa dev on the descent forcast page allows only for a rather high temperature(can´t recall the value) and I think it is for descent planning only.

Anyone in the know? Meanwhile I am back to good old V/S.

Regards, the Walter

[Empty Cruise]

In the DESC FCST page, you can enter the ISA deviation & QNH. This should allow the FMC to compute geometric altitude. However, I don't have the FCOM at hand right now, so no reference @ the moment.

Will come back once I've either comfirmed or disproven my recollections :O

Brgds
Empty

[Crossunder]

Hm. Although we have Universal FMS' installed - I can tell you that they have a special "TEMP COMP"-feature that will allow you to enter the destination airport ground temperature. The box then calculates the new altitudes automatically...
Should be the same for every manufacturer/software??? Haven't you been given the chance to practise this in the flight stimulator on your PC/OPCs?

[bigmountain]

You will generally be safer if you do not modify the altitudes in the FMC. You will generally get the reason as " chances of corrupting " the Fmc. Although the likely scenario would be the chance of error insertion while modifying the altitude constraints.

The best way to fly a VNAV is to fly an approach procedure that is approved to fly under cold weather conditions instead of standard ISA temp. As more and more carriers are discovering VNAV you will see more approach charts offering temp limits for using VNAV e.g valid uptoISA -15 0 or ISA -30 etc.
Ideally that would be the way to go .Ie dedicated VNAV approach procedures ,however we still need to cater for the larger GA aicraft operators who still employ the duck and drive approach for Non Precison approaches. If only a cheap solution for VNAV becomes available will we see a global push for stabilized constant descent approaches, and would ultimately be reflected on approach charts

BM

[Walter Sobchak]

to Crossunder:

Hm. Although we have Universal FMS' installed - I can tell you that they have a special "TEMP COMP"-feature that will allow you to enter the destination airport ground temperature. The box then calculates the new altitudes automatically...
Should be the same for every manufacturer/software??? Haven't you been given the chance to practise this in the flight stimulator on your PC/OPCs?

The Smiths FMC offers only ISA DEV/QNH alteration on the DESCENT FORCAST page and I can´t find anything in my documentation concerning approaches.

to bigmountain:

As long as I don´t know for sure how to modify correctly I am back to V/S and correct it manually during cold weather.

Thanks for the replies

Greetings, the Walter