737NG VNAV and Temperature Corrections
 

Our current SOP's do not allow use of VNAV during NPA's with the temperature below 0 Celcius. I was wondering how many operators are allowed to correct for temperature on the FMS LEGS PAGE (EG: the FAF and IF if required)) and then fly the approach in VNAV.

I am also looking for information related to the understanding of geometric path. I assume that with u10.7 installed this option is enabled but not 100% sure.

Many thanks in advance.

We do not have such a restriction, but airmanship dictates an appropriate correction is applied. It's surprising what a difference it makes, and you can confirm it with radio altimeter (when it is not indicating -8'!). We wouldn't fly such approaches in VNAV at that stage though.

We can fly VNAV approaches although IAN is preferred for NPA. Temperature corrections for all minimum altitudes have to be used though (MDA, MSA etc) and we are free to modify the waypoints to reflect the cold temperature correction for VNAV Approaches.

However for IAN approaches a modification of waypoints is not allowed as the FMC supposedly does an automatic correction, a correctly filled in Descent Forecast page is required for that.

Would you use VS following the VNAV Path or the conventional descend to MDA and wait until MAP?

We wouldn't use LNAV or VNAV ever for such approaches. Always VS- that is the only way we do it in the sim.

We fly VNAV with temperature below zero degrees Celsius.
VNAV is the company preferred non-precision app.
We cold weather correct all altitudes below MSA in the legs page, except the runway point, where you cannot change the altitude. And it works a treat…

Thanks for the information. That is the way we did it on the 767 and it worked quite well. It is not uncommon to see -30 Celcius in January in many parts of Canada which leads to a large altitude correction.

There are legal issues with modifying procedures from the nav database. Unless you are specifically authorised to do it. DON'T!

FE Hoppy is dead right! You may, if you choose, amend Database data contained in the FMC, but, after doing so, VNAV approaches are FORBIDDEN!!!:eek:

VNAV approaches (where available AND approved) are legal, so long as the FMC data has not been amended.:ok:

Regards,

Old Smokey

Seems to depend on legislation under which you operate. We have this rock hard "forbidden", or better "not allowed", only for amended IAN approaches, but not for VNAV ones. For those a cold temperature correction may be added and must be added if below -30° C.