Originally Posted by
Raptor71
First, is it necessary to correct the DA and platform altitude for cold temperatures as long as the OAT is above the minimum published temperature?
Not when flying LNAV/VNAV, but yes when flying LNAV or other types of non-precision approach.
Besides, in your airline do you add 40/50ft to comply with the CDFA technique?
Company specific, not required in some cases.
If we do not correct these altitudes, is it acceptable to fly a lower angle of descent?
Yes, that is what procedure is designed to do, keep you safe up to a certain temperature.
Does the procedure guarantees obstacle margin as long as the temperature is above the minimum published temperature? this margin decreasing when we are close to the mini published T.
Yes as per above
What happens if I decide to correct the altitudes (platform) and in consequence to fly the published angle of descent? Is that acceptable?
This is required technique below MSA when using LNAV or other non-precision approaches, not LNAV/VNAV
Finally, how does the system reacts if I put the temperature and QNH in the DESCENT FORECAST page?
Temperature correction for descent page and PERF INIT are different in the sense of what is trying to be achieved.
For climb we want to predict a Mach Number at altitude and an ISA deviation is used to correct IAS and initial Mach number at the forecast altitude/wind/temperature/level.
For descent, the ISA DEV is the average ISA deviation during descent, something quite difficult to predict and not something provided by many operators as close to ground ISA deviation can be significantly different from aloft. This correction only has an impact when ISA deviation throughout entire decent is +15C or greater, therefore most airlines ignore this entry unless flying in very hot regions of the world.
QNH on the other hand is used to correct for difference in altitude during transition from flight levels, therefore it moves the T/D point to take advantage of this information. it also is used for non-barometric approaches like using VNAV, which make use of this information. The 737NG is a non-barometric capable aircraft only, whereas the Airbus 320 is Baro-Aided and thus does not "need" this information.
I know that some FMC's will make some kind of corrections to keep optimum obstacle margin if temperature decreases close to the minimum temperature published for the procedure. Does it "update" the FAP ( making it closer to the runway)?
No, it does not