Originally Posted by
underfire
Dutchman, What does the chart say? Does it say "uncompensated baro-vnav"?
When we design the procedure, there is the NA above and below temperatures. If you are between the NA above and NA below, the OCS has been validated to avoid the obstacles, from IF onwards.
In FAA land, the NA below takes you to a 2.75 degree glidepath (ICAO goes to a 2.5 GPA) The NA above takes to you a 3.5 GPA.
So uncompensated, you are between basically 2.5 and 3.5 degrees and the OCS has been validated for this.
Therefore, if you are within the temperature boundary, you do not need to correct the FAF nor the DA/MDA altitudes. Again, if the chart states uncompensated baro vnav, which I think they all do, or it is assumed uncomp.
IF you get outside the temperature limits, you must either not use the procedure or correct...
Bottom line, when you use the procedure, you do not have to compensate if you are within the temperature boundaries. IF your system has the ability to compensate, all the better, but not req'd. The FMS, not the driver outside of the FMS.
Keep in mind, if you simply add a factor to the altitude, say the FAF or DA, while your system remains uncompensated, that is a problem. This is why many ATC vectors will not allow for compensation, due to the blend of ac with the system ability or not, and not to rely on the drivers to be consistent in the correction .
Thanks for your reply. It's really not too complicated after all. The 737s in our fleet don't compensate so we'll just fly the Baro-VNAV approaches without correcting FAF and DA down to the minimum temperature, and below this minimum we'll go with the LNAV minima and correct all minimum altitudes.
With regard to your last remark, we don't correct while receiving radar vectors, unless (!) one of the minimum altitudes in the procedure is below the minimum vectoring altitude (MVA). Without checking I'm not entirely sure if this is stated in the FCOM or just company policy though.
@
FlightDetent
That's exactly the reason why I believe adjusting the DA on a Baro-VNAV approach doesn't make sense in the first place, because of this increased distance to the threshold. But yeah, it's good for getting a better understanding of what we're actually doing, so cheers for thinking along with me!