gps altitude - best use of equipment
Thread Starter
Joined: Jun 2009
Posts: 21
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From: switzerland
B787 IAN Glidepath
Instructors claim that this FMC calculated glidepath, which is used like an ILS-glideslope for ALL non precision approaches where the database publishes an angle is NOT prone to cold weather corrections which suggests for me:
Boeing is using GPS altitudes for "target altitude at this point" when on final approach course!
Boeing is using GPS altitudes for "target altitude at this point" when on final approach course!
Joined: Mar 2011
Posts: 1,407
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From: engineer at large
Different issues here.
First off, for non-precision approach such as RNP, it is non-compensated baro for the altitude, which is very temperature based. Effective Glidepath is why there are temp limits for the procedure on the charts.
Depending on the FMS, the GPS altitude is calculated different ways, but all of them relate to the WGS 84 ellipsoid, not actual terrain (HW uses a lookup table from a grid, while GE uses a calulated solution)
The Boeing FMS does not use the GPS altitude for final approach unless you have a ground based augmentation such as GBAS.
First off, for non-precision approach such as RNP, it is non-compensated baro for the altitude, which is very temperature based. Effective Glidepath is why there are temp limits for the procedure on the charts.
Depending on the FMS, the GPS altitude is calculated different ways, but all of them relate to the WGS 84 ellipsoid, not actual terrain (HW uses a lookup table from a grid, while GE uses a calulated solution)
The Boeing FMS does not use the GPS altitude for final approach unless you have a ground based augmentation such as GBAS.
