LPV approaches

[JammedStab]

Is this statement correct, I thought that even on an LPV approach the CDI sensitivity remains constant.

"In most cases, GPS CDI sensitivity stays the same regardless of the distance to a waypoint (so long as the receiver remains in the same mode). There is, however, one exception to that rule. When a WAAS-certified receiver is being used to fly an LPV approach, once past the FAF the CDI needles will behave like localizer/glideslope needles, becoming more sensitive as the aircraft proceeds down the final approach course."

[davidjh]

LPV approaches indeed are like ILS approaches because the lateral and vertical paths from the FAF to the MAP are angular in their displacement I.e. the deviation limits don't remain the same as they would in a "normal" GPS approach, which are linear. An example in the vertical path is: normal GPS approach flown 1dot below vertical path deviation indicator equals 75' low at the FAF and, if it remains a 1 dot deviation, will equal 75' low at the minimums. If this was an LPV approach, 1 dot below equals 180' low at the FAF and (depending on which source you use for information) only 18' at the minimums.

Hope that helps.

[g450cpt]

I have to second David's statement. LPV is in fact angular rather than linear, therefore the closer you are to the runway the more sensitive the CDI becomes.

[aterpster]

In LPV the technical specs require it to behave like ILS. It is angular and decreases to approximately +/- 350 at the runway threshold.

Some avionics vendors (Garmin being one of them) elect, though not required to, treat a WAAS navigator the same on an LNAV final approach segment, although the integrity and alerting is not as tight with LNAV as it is with LPV or LP. With LNAV the integrity and alerting is 0.3 n.m. at the runway threshold although the CDI scaling is +/- 350 feet just as with LPV and LP.

[Pontius's Copilot]

In European regulation, EU-Ops AMC 20-28 (guidance on LPV/SBAS approaches) the following appears in 7.1 Required Functions -

3) The deviation display must have a suitable full-scale deflection based on the required track keeping accuracy. The lateral and vertical Full Scale Deflections are angular and associated to the lateral and vertical definitions of the FAS contained in the FAS Data Block.

This is different from LNAV (RNP APCH) approaches where CDI scaling for final approach is indeed linear at +/-0.3nm full scale. AMC 20-27 refers.