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An air carrier pilot questions the practice of flying ILS precision overlay approaches to DH minimums using GPS; LNAV; and RNAV when RNAV approaches using the same GPS; LNAV and RNAV adhere to much higher MDA minimums.
| Attributes | |
| ACN | 836540 |
| Time | |
| Date | 200905 |
| Aircraft 1 | |
| Make Model Name | Commercial Fixed Wing |
| Operating Under FAR Part | Part 121 |
| Flight Phase | Final Approach Initial Approach |
| Flight Plan | IFR |
| Person 1 | |
| Function | Captain |
| Qualification | Flight Crew Air Transport Pilot (ATP) Flight Crew Multiengine Flight Crew Instrument Flight Crew Flight Instructor |
| Experience | Flight Crew Last 90 Days 250 Flight Crew Total 15000 Flight Crew Type 3000 |
| Events | |
| Anomaly | Deviation - Procedural FAR Deviation - Procedural Published Material / Policy |
Narrative:
This is not a specific incident per se; but rather a procedure that I believe is flawed and unsafe. Our air carrier's procedures allow us to use a GPS overlay (LNAV/VNAV) for any published approach which has a glide path angle; a ff (final fix) or MM (missed approach fix). If a DH is published; our procedures further allow us to fly this approach in LNAV/VNAV down to the published DH. I believe this is unsafe. To give a specific example; please look at the approaches into lax for runway 24R. The ILS 24R has minimums of 320' (200 above TDZ). The GPS runway 24R has minimums of 604' (484'). Both approaches have identical final approach courses; and identical missed approach procedures; so the terrain criteria for the terps approach buffer zones should be identical. The question; then; is if I am flying a path overlaying the ILS (using only my GPS in LNAV/VNAV mode...the ILS does not have to be monitored or even operational); why does this allow me lower minimums than flying the identical path; but calling it a RNAV approach? This cannot be the intent of allowing us to use GPS overlay approaches.
Original NASA ASRS Text
Title: An air carrier pilot questions the practice of flying ILS precision overlay approaches to DH minimums using GPS; LNAV; and RNAV when RNAV approaches using the same GPS; LNAV and RNAV adhere to much higher MDA minimums.
Narrative: This is not a specific incident per se; but rather a procedure that I believe is flawed and unsafe. Our air carrier's procedures allow us to use a GPS overlay (LNAV/VNAV) for any published approach which has a glide path angle; a FF (Final fix) or MM (missed approach fix). If a DH is published; our procedures further allow us to fly this approach in LNAV/VNAV down to the published DH. I believe this is unsafe. To give a specific example; please look at the approaches into LAX for Runway 24R. The ILS 24R has minimums of 320' (200 above TDZ). The GPS Runway 24R has minimums of 604' (484'). Both approaches have identical final approach courses; and identical missed approach procedures; so the terrain criteria for the TERPS approach buffer zones should be identical. The question; then; is if I am flying a path overlaying the ILS (using ONLY my GPS in LNAV/VNAV mode...the ILS does not have to be monitored or even operational); why does this allow me lower minimums than flying the identical path; but calling it a RNAV approach? This cannot be the intent of allowing us to use GPS overlay approaches.
Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.