Both the LNAV and the LP have the same ROC (Required Obstacle Clearance) of 250 feet on the final approach segment between the FAF and MAP. The difference is that the LNAV has a much wider surface that is evaluated for obstacles. The primary protected area LNAV is +/- 0.6 NM either side of the course centerline. The LNAV approach requires an integrity value of half the protected area, or +/- 0.3 NM (556 meters).
The LP is an SBAS approach and takes advantage of the greater accuracy available with an SBAS. The area protected is in the shape of a trapezoid that at 8.26 NM (50,200 feet) starts at +/- 1 NM wide and tapers uniformly from this point to a point 200 feet before the threshold where it reaches +/- 700 feet. If the FAF is located at 5 NM from the threshold, the taper is down to +/- 0.6 NM. The main point is that obstacles can affect the LNAV, but not the LP if they fall outside of the LP trapezoid but inside the LNAV rectangle. If the controlling obstacle is inside the LP and LNAV protected area, there is no advantage to the LP MDA and it will not be published. The required integrity value for the LP is +/- 40 meters.
Anytime an LP is published, there will always be an LNAV minimum line on the same chart with a higher MDA. Also, before the LP is published, the runway is evaluated for an LPV and found to be not suitable for a vertically guided approach or the LPV would have been published instead.
For software Versions 3.3 thru 5.03 of the GNSxxxW, LNAV approaches could have the advisory glidepath or LNAV+V. The advisory glidepath was not evaluated for use below the MDA. For these software versions, LP was supported, but without any advisory glidepath. This often meant that after an LP was added to an approach, the LNAV lost its +V. This is due to the hierarchy of the procedure selection in a WAAS GPS, if there is only an LNAV, then +V is possible. If there is an LP, it is chosen if the integrity is OK and if not, then the GPS downgrades to LNAV without +V. All downgrades are to LNAV and vertical guidance is never available on downgrades. Pilots objected to getting an LP and losing the +V that they had used for years on a given approach.
With software Version 5.10, Garmin added +V support for LP procedures. This means advisory vertical guidance may be available on the LP as long as there isn’t a downgrade to LNAV. Not all LNAV or LP procedures will have +V available as some are found to be unsuitable for +V during flight test. During flight test, one of the tests performed is to fly the LP or LNAV with +V and below the glidepath with one dot fly up. If a TAWS terrain or obstacle warning occurs, the test pilot notes this in his report along with any other observations that the test pilot deems that +V is not suitable. When this report is evaluated, the approach chart has the VDA (Vertical Descent Angle) removed from it and the database is coded to not provide an advisory glidepath.
Wow, thanks for that informative reply, NCYankee. You are super-savvy and your posts are always top quality!
With software Version 5.10, Garmin added +V support for LP procedures
Brilliant post. And very relevant to Europe, especially the UK where a lot of +V approaches went missing.
Is this situation mirrored on the GTN750?
Is this situation mirrored on the GTN750?
Yes, but the software hasn’t been released. Same for G1000 although since the G1000 STC is owned by the airframe manufacturer, it will take even longer.
This is such a pain in the G1000 PA-46. Airframe mfrs are miles behind technology development.
