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3 Answers

Auto Pilot LPV Flightpath Intercept From Below

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Hi All:  I am somewhat new to LPV approaches (GTN 750 coupled to 800 IFCS).  My IFCS (Integrated Flight Control System) like most autopilots, I believe, require the GS/Flightpath to be intercepted from below to allow the AP to capture the GS/Flightpath.  Take a look at the approach plate for KLGU (Ogden Utah) RNAV (GPS) RWY 35.  The 11,400 (underlined) holding pattern  altitude or altitude from IAF OGD VOR, along with the minimum altitudes at each subsequent fix don't really allow you to ever get below the flight path (I am flying this in my Cessna 421B at an approach speed of 120 knows, making this a rather steep approach that I have to configure for gear and flaps down and power back to even come close to getting below the flightpath in order for the AP to capture the flightpath).   Any thoughts or suggestions on how to better enable me to get below the flightpath of LPV approaches so the AP can properly capture the GS (in this case flightpath)?

Thanks, G. Silver


3 Answers

  1. John D Collins on Dec 16, 2014


    There is nothing particularly steep about this approach, as descent along each of the segments is below a 3 degree slope (318 feet per NM). On any LPV approach (same for ILS and LNAV/VNAV), one must comply with all of the step down minimums outside of the FAF using the barometric altimeter. Using the GS for vertical guidance to clear these stepdowns does not assure you will comply. This is because the GS is relatively fixed in space but the altimeter is affected by temperature. On a hot day, it is possible that following the GS will place the aircraft below the stepdown minimum, which is not permitted. The only leg segments where the GS is authorized to be used, is the one that ends at the lightening bolt GS intercept symbol, usually shown just before the FAF.

    Although many autopilots permit intercept of a GS from above, this is not desired for a variety of reasons. I would fly the approach from the hold already configured for landing as the descent will be relatively constant and close to the final approach descent rate to make the stepdown fixes. On the leg between OBNUE and the FAF, I would use a higher descent rate to enable the aircraft to get stabilized at the GS intercept altitude with sufficient time to satisfy the autopilot for the GS coupling. This leg requires a decent rate of 292 feet per NM which would be just under 600 FPM at 120 Kts. I would use a rate higher than that, between 800 and 1000 FPM

    When flying an LPV or other RNAV approach with vertical guidance, the GS does not appear until you are flying the leg that ends with the FAF. On this procedure, it will be just as you pass OBNUE, so you won’t have a GS prior to that point anyway on an LPV. With most autopilots, the GS needs to be a valid signal and you need to be below the GS for a period of time in order for the autopilot to couple to the GS.

    You can force the GS to appear earlier on the procedure, but this is a bad idea as you must still comply with the stepdown minimums and these waypoints will no longer be displayed.

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  2. Russ Roslewski on Dec 16, 2014

    While the segments from JIPOS to the FAF (UHINE) are below the maximum descent gradient permitted, I understand your concern that there isn’t a whole lot of wiggle room there. It’s not like a nice, flat intermediate segment prior to the FAF, and that’s what makes an approach like this benefit from a nice, thorough briefing.

    Each segment prior to UHINE has a required descent gradient of just udner 300 ft per nm – around 295, and it’s pretty consistent for each segment. Meaning that a steady descent starting at JIPOS will get you there. But since the descent gradient is LESS than the 318 ft per nm of the final segment, that means that each fix is already slightly below the glideslope. If you hit OBNUE right at 8400 you should be below the glidepath and the autopilot should be able to couple from below like it wants to do.

    However, like John states, the temperature can have an effect. If it’s a hot day you may actually be above glidepath while being at 8400 (indicated) at OBNUE (this is the opposite case from what he discusses). In this case, the only thing to really do is to hustle it a little bit down to 7000 prior to UHINE.

    So, to make this approach work with the autopilot intercepting from below, you really have to nail your altitudes. Not an approach for the sloppy pilot!

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  3. John D Collins on Dec 16, 2014

    A temperature of around 73 F would place the glidepath at 8400 feet, although the true altitude would be 8622 feet.

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