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Thread: Questions about VFPL functionality

  1. #1

    Questions about VFPL functionality

    I am trying to better understand how VFPL works on my AF-5600 to be able to arrive at a pattern (or other) altitude using a reasonable rate of descent.

    It looks like the simplest way to use VFPL is by having crossing altitudes set for all waypoints in which case one takes off with the plane in VFPL mode and as long as the destination has a BOD defined (say, 2 miles away to cross at TPA), the EFIS will manage the descent profile appropriately. The catch that I see in this is that one has to define a TOC.

    My normal procedure after takeoff would be to climb to cruise altitude as quickly as possible (unless there are airspace restrictions like an overlying Class B/C or if I need to manage CHTs) so I usually do not have any TOC location. In this scenario (with a very simple direct route from the origin to destination airports), the AP is set to ALT mode with the ALT bug set to the cruise altitude (obviously, VFPL has been off so far). Once I am at cruise altitude, if I set a BOD and turn on VFPL mode in the AP (the ALT bug is still set to my cruise altitude), would it then start automatically descending at the TOD or would I get a cue that I need to change the ALT bug to the target crossing altitude so that the EFIS can direct the AP to start a descent?

    Or, do I need to create an additional waypoint ( between my current cruise altitude location and the TOD) with a crossing altitude to be my current cruise altitude, switch the AP to VFPL mode (which shouldn't do anything right away because I will already be at that altitude) at which point it will then be in VFPL mode and will initiate a descent at the correct TOD?

    I guess this is a long-winded way of asking how I can switch to VFPL mode to appropriately start the descent after using the ALT mode as usual to get up to cruise altitude (without having to come up with a TOC and crossing altitudes for any intermediate waypoints).

    A related question is how does the ALT bug function when the EFIS is in VFPL mode?

    Thanks,

    Vas

  2. #2
    Once I am at cruise altitude, if I set a BOD and turn on VFPL mode in the AP (the ALT bug is still set to my cruise altitude), would it then start automatically descending at the TOD or would I get a cue that I need to change the ALT bug to the target crossing altitude so that the EFIS can direct the AP to start a descent?
    If you enter a VFPL profile to the currently active leg, it will push the TOD out all the way to the end of the leg, as far out as it can with respect to your IAS/VS bug settings to arrive at target altitude at the very end of the leg, less horizontal crossing entered. You will need to manually lower the altitude bug sometime before it begins to descend because the autopilot will not descend below the altitude bug in VFPL mode.

    A related question is how does the ALT bug function when the EFIS is in VFPL mode?
    1) altitude bug will be followed and AP will behave as if in ALT mode for any legs where you accidentally or intentionally did not enter a VFPL profile but VFPL mode is active all the same
    2) AP will not descend below the altitude bug in VPFL

  3. #3
    Quote Originally Posted by Jonathon View Post
    If you enter a VFPL profile to the currently active leg, it will push the TOD out all the way to the end of the leg, as far out as it can with respect to your IAS/VS bug settings to arrive at target altitude at the very end of the leg, less horizontal crossing entered. You will need to manually lower the altitude bug sometime before it begins to descend because the autopilot will not descend below the altitude bug in VFPL mode.


    1) altitude bug will be followed and AP will behave as if in ALT mode for any legs where you accidentally or intentionally did not enter a VFPL profile but VFPL mode is active all the same
    2) AP will not descend below the altitude bug in VPFL
    Thanks, Jonathan!

    To follow up on my question, it looks like the best way to move to VFPL mode would be to set a VFPL profile for the current leg once I reach cruise altitude and then change to VFPL mode?

    Once I set up the appropriate VFPL profile for the current active leg and there is a VFPL profile for the destination, am I to conclude that setting the ALT bug to a lower altitude at that point will not cause the plane to start descending right away (which it otherwise does if the ALT bug is normally set to a lower altitude)?

    Also, will there be any kind of a reminder message asking me to set the ALT bug to a lower altitude before the TOD in case I forget?

    Vas

  4. #4
    it looks like the best way to move to VFPL mode would be to set a VFPL profile for the current leg once I reach cruise altitude and then change to VFPL mode?
    Yes this would be fine

    am I to conclude that setting the ALT bug to a lower altitude at that point will not cause the plane to start descending right away (which it otherwise does if the ALT bug is normally set to a lower altitude)?
    Correct when VFPL is active, the autopilot will not chase the altitude bug.

    Also, will there be any kind of a reminder message asking me to set the ALT bug to a lower altitude before the TOD in case I forget?
    Yes an annunciator warning will pop up advising you to lower the alt bug to at or below the altitude programmed for the active leg's VFPL profile.

  5. #5
    Great! Thanks, Jonathan!

  6. #6

    Dynons Decent required function

    Just flew in a Dynon equipped Aircraft and they have a very nice function. called decent required. Its a small field along with ETE WPT etc. Gives FPM required to recent the Airport. So Dynon pilot just glance over at that and when they see 500FPM start their decent. Less if the have passengers, higher if they are doing the RV space shuttle decent. It would be neat to have.

    Paul

  7. #7
    I would favor that feature.
    Dave Ford
    RV10

  8. #8
    I'd really like that feature too. I do have the IFD set up for that functionality on a user defined field, but then I have to put the VFPL into both the AFS and IFD. I typically just use the AFS flightplan.

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