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Thread: Descent Profiles

  1. #1

    Descent Profiles

    I talked to Ken, advised that the current descent profile is too agressive. A normal glide for an approach ( at lower sirspeed ) is a 3 degree angle. However, at higher speeds, this becomes a bit hard on the ears. A good formula is to take 1/2 your groundspeed in knots, add a zero to it, and that is the required rate of descent to maintain a 3 degree glide. For example, an RV cruises at about 160 knots, and descends at about 175+ knots, that equates to about 800-900 feet per minute to hold a 3 degree glide, therefore, a 2 degree glide is much easier on the ears and is compatible with the performance of most experimentals. Also, for most RV's and similar aircraft, a descent profile of about 5 miles per 1000' feet works quite well. I REALLY hope Advanced adresses the verticle navigation, and also comes up with a "green bannana" indicating where on the map the top or bottom of the verticle profile will be reached, just like all the other EFIS manufactures. Having gone through the growing pains and doing beta testing for Blue Mountain ( similar fate of the DoDo bird) I am anxious for these features to work as advertised to enhance Advanced market share and survivability. BTW, for those who like formulas, another method to compute a 3 degree glide is to take the distance to touchdown and multiply it by 3, and the result should be the altitude above touchdown, i.e. 5 miles from the runway, 1500 AGL, 9 miles = 2700 AGL, sink rate needs to be 1/2 the ground speed plus a zero, i.e. 100 knots - 500 feet per minute, works real good, lasts a long time! Regards,

    Gary Brown ATP A&P I.A. CFI DAR (Continental Airlines - Retired)
    Last edited by garymail; 08-04-2011 at 06:05 PM.

  2. #2
    Hi Gary,

    On the way to Oshkosh I setup a 3 degree glide-angle synthetic approach to Owatanna, MN. I found that it wasn't the 3 degree angle that was causing the problem, but rather the way we were generating it. We use the current speed to calculate the TOD, rather than having some kind of increased speed factor of lets say 12% (120kts in cruise, 135kts on decent). That causes us to draw the TOD too close in to the destination, resulting in an increased descent rate required to meet the target altitude. We allow the user to set the glide-angle they desire from 0 to 4 degrees. Since most IFR approaches with vertical navigation (LPV, LNAV/VNAV, ILS, etc) have a 3 degree glideslope, I would assume most customers would want to use the same angles for synthetic approaches. However, we allow you to adjust it based on terrain, obstacles, and user preference.
    In short, we are still working on the vertical navigation portion of HITS and the descent profile. It is feedback from users like yourself that help us perfect it. Thank you
    ---
    Trevor Conroy
    Technical Support
    Advanced Flight Systems
    Support@Advanced-Flight-Systems.com

  3. #3
    Is there a handbook or guidelines out regarding HITS operations and your flight planning function?
    I find a descent rate of 400ft/min is comfortable and is what airliner pressurisation systems are set at. It does however require the pilot to do bit of math to plan a continuous descent. I guess these values can be manually set on the EFIS.

  4. #4
    For an RV, use about 5 miles per 1000 feet; or two minutes per thousand; for the approach phase, you can use a 3 degree glide, that equates to 3 times the distance to touchdown in altitude AGL, the sink rate to hold that glide is 1/2 of your groundspeed in knots plus a "zero" for verticle speed, ie 90 knots equals 450 feet per minute sink rate. We used to do NDB approaches all over the pacific in 727's using the math for the profile, worked great if you can cipher quickly! 150 knots approach speed = 750 feet per minute sink rate.

    Regards,

    Gary
    Last edited by garymail; 08-15-2011 at 09:45 AM.

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