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4 Answers

Non precision approach like a precision approach

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Aerodynamics, Instrument Rating

How would you calculate when to initiate a descent and your rate of descent while shooting a Non-precision approach like a precision approach? In other words how can you calculate your rate of descent and Top of descent in order to shoot a non-precision approach using a Continuous Descent Angle Approach (method) instead of using a step down procedure? Assuming we are flying a non FMS equipped light aircraft with a 90 knots approach speed

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4 Answers

  1. Best Answer

    Wes Beard on Jul 19, 2013

    A standard 3 degree glide means the airplane descends 320′ for every mile.

    Take your distance from the threshold in miles and multiply by 320′. Add that number to the threshold crossing height and you would start your descent at the distance you chose at the altitude you calculated.

    See my blog post on Constant Approach Non Precision Approaches.

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  2. John D. Collins on Jul 20, 2013

    How to determine the rate of descent for a constant angle descent path is relatively simple. First, determine the number of feet needed for the descent and divide by the distance in NM. This provides a gradient in feet per NM. Multiply this times your ground speed divided by 60 to obtain your descent rate in feet per minute. At 90 Knots, the speed in NM/Min is 1.5 NM/Min (90/60). To lose 1800 feet in 5 NM is 360 ft/NM, and to convert that to a rate of descent, multiply by 1.5 for 90 Kts = 540 ft/Min. For a three degree descent angle at 90 Kts using Wes’s rule of thumb, 320 * 1.5 = 480 Ft/Min.

    A separate question involves when is it appropriate to use a constant angle descent on a non precision approach (NPA) and when it isn’t. A NPA is not designed to consider a constant angle descent when below the MDA. Some approaches have clear visual segments and others can have serious obstacle issues. At night these obstacles may not be able to be seen and a CFIT can result if you fly the constant angle to the runway. Furthermore, most NPA runways do not have approach lights and are difficult to spot in low visibility. In Wes’s excellent blog post, he shows an example of a NPA to RWY 24 at Teterboro and he recommends using a constant angle descent to 550 feet at which point if the runway is not in sight, to initiate a missed approach. The minimums for the approach are a MDA of 500 MSL and a visibility of 1 SM. At the point where he recommends the go around decision, the visibility would have to exceed 2 SM to successfully complete the approach with a landing. There is nothing wrong with this except that it would only leave an instant for the decision to land when the visibility is just over the 2 SM. So a counter argument for using the dive and drive is that it would permit the aircraft to stabilize at the MDA or slightly higher and to have additional time to search for the runway while at the safety of the MDA.

    As a side note, MITRE did a study on the safety of dive and drive verses constant angle method and found that it was much safer for a turbine aircraft to use the constant angle method, by at least a 2 to 1 factor. They then conducted a similar study for general aviation piston aircraft and could not find any statistically significant difference in the two methods, in fact the dive and drive had a lower accident rate. A lot of this is explained by the different kinds of airports and runways that general aviation uses verses the airlines. Most of our runways are shorter, more obstacle challenged, poorer marked, and much poorer lit with fewer approach light systems. We are more likely to have trouble differentiating the Wal Mart parking lot from the runway and need the extra time to clearly determine that what we are looking at is actually the runway we want to land on.

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  3. Bob Watson on Jul 20, 2013

    RE: John’s comment about night approaches: “We are more likely to have trouble differentiating the Wal Mart parking lot from the runway and need the extra time to clearly determine that what we are looking at is actually the runway we want to land on.”

    It didn’t take too long into my piloting experience that if an area was brightly lit (e.g. the WalMart parking lot) it almost was never a place to put your airplane. The airport was almost always the darkest patch in the area. Eventually, when lined up with the runway, the lights appear, but it’s always an act of faith in navigational aids up to that point.

    The worst-case scenario is when there’s a well-lit area in the approach path to the airport. In one case for me, that area was a harbor (and I didn’t have a seaplane). Fortunately, I figured out that mistake in time. After a long, IFR flight, it can be tempting to take the first “airport-looking” space that appears in the windshield.

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  4. Jh on Oct 05, 2013

    To calculate rate of descend take G.S divided by 2 times 10
    Ex. 120 kts divided by 2 = 60 * 10 = 600 feet rate of descend
    For top of descend
    Take difference altitude times 3 plus 10

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