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Adding 1/2gust factor to approach speed

Asked by: 3370 views Commercial Pilot, Flight Instructor, General Aviation, Private Pilot, Student Pilot, Weather

Manufacturers recommend adding 1/2gust factor to approach speed and yes it truely makes airplane fly smoother and stable approach onto runway.

However I was looking for the science behind it and could not find credible source.

I would like to share ideas and your knowledge on this matter !

 

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3 Answers



  1. Mark Kolber on Apr 01, 2016

    What happens to airspeed when a gust (aka shear) takes place? How does that affect the relationship between the aircraft’s stall speed and a standard 1.3 Vso short final airspeed?

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  2. Brian on Apr 05, 2016

    “How does that affect the relationship between the aircraft’s stall speed and a standard 1.3 Vso short final airspeed?”

    I’m going to play devils advocate and say not much for most. Take a look at the common 172sp used for training. The CAS stall speed is 48, making the CAS 1.3 Vso equal to ~60.4 KIAS. The short field approach speed is 61 KIAS. So far so good.

    Now let’s look at this aircraft in a typical instructor/student configuration: Basic empty weight of 1642, 3/4 tanks (~40 gallons or 240 pounds), and two people plus bags ~350 pounds. Actual weight 2232. Let’s say some fuel was burned and end at 2200 just to make the next step nicer looking.

    Correct stall speed for weight we have sqrt (actual weight/gross weight) * Vs (in calibrated speed). We get 44.58 * 1.3 = 57.9 or 58 KCAS. Convert to IAS for a speed of roughly 55 KIAS.

    So here we are with a 1.3 Vso speed appropriate for our weight of 55. Normal landing speed is 60-70. Most schools I’ve seen have students on the high side of that and then add half the gust factor onto that speed. So even if it stops here you’re looking at the majority (especially those folks at the mills) flying a 172 at 75 knots on final with a 10 knot gust factor.

    It, sadly, get’s worse when we combine this with the silly idea that we need to fly final on a 3 degree path and, because it’s gusty many folks use partial flaps, suddenly we have an approach at 80-85 with a Cessna using more runway than most small jets.

    This isn’t to shoot down your idea here Mark, as I agree with you. Given a few caveats, 1 – do it based on an appropriately weight adjusted speed. And 2 – do it based on an actual 1.3 Vso. The majority of landing incidents (by 3 fold of any other incident as per the latest Nall report) occur due to loss of control on roll out.

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  3. Brian on Apr 05, 2016

    Apologies, messed up the clean gross weight speeds. CAS stall is 48. 1.3 of that speed is 62.4. Using the CAS->IAS chart the KIAS is 60.4. The book published is 61 KIAS.

    In other words:

    Dirty stall gross calibrated – 48
    Dirty gross calibrated 1.3Vso – 62.4
    Dirty gross indicated 1.3Vso – 60.4
    Published gross indicated short field speed – 61

    Dirty stall @2200 calibrated – 44.58
    Dirty @2200 calibrated 1.3Vso – 57.9
    Dirty @2200 indicated 1.3Vso – 55 (56 depending on how you wish to interpolate)

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