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3 Answers

40:1

Asked by: 6420 views Commercial Pilot, FAA Regulations, Flight Instructor, General Aviation, Instrument Rating

Can anyone plz explain what is 40:1 rule for IFR flight?

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3 Answers



  1. John D Collins on Sep 30, 2016

    I am not aware of a 40 to 1 rule for IFR. There is a 40 to 1 slope used to evaluate obstacle clearance on departures and climb. Standard IFR climb rates of 200 feet per NM are assumed, so 40 to 1 is 151.9 feet/NM, providing 48.1 feet of clearance for each NM if an aircraft climbs at the standard performance rate/NM.

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  2. david4790 on Sep 30, 2016

    Yes i mean the 40 to 1 slope. So for every 1 feet we will have obstacle clearance of 48 feet if we maintain standard climb 200NM?

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  3. Best Answer


    John D Collins on Sep 30, 2016

    Well sort of. I assume you meant for every one NM there is a 48 foot buffer, That 48 feet per NM is cumulative, so at 2 NM, it is 96 feet, and at about 21 NM it adds up to 1000 feet, which is the standard obstacle clearance used when enroute (except in the mountains).

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