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5 Answers

Definition of the 91.175 Normal Rate of Descent

Asked by: 730 views Flight Instructor, Instrument Rating

I can't find a definition of the 91.175 "normal rate of descent" in the regulations, AIM, or FAA pubs. I have been using a descent gradient of 400 ft/NM to plan the point at which descent below the MDA may be initiated when no VDP has been designated, the runway environment is in sight, and flight visibility meets the specified minimum. 400 ft/NM is mentioned in the Instrument Procedures Handbook as a criterion for "circling only" approaches, so this seems reasonable.  Is there any authoritative definition? 

5 Answers

  1. Mark Kolber on Dec 12, 2017

    Sometimes it’s just English.

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  2. KDS on Dec 12, 2017

    Mark hit the nail on the head, but I’ll elaborate a bit.

    It’s one of those terms like “reasonable”, “reckless”, and “careless” that varies depending upon the person judging the act. Sort of like the old saying that “beauty is in the eye of the beholder”.

    I was once doing a preflight with a very old and experienced pilot. We came to a spot where a certain connection had a tolerance of so many hundredths of an inch. I just rhetorically commented about how in the heck am I supposed to tell if it meets a tolerance like that. He said something very simple, but very wise. He said, “I look at it and shake it and if it doesn’t look like and shake like the connection on the thousand preflights I did before this one, then I call a mechanic.” So, when you think about your descent, just ask yourself, does this look like the last hundred descents to land that I did.

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  3. John Scarry on Dec 15, 2017

    Like Mark said, what rate of descent do you normally use? If I follow the VASI or PAPI it is around 500 fpm in the planes I fly. Likewise, tracking the glideslope on the ILS is just over 500 fpm. So for the planes I fly, a normal rate of descent is in the neighborhood of 500 fpm.

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  4. Bruce Mamont on Dec 15, 2017

    Perhaps some context for my question would help: the question came up originally when a pilot asked when he could start a descent below MDA during a circling approach to comply with 91.175, continuously maintaining a position using a “normal rate of descent”. An opinion was offered that a descent “rate” (really a gradient) exceeding 400 ft/NM would be…not normal. I wondered if this number appeared in the FAR, AIM, TERP, or an FAA legal opinion. It does appear in the Procedures Handbook as a criterion that restricts an approach to “circling only” minimums, implying that a steeper gradient approach wouldn’t be normal. The optimal”approach angle of 3 degrees (318 ft/NM) would require a descent during a circling approach to start pretty “early”, given that you might be around 500 AGL at MDA. I wondered what examiners might be using as “normal”, other than their own judgment, to determine whether an instrument candidate was descending too early or steeply on a practical test.

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  5. KDS on Dec 16, 2017

    The short answer to your question is that there isn’t anything written that gives a quantifiable standard to an examiner.

    It’s a bit like something an electrician said to me recently. He was showing me some wiring and saying that the code required wires to be secured in a workman like manner. But, there is no hard and fast definition of “a workman like manner”.

    A better question might be has there ever been an enforcement action based upon a violation of that regulation. While it’s impossible to know for sure, my money says that any enforcement that ever included that regulation was following an accident or maybe a complaint from someone who got buzzed short of a runway.

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