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12 Answers

Statute or nautical?

Asked by: 6653 views , ,
FAA Regulations, General Aviation

I've wondered for years why FAA, in writing their regulations, uses "statute" miles in one regulation, and "nautical" miles in others. Anybody??? 

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12 Answers



  1. Kris Kortokrax on Dec 02, 2019

    I think you have to look at the individual regulation.

    For instance, if we are talking about visibility for weather minima, 3 statute mile of visibility allows us to go fly in lower weather than if they required 3 nautical miles of visibility.

    For speeds, 250 knots below 10,000′ allows us to fly faster than if they limited us to 250 MPH (statue).

    It seems that they are giving us the most bang for our buck in each circumstance.

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  2. KDS on Dec 03, 2019

    I believe the year in which the regulation was written factors in there. It goes along with the fact that airspeed indicators used to be marked in MPH, but newer aircraft use Knots. I believe it also relates to what it interfaces. In the case of weather, that is the way the weather people report the weather so if the weather reports came out in one measurement and the regulations were written for another, that would be a mess.

    I know that back about 50 years ago control zones (what you would think of as Class D airspace) were defined as having a radius of five statute miles. Today, anything of that nature would be in nautical miles.

    Lastly, I’ll add this. I had a friend who was a very wise FAA inspector. A big part of his job was to go around giving safety presentations. If anyone asked a question that started with “Why does the FAA ……..”, he would cut them off and say that the one thing he couldn’t answer is why does the FAA do anything. It’s a bit like the old saying about sausage. Just enjoy it and don’t ask how it is made.

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  3. Mark Kolber on Dec 03, 2019

    KDS, I agree with that inspector to the extent that if he can’t answer it he can’t answer it. and there are some reasons which we really can’t guess to much on. But I disagree with the implication that asking or wondering or *thinking* about it is useless. even if there is no clear answer, thinking marks the difference between rote and the higher forms of learning. Between memorization and incorporation..

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  4. KDS on Dec 03, 2019

    I certainly was not implying that people should not think. If that impression was conveyed, it’s purely attributable to my inability to express myself well in writing. Rest assured, there is no greater advocate for thinking than I. In fact, I’ve reached the stage in life where I often sit and do it for hours.

    To ensure any such thought is banished, I’ll steal a phrase used by Professor Harold Hill in Meredith Wilson’s The Music Man and say THINK, BOYS. THINK!

    However, in defense of wise old FAA inspectors, I’ll add this in conjunction with Mark’s words. An FAA inspector who tells everything he knows in public, or worse yet, everything he thinks, will in the future be much wiser, a little older, but quite possibly no longer be an FAA inspector.

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  5. Mark Kolber on Dec 04, 2019

    KDS, I wasn’t disagreeing with you. I was disagreeing with the possible effect of someone cutting off questions dismissively.

    Of course, I dint know that’s what he did since I wasn’t there.

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  6. KDS on Dec 04, 2019

    No problem Mark. I wasn’t taking it as disagreement.

    If we were sitting together in private I could provide some good examples, but those same words would be inappropriate to an Internet forum.

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  7. Gary S. on Dec 06, 2019

    Since asking the question several days ago, I’ve had the idea that going to a proper book on navigation would probably answer the question. You know, like the ones the military issues to student navigators. I’m tied up right now but when I get time I will try to find such a manual.

    I’m not an intellectual but I do wonder about things sometime. This just happens to be a question I have wondered about for years. Wish an old military navigator would sign on!

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  8. Kris Kortokrax on Dec 08, 2019

    The answer is not related to either a timeframe or the weather service.

    My book of regs from 1970 shows visibilities in statute miles and speeds in knots.

    The 250 knot speed limit was first proposed in May 1961 when airspeed indicators (at least in the Tri-pacer I owned) still showed MPH. It did and still does show both Knots & MPH in the reg.

    97.5, which deals with instrument approaches, mandates that statute miles be used for visibilities (other than RVR). AC 00-45H, Aviation Weather Services, states that visibility in METARs is to be reported in statute miles. The weather service used to report temperature in degrees Fahrenheit, but now reports it in degrees Celsius. This would show that if there is a change needed (or required by policy), the weather service will adjust.

    Also, there are instances where the pilot’s determination of visibility is used. The weather service would not be involved in this.

    With regard to control zones vs. Class D, the 5 statute mile radius for control zones is effectively the same as the 4.4 NM radius for Class D.

    While 97.5 applies to instrument approaches, it seems that the FAA is following the convention of using feet and Statute miles for visibility and Nautical miles for all other mileages.

    While I haven’t yet found a reference, it seems that velocities (airspeeds, wind apeeds) use knots.

    I would be interested in seeing a reg that did not follow that convention.

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  9. KDS on Dec 08, 2019

    The original question was “why” does the FAA write the rules the way they do. We already know what the rules are.

    As far as the dimensions of Class D airspace, take a look at JO 7400.11D Subpart D, and you’ll see that Class D airspace is as individually specified in the order as opposed to a fixed number, which it was before.

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  10. Kris Kortokrax on Dec 08, 2019

    You are absolutely right about Class D airspace. I seem to remember something about the 4.4 NM during the transition, but don’t have the time to go back and test my memory.

    There is still language in 91.126 & 91.127 about contacting a tower in E or G airspace with in 4NM. Maybe that stuck in my mind.

    Regardless if they intended it or not, the use of Statue for visibility and Knots for speed does give us more leeway to operate and they do seem to be consistent in that usage.

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  11. KDS on Dec 09, 2019

    You and I are have very similar backgrounds over a similar timeframe and I too remembered the 4.4 NM figure.

    I “THINK” what happened is when the FAA converted to the alphabet airspace system, they started every Class D airspace at 4.4 NM. Then as time and analysis allowed, they were converted to what they are today. The actual conversion from 5 SM to nautical is 4.34488120950325 NM. Mathematically that would round down to 4.3 but the FAA went the more conservative route and rounded up. Just knowing how the government works, I’d bet that the lines on the charts didn’t move until later, but the legal definitions took effect then. So, for a while 4.4 NM was the legal definition and the non-government publications picked up on it.

    I’ll just state for clarification that I “THINK” that is what happened, but I don’t really know. I should also add that I didn’t come up with that 15 place number through a calculator, not my math ability.

    Also, you mentioned your 1970 book of FAR’s, so I’ll add a story to that. Our minister ran a church donation shop. Someone donated a stack of 1968 aviation magazines and a 1968 book of FARs. He gave them to me for Christmas. I really appreciated it because I found it very enjoyable to go through those magazines. Then I put the 1968 FARs in my bookcase at work. My intention was to be able to pull it out during a discussion for humor as if that is how out of date I was. Then it just happened that a discussion about 30 day VOR checks. I mentioned in passing that the check used to be required every ten days or ten hours in service, whichever came first. Nobody would believe me, and I actually put that yellowed book of regulations to use. So, hang on to that book, you just never know when it might come in handy.

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  12. KDS on Dec 09, 2019

    EDIT: DID come up with that 15 place number through a calculator, not my math ability.

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