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In looking at all the sources of aviation data, including POH's, instrument dials, instructional materials, FAA publications, METAR's, flying magazine stories, YouTube videos, etc., I see no standard usage of either knots (nautical miles per hour), or mph (statute miles per hour). It seems many people use them not interchangeably, but mix them up seemingly randomly.

Is there any kind of agreed upon standard about when to use which system of units? It's easy enough to convert one to the other (well actually a lot easier to convert knots to mph), but it seems this is just a latent trap for mistakes in navigating and, more importantly, in airspeed errors.

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Knph is what you see on aircraft instruments and hear over ATC. Mph is probably used just so that a common person would know what is happening. – SMS von der Tann Mar 24 at 18:18
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I agree with @SMSvonderTann, all the instruments, FAA publications, METAR as well as the ceritifications criteria (e.g.: stall speed > 61 kts for CS23) etc are given in knots, not in status miles per hour! Gliders and russian airplane though use the metric sstem, Km/h for speed and m/s for rate of climb. – GHB Mar 24 at 18:35
    
My airplane's ASI reads in mph. – Jonathan Walters Mar 24 at 20:41
up vote 3 down vote accepted

According to the FAA Flight Navigators Handbook (the Flight Navigator Cert is rarely issued any more to my understanding but none the less).

The standard unit of distance for navigation is the nautical mile (NM). The NM can be defined as either 6,076 feet or 1 minute of latitude

It goes on to talk about speed

Closely related to the concept of distance is speed, which determines the rate of change of position. Speed is usually expressed in miles per hour (mph), this being either SM per hour or NM per hour. If the measure of distance is NM, it is customary to speak of speed in terms of knots. Thus, a speed of 200 knots and a speed of 200 NM per hour are the same thing. It is incorrect to say 200 knots per hour unless referring to acceleration.

It seems it comes down to what you measure distance in, then simply report the appropriate speed units.

Most pilots will calculate (and talk in units) based on the instrumentation in their plane. Some airspeed indicators contain both knots and MPH however its not uncommon for them to contain only a single unit (generally older GA planes is where I have seen MPH instruments). POH's will generally contain what ever unit the air speed indicators in the original build of the plane contained (some POHs will contain both units). Just to show some examples, Mooney reported speeds in MPH in their older POH's while say a Piper from that era shows Kts in its POH. For example this indicator contains both units.

enter image description here (source)

In the end of the day its most important that the units are not mixed up and or used interchangeably.

As for the errors in airspeeds this is one of the reasons that air speed indicators have color queues to indicate things like stall range, flap extend range and smooth air operating range. This way if a pilot does accidentally confuse units there are other things to rely on. (Side note there are some important speeds that do NOT appear on airspeed indicators that should always be noted).

-Edit- To answer the comments below.

I report distance in NM since thats what foreflight uses on its ruler function as well as its ring function (at least the way I have it set up). Most plotters I have seen are in NM so even if I had a paper map I would be reporting NM as well.

Im not sure the FAA has official declared that Knots are the only acceptable unit. MPH airspeed indicators are still considered air worthy so they must at least accept it on that level. There have been pushes to standardize instrumentation after the issues caused by the eastern vs. western attitude indicators but with the advent of glass cockpits most of these things can now be configurable in software and changed by a simple setting. The FAR's (such as 91.117) report speed in both KTS and MPH and that is the letter of the law from the FAA.

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Not that it's a huge difference, but when pilots report positions in flight, do they really mean NM as opposed to statute miles? So a 2 mile final is ~12,000 ft and not 10,500 ft? – PJNoes Mar 25 at 21:15
    
That's interesting about the Mooney. I wonder if the convention has slowly morphed over the years or was it by some decree of the FAA? – PJNoes Mar 25 at 21:19

Knots are used almost universally for marine and aviation navigation because a nautical mile relates directly to mapping of the earth. A nautical mile is equal to 1 minute of latitude making it very easy to use Knots for speed and nautical miles for distance when already the earth's mapping uses degrees and minutes of latitude. Each degree of latitude is equal to 60 minutes of latitude.

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