# What is the official unit of airplane/aviation speed?

Some units I read/found are commonly used in aviation: mach, knot (nautical mile per hour), mile per hour (mph), kilometer per hour (km/h or kph).

"Official" meaning like the units used by the flight data recorders, if a pilot makes a report of their speed, and so on.

• Related, maybe a dupe? Jan 26 '19 at 20:05

### Flight data recorders (knots)

ICAO (Annex 6 Part 1) wants the FDR data to be readable in engineering units (knots, feet, etc.). An example from Appendix 9:

2.3.3 Documentation concerning parameter allocation, conversion equations, periodic calibration and other serviceability/maintenance information shall be maintained by the operator. The documentation needs to be sufficient to ensure that accident investigation authorities have the necessary information to read out the data in engineering units.

FAA, EASA, and AAIB, for example, adhere to that:

FDR documentation requirements The FDR records binary data which needs to be decoded. Using a ground replay system, the binary data can be converted to engineering units (knots, feet etc.) by referencing a detailed document specific to the aircraft installation. The generic name for this document is the Data Frame Layout (DFL).

### Air and ground operations (knots, "temporarily")

ICAO Annex 5 is all about SI (km/h for airspeed, m/s for wind), but permits the temporary usage of knots:

So officially knots is temporary, but it's what you'd expect from most (not all) nations and it's not going anywhere anytime soon. Mach is a ratio (not a speed), and is used in position reports in oceanic/non-radar environments, for example. Mainly because that's what the airplane normally targets above a certain altitude that is usually below cruising level.

There are also various speeds (for the same unit), for example: ground speed, true airspeed, and indicated airspeed. A controller typically sees the plane's ground speed, but requests an indicated airspeed -- because of how the different speeds work it's a lot more practical for both the pilot and controller to use indicated airspeed.

• Mach is ratio. But denumerator is m/s, the speed of sound. So, Mach 0.8 is 0.8*340m/s. Mach 2 is 2*340m/s, and so on. Jan 27 '19 at 0:14
• But thank you anyway for the clear explanation. I got it that km/h and knot is two official speed unit used in aviation. Jan 27 '19 at 0:16
• @AirCraftLover Most situations in which you will want to use a Mach number, the speed of sound will not be $340 m/s$. Keep in mind that $M=\frac{v}{a}=\frac{v}{\sqrt{\gamma R T}}$ Jan 27 '19 at 13:28
• Yes I know that the higher altitude the smaller speed of sound. I just want to make simple argument regarding the mach as speed unit. Jan 27 '19 at 14:35

Knot. Pilots think and communicate with ground personnel in knots. HUD tells you knots. Textbook tells you to increase speed to 500 knots after takeoff, and land at 150 knots.

• You might want to add a US in front of Pilots, Textbooks and Instruments. Jan 27 '19 at 12:11

As usual it calls for a Well, But... answer.

The legal unit is worldwide km/h, kilometer per hour.

For all practical purpose most countries use knots, except when it comes to high altitude flight, when the mach number may be used in reporting.

• "The legal unit is worldwide km/h": Which law are you referring to?
– mins
Jan 27 '19 at 10:26
• The law of nature :)) But serious, Measurement is worldwide defined in SI units. Satute and nautical mile are since quite some time no longer based on their own definitions, but defined based on the meter. Practical usage of miles and thus knots is therefore legally use of km (and km/h). Jan 27 '19 at 12:10
• As far as I understand, ICAO is the international body for recommendations in international aviation. Then each country writes an AIP where it states any deviation from ICAO recommendations. Regarding units, this is part of Chicago Conv. Annex 5 which states that there is no date to stop using foot or nautical mile or knot. Only China and Russia have switched to SI units if I remember correctly.
– mins
Jan 27 '19 at 19:10
• @mins Mind to read my reply at whole? Even the mile is nowadays defined by using SI units. it's no longer a defined as a latitude minute, but 1852m. Isn't it? Thus the legal base is the meter. And That's as well why I diferentiated between legal definition and practical use - much like ICAO does. Jan 27 '19 at 19:12
• @mins, just to nitpick: China and Russia never 'switched' to SI units (in aviation), they always used them. (Continental) Europe did too, but switched to knots/feet after WWII due to US/British influence.
– Zeus
Jan 29 '19 at 1:20

As said above, Knots are used internationally mainly for convenience. 1 Knot =1 Nautical Mile/per hour. And a Nautical mile is 1 degree taken at a meridian i.e a great circle. This relation comes quite handy for navigation since if you traveled at 1 kt you’ve done 1’ of arc in 1 hour....

• So, the distance of nautical mile will be different between equator and near the polar, right? Jan 27 '19 at 17:36