# What is the measurement system used in the aviation industry?

I saw this question from History Stack Exchange and noted that the US is still using the imperial measurement system such as feet, miles and pounds.

Given that a plane may need to fly from one country to another, is there any standardisation across the globe? Does planes manufactured by Boeing use one system and planes made by the Airbus another system? If not, wouldn't it be very easy for pilots to make calculation error?

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What about the literal nuts and bolts? I assume they are all metric? Is there anything in aeronautics engineering that is still imperial? – RoboKaren Jul 11 '14 at 15:32
Actually, the US uses American customary units. The American fluid ounce is slightly different from the imperial one but the more significant differences are that an American pint is 16oz (imperial is 20) and, correspondingly, an American gallon is ~20% smaller than imperial; an American hundredweight is 100lb (imperial is 112, despite the name) so an American ton is 2000lb (imperial is 2240) – David Richerby Dec 6 '14 at 12:12

Outside US, SI is generally used for everything except altitude, distance and speed.

• Altitude: 1000 ft happens to be reasonable vertical separation, which is somewhat easier to calculate with than the corresponding metric figure 300 m.
• Distance: There are two quantities that were never converted to decimal. Time and angle. Since we didn't switch to gradians, it makes sense to keep nautical mile, which corresponds to 1 minute along meridian or equator, as it makes calculating distances from navigational maps somewhat easier. If the maps were marked in gradian coordinates, 1 gradian would be 100 km and kilometres would be used.
• Speed: Obviously based on the unit of distance in use.

In Russia, Commonwealth of Independent States (most former Soviet countries) and China (and I am not sure whether some other Asian countries), metric system is used for everything. I believe in Russia they recently switched to flight levels based on feet, but they do use metres below transition altitude.

Before second world war, aircraft built in continental Europe usually had instruments in metric units as well. However before the war aviation was more advanced in the USA, instrument procedures were developed in USA and after the war there was surplus of US-built planes, so most of the world just adopted the US procedures including the units they used as everybody using the same units was more important than any personal preferences. Without these reasons we'd be probably using metric in Europe for everything too.

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SI actually specifies the radian, not the gradian, as the unit of angle. – David Richerby Dec 6 '14 at 12:14
@DavidRicherby: Yes, but I said angles were not converted to decimal, not to SI. Radian would be impractical for latitudes, longitudes and bearings. – Jan Hudec Dec 6 '14 at 16:35
For altitude, I once saw a argument telling there is no advantage of meters over foot, and no advantage of foot over meters: if we use meters, the reasonable vertical separation easy to calculate would be 250m, and the only reason we choose one over the other was the prevalence of US-built aircraft. I don't know what to think of this argumentation. – Manu H Feb 10 '15 at 8:47
@ManuH: Sure it is. That's what I said in the last paragraph too. I also added to the last paragraph mention of the US procedures, since instrument rules were developed in US. – Jan Hudec Feb 10 '15 at 9:01

I can tell you that all of the airplanes that I have flown (which are US registered, but some are French built) use the following:

• Feet for height
• Knots for airspeed (Nautical Miles/Hour)
• Some old airplanes still use Miles/Hour (MPH)
• Nautical Miles or feet for distance
• Statue Miles or feet for weather (visibility and RVR)
• Mach for high speed flight
• Inches of Mercury for altimeter settings
• Most of the airplanes that I have flown can actually switch between "hg and hPa (mb).
• pounds/square inch for pressure
• Pounds for the fuel quantity on the aircraft (read by the fuel gauges)
• Pounds/hour for fuel flow
• Gallons to order/purchase fuel

These are pretty standard in most of the Americas (North/Central/South), but in certain parts of the world some of the units are different:

• Meters for height
• Kilometers or meters for distance and weather
• HectoPascals (hPa/Millibars) for altimeter settings
• Kilograms for fuel quantity on the aircraft
• Kilograms/hour for fuel flow
• Liters to order/purchase fuel
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Russia, CIS countries and China are the main notable exceptions that use the metric system I believe. – Thunderstrike Mar 22 '14 at 18:31
@Manfred For altitude, yes. For altimeter settings, and runway distance, visibility, etc metric is far more common. – Lnafziger Mar 22 '14 at 18:45
All METARs I've seen anywhere outside America had metres for visibility and hectopascals (=millibars, but hPa is more common name) for altimeter settings. – Jan Hudec Mar 22 '14 at 21:41
@JanHudec Canada and Mexico both use SM for visibility, and both of those plus South America use "hg for the altimeter. The Bahamas and most of the Caribbean are the same. – Lnafziger Mar 22 '14 at 22:31
@Lnafziger: That's why I said America and not USA. – Jan Hudec Mar 23 '14 at 10:34

Unless Boeing has recently changed to the metric system in their design work, the short answer to the question as to whether Boeing and Airbus use different systems insofar as aircraft manufacture is yes. I'm only familiar with Boeing up through the 747-400. Perhaps others can provide an answer for later models.

The Boeing Weight and Balance Control and Loading manuals I have do provide kilogram pages following the pound pages, but that the pound is the controlling unit is also stated.

Interestingly, for moment limitations, the manuals follow the inch-pound pages with inch-kilogram pages. In other words, they're mixing systems, which I believe to be unwise as the net effect is to introduce a third, hybridized system.

In the past, many 747 freighter flights that I flew were slightly over gross at takeoff due to the fact that the common conversion used for kilograms to pounds was 2.2 when the actual value is 2.20462262.

As I remember, only the U.S. and Liberia are still non-metric officially. There is a political dimension to this which you can explore at http://en.wikipedia.org/wiki/Metrication_in_the_United_States.

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– Michael Kjörling Jul 11 '14 at 12:04
Burma is also non-metric, though they're making moves towards metrication. – David Richerby Jul 11 '14 at 18:50