# 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?

• 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.

• 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
• 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.

• – a CVn 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
• inch-kilogram is not just mixing measurement systems, it's actually the wrong unit for a moment. Kilograms measure mass; a moment is a force at a distance, not a mass at a distance. The correct (but still weirdly mixed) unit would be the inch-Newton. – Joe Abley Nov 7 '17 at 20:32

A common misconception is that pounds and kilograms, or nautical miles and kilometers are similar units. They are not actually measuring the same thing. The pound is a unit of force not mass, kilogram is a unit of mass not force.(newtons are the SI unit of force, and slugs are the "imperial" unit of mass) kilometers and statute miles are units of fixed distance and do not cleanly divide into the Earth-geoid's circumference, nautical miles are a fixed integer fraction of the circumference. What matters most to an aircraft(which tend to have minimal acceleration once air born) is not the mass but the lifting force, drag force, and motive force(thrust).

As for nuts and bolts etc, all that matters is that an industry standard, "size B" just needs to be consistent between makers and batches, the units used to measure the tolerances of a size B make no difference. The set of standards to be used is purely one of economics, this is why all plumbing in buildings around the world is based on inches, it is a standard so all spare parts fit and mass production makes it low cost. That is what counts, that you call a pipe one inch, 26mm, or size 37G doesn't change the fact that it has a known diameter tolerance thread pitch, thread form, and is compatible between manufacturers.
When it comes to machining many machines have, for example, set gear ratios for either metric or SAE forms (maybe carriage movement per rotation of a lathe) and these are very costly machines that last for many decades so just switching may not be cost effective, especially if all the machines need to switch at the same time. American auto makers attempted to mix metric and inch withing single vehicles during the '70s and '80s to ease the transition costs and it was a nightmare for maintenance mechanics.(really one of the several reasons for the downfall of Detroit)
The issue is eg a 12mm screw is very close to a 1/2" screw, close enough for general strength but not close enough in thread shape to interchange but too close in size to notice by eye. Also you need two full sets of tools to cover the same size/strength range.

• Hmm... Wikipedia (and general usage) disagrees about pound: mass is lb, force is, strictly speaking, lbf. (Similarly kg and kgf). Of course, the standard unit is N, but more often than not kg (implicitly kgf) is preferred. As for nautical mile, it had been originally defined as a fraction of Earth circumference (likewise the metre!), but the current official definition is 1852 m exactly. – Zeus Jul 5 '18 at 0:35
• Wikipedia is not a solid authority. The label lbf is used for clarity. Kgf is a crude approximation it is not a defined base unit, the newton has a proper definition: the force needed to accelerate one Kg by one meter per second per second. A pound-force is the amount of force required to accelerate a slug at a rate of 1 ft/s^2 – Max Power Jul 7 '18 at 5:58
• " No name has yet been given to the unit of mass and, in fact, as we have developed the theory of dynamics no name is necessary. Whenever the mass, m, appears in our formulae, we substitute the ratio of the convenient force-acceleration pair (w/g), and measure the mass in lbs. per ft./sec.2 or in grams per cm./sec.2. " — Noel Charlton Little, College Physics, Charles Scribner's Sons, 1928, p. 165. – Max Power Jul 7 '18 at 6:03