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