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Why is there a variation in aircraft flight instruments of aircraft from Western & Russian origin?

  • Foot–pound–second system (FPS)

  • Meter-kilogram-second (MKS)

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  • $\begingroup$ Interesting question. But according to this aviation.stackexchange.com/questions/2566/… you are asking for a reason for a non-fact so it's impossible to answer. China isn't entirely metric in reality because a few "native" units are still in use, like their version of inch (33mm), foot (330mm), mile (500m), etc. So I would say the resistance of introducing "yet another standard" is understandable when metric is "yet another" already itself. $\endgroup$ Commented Nov 12, 2017 at 16:23
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    $\begingroup$ Units of mass isn't uniform. Western aircraft can be switched over to kg by operator's preference, usually just a setting for the software. (Such a changeover caused the Gimli Glider accident) Further, many western aircraft have a metric display button for flying in metric airspace. $\endgroup$
    – user71659
    Commented Nov 12, 2017 at 18:33
  • $\begingroup$ @mins It wasn't. The fueler always expected liters. The crew used the conversion factor for liters to pounds then pounds to liters, when they were computing liters to kg, then kg to liters. This was because the 767 was configured for kg instead of their earlier pounds. $\endgroup$
    – user71659
    Commented Nov 12, 2017 at 18:52
  • $\begingroup$ According to ICAO, the use of non-SI units is a temporary measure, as most countries in the world use SI units in other domains, including research and engineering (US being a notable exception). However Western aviation organizations are very reluctant to change feet into meters. Related: Standard Application of Units of Measure in Civil Aviation. $\endgroup$
    – mins
    Commented Jul 17, 2023 at 9:30

2 Answers 2

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Why is there a variation in aircraft flight instruments of aircraft from Western & Russian origin?

As so often in aviation it's a result of WWII. In continental Europe, and this includes Russia, MKS was until the end of the war standard, while the US and GB used FPS. After the war, Germany as major standard proponent in aviation 'vanished', flying in western Europe was basically defined by US measures - including measurements, while on the other side of the iron curtain MKS was continued. This meant not just the USSR, but all eastern European states. China in turn acquired most of its flight technology from the SU, including regulations and measurements.

Why do Western aircraft manufacturers [...] while Russian aircraft manufacturers [...]?

It's also less an issue of manufacturers, as of regulations within these states. Boeing is happy to deliver 787s to Russia with all metric instruments - the same way Suchoi would frolicking to equip Superjet 100s with FPS to be sold in the US :))

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The short version is that "Western" countries (nearly all countries, in fact) use knots and such because that's what we started with and, once that became entrenched, the cost of retraining every pilot and reequipping every plane--and the inevitable confusion and crashes during that transition period--greatly exceeds the benefits.

The USSR, PRC and a few others were able to use metric units because they didn't need to care about compatibility with the rest of the world--or perhaps saw the lack of compatibility as a good thing, helping to keep them isolated.

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