Aviation’s Crazy, Mixed Up Units of Measure

and how pilots keep them straight!

AeroSavvy - Aviation Units

One of the challenges of international flying is handling different units of measure in different countries. In aviation, the battle between imperial and metric units continues. Feet, meters, statute miles, nautical miles, inches of mercury, millibars, hectopascal, knots, meters/second – it can get a little confusing! Read on and I’ll scramble your brain with international aviation units!
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AirAsia Flight 8501 – Shame on you, Fox News!

On Sunday, December 28, 2014, Fox News host Anna Kooiman speculated that the metric system and other aviation units might have played a role in AirAsia 8501’s tragic crash [story and video].

International pilots are extremely well versed in all the aviation units discussed in my article below. We use these units daily and can juggle them in our sleep. The AirAsia 8501 flight crew was on their own turf, flying a familiar route. To blindly speculate that they became confused about units of measure is absurd.


Altimeter Calibration: mb, hPa & inHg

Altimeters can be set using hPa or inHg.
Altimeters can be set using hPa (mb) or inHg.

In order to calibrate our altimeters for varying atmospheric pressure, we set the current pressure reported by ground stations. Pressure is reported in inches of mercury (inHg), hectopascals (hPa), or millibars (mb). North America and Japan use the inches of mercury measurement. Hectopascals dominate the rest of the aviation world (millibars and hectopascals are equal). For years, altimeter manufactures have provided two calibration windows for hPa and inHg. This makes setting our altimeters easy no matter where we fly.  Just dial the reported pressure into one of the windows and you’re all set! Newer, digital altimeters have a button to switch between units. I like the older ones, they’re easier to set.

Measuring Altitude: Feet vs. Meters

Here’s where things get tricky. Because of the proliferation of American and British aircraft during the early years of aviation, the imperial foot became standard for altitude measurement. China (PRC), North Korea, and Russia, however, use meters for altitude measurement.
[Update: Russian high altitude airspace changed to Flight Levels calibrated in feet. In 2017, all Russian airspace from the surface up, began transitioning to feet.]


Converting meters to feet. Click for larger.
Converting meters to feet.  Click for full size.

As luck would have it, I often fly over Chinese, Russian, and North Korean airspace. The altimeters in our aircraft are calibrated in feet. When flying into metric airspace, we use a conversion card. When Shanghai Control clears us to descend to 3600 meters, we check the card and descend to the equivalent: 11,800 feet. Does this sound like a pain in the you-know-where? It is!

During rush hour at airports like Shanghai and Beijing, the controllers rattle off clearances very fast. They will often assign us a heading, speed and metric altitude all in one shot. Doing the metric altitude conversion adds an additional task to an already challenging environment. We stay on our toes in metric airspace!

Distance Across The Ground

World-wide, the nautical mile (nm) is the standard for measuring the distance an aircraft travels across the ground. That’s nice and easy and it makes me happy. 🙂

Other lateral measurements are a mess. Most of the world measures runway length in meters while North America uses feet. Most of the world measures airport visibility in meters. North America? Not nautical miles, not meters, but statute miles! Huh?? Not to worry, North America changes back to feet when measuring Runway Visual Range (runway visibility measured with a laser), while the rest of the world sticks with meters. Confused? I sure am, and I do this for a living!

Wind speed: Knots vs. Meters/Second

Windsock2When reporting weather, airports in China and Russia state the surface winds in meters per second (m/s). They love this metric stuff! The rest of the world reports wind in knots (nautical miles / hour). Converting is easy:

1 m/s ≈ 2 kts

Double the m/s and we are close to knots. If Shanghai tower tells me the winds are 7 meters per second, I know they are blowing just a little shy of 14 knots. I can land in that much wind. Any more than that and I’ll let my first officer do it!

Is flying safe with all these different units?

Yes!  Pilots flying international routes deal with this assortment of units daily. We can juggle them in our sleep. While it’s certainly an added challenge, it’s an important part of our occupation. There are very well defined standards and procedures that pilots use to work with the different units. Flying continues to be the safest form of transportation available.

What’s the solution?

Wouldn’t it be easier if all countries agreed on one standard? Absolutely! Meters, feet, furlongs, parsecs… It doesn’t matter what units we use, as long as we all use the same ones.

Having a world-wide standard is a far better option!

The International Civil Aviation Organization (ICAO) is the governing body that makes official aviation recommendations. It might surprise a lot of pilots that for years, ICAO has recommended that the aviation world move completely to metric units (SI Units):

  • Meters
  • Kilometers
  • Kilometers per hour
  • Meters per second
  • Liters
  • Hectopascals

Yep! No more knots. No more feet. The future of aviation is supposed to be 100% metric.

Maybe.  Someday.  Don’t hold your breath.

Back in the early 1970’s, America was on board with ICAO and ready to make the jump to metric. A presidential order was made and Congress passed the Metric Conversion Act of 1975. We were on our way!

…but someone deactivated the METRIC drive.

By the mid 1980’s, America’s move to the metric system was dead and so was a metric aviation standard.

If you’re curious about why metric died, check out this book review for good background info:  Death by Inches – The battle over the metric system in America.

ICAO acknowledges that the current, haphazard system is so widely used that it will be difficult to switch. Costs for equipment upgrades and training would be staggering, so ICAO has put the transition on hold. The official word on imperial units: “Termination date: not established.

Regardless of ICAO recommendations, we are stuck with our confusing mix of units.

Extra reading for those who are really AeroSavvy:

ICAO publication that proclaims the Metric System as the standard (but concedes that the current system will be with us for a while): ICAO Annex 5 – Units of Measurement (PDF)

Wikipedia: Metrication in the United States




  1. I must fess up; I just found your blog serendipitously while searching (for the umpteenth time) for anything new on Captain Kent’s Cockpit Chronicles. I hope all is well with him, a very good writer and photographer.

    I’ve enjoyed catching up on your site to the point that it has taken my attention away from the first week of NFL football, no small feat.

    Your articles are funny, witty and make things easily understandable. I have always been interested in aviation but my connection to the field has been rather peripheral at best, as an Aviation Medical Examiner for 31 years up to my retirement a couple of years ago. I always enjoyed talking to the pilots who ranged from crop dusters to week-end fliers to airline pilots. If you haven’t done so already, perhaps an article addressing the medical certification requirements of an airman would be of interest.

    I look forward to future articles.

    • Hello, Doc!

      Thanks so much for reading and thanks for the very kind comments. I really like your article idea and it’s now on my list of things to do!

      Be sure to tell your friends a out AeroSavvy!

    • Not Just Japan and US, in México we use inHg , feet , NM and knots too in aviation, it´s strange because if I drive my car I use km, and in everyday things we use metrical system, but in altitude I get so used to feet that in meters I don´t get a mental reference, it´s like a got a different mindset when driving or flying.

  2. Dunno about aviation issues but in both the UK and South Africa, engineers still use the imperial system but converted to metric. So I buy aluminium tubes (or whatever) as 25.4 mm diam which surprisingly converts to exactly 1 inch. Most other metric measurements are straightforward imperial conversions. So now we can say we have converted to the metric system. Duh !

  3. My question is are airplanes built in metric and imperial? Does Boeing build imperial, and airbus metric? What about GE engines versus Rolls-Royce engines? And while were on the Topic? Why are jet engines not measured in horsepower everything else seems to be?

    • Hi Greg, good questions.

      Although I haven’t reseached it and wouldn’t bet my life on it, I think it’s a safe guess that most (if not all) aircraft and engines are now designed in metric. The metric system is the world standard for engineering. Even “American Made” aircraft and engines have many components designed and manufactured all over the world. It would make sense for all the engineers to speak the same language.

      Thrust vs. Horsepower: here’s a great discussion about that: http://www.airliners.net/aviation-forums/tech_ops/read.main/14154/
      The first response should answer your question.

      Thanks for reading!

    • In the 1960’s the Royal Australian Air Force had Dassault Mirage’s in their fleet. A number of local modifications were made to them so the result was that the basic aircraft was metric but all the local mods were Imperial…

  4. The basic question to ask is why on earth, or heaven, is the metric world flying in feet and all other medieval unit? There is an answer, but it is an ugly one!

    • Agree. If they don’t use the unit, why should we use their peculiar name for it?

      Peculiar that a country that fought a war to escape a certain Empire should still be so wedded to “Imperial” units …

  5. As you rightly say, ‘don’t hold your breath’ for reform anytime soon. Even with the adoption of PBN/RNP, where there would have been the ideal opportunity to tidy up many of the inconsistency, the mixture of standards remains. When I did my flight training in the mid ’80’s we were hammered with the need to double check conversions, coming just a few years after the Gimli Glider incident (the 767 that ran out of gas in Canada due to a pounds versus Kilograms conversion error).

    • Hi AJ,

      Funny you should mention the Gimly Glider…

      A few weeks ago, I was at a Canadian airport in the 767. We had just been fueled and I was handed the fuel receipt. We use the receipt to cross check our fuel load and the ship’s fuel gauges. The fueler was new and had the pounds, gallons, liters and kilograms all mixed up on the ticket. I looked at my FO and said: “Remember the Gimly Glider?”

      We had the fuel people pull the actual numbers off the fuel truck’s meter so we could calculate our upload. We always have to be careful!

      Thanks for reading!

  6. In November 2011, Russia, Kazakhstan, Belarus, Uzbekistan, Turkmenistan, and Kyrgystan stopped using Metres for altitudes, and switched to RVSM, bringing them into line with the rest of the world who also use feet-based flight level altitudes.

    This means that in 2011, the world went from having 9 countries using Metric altitude reporting, to having 3 countries using Metric altitude reporting.

    It appears that instead of encouraging more countries to use metric measurements – the opposite is happening.

    There’s good reasons not to use metric altitudes. China has tried to increase their capacity by reducing vertical separation (like RVSM) but continue to cling to their metric altitudes, so now the valid east/west split is so complicated they have to publish their hemispherical rule all the time… valid eastbound levels being 8900m 9500m 10100m 10700m 11300m 11900m 12500m & 13700m – Not sure I see much of a pattern there.

    Meanwhile RVSM in feet gives valid eastbound levels being FL290, FL310, FL330, FL350, FL370, FL390, FL410 … aka “Odd” levels. Westbound is “even” levels ie: FL300, FL320, FL340 etc.

    That said, the funky mix of Feet for altitude and height, Metres for length of runways. Nautical miles for distance and speed. Kilograms for weight, Litres for volume etc is also quite common around places that aren’t the USA.

    As long as the numbers are easy when they need to be, it’s all great. Applying the “3 times table” to find a Nautical Mile distance someone at a certain altitude in feet should start descending is one of the best tricks of all time, afterall.

    • Hi Trent,

      Thanks for the great comments. The world is definitely sticking with feet-based QNH and RVSM (Reduced Vertical Separation Minimum) cruise altitudes. As long as everyone is standardized, safety is improved.

      China has a very odd system, indeed. The Chinese “Metric” flight levels (8900m, 9200m, 9500m, 9800m, etc) may seem random, but there is a very distinct pattern to them. China’s military requires the use of meters. The rest of the world has gone RVSM with 1000 feet separation. In order to satisfy everyone, the “metric” altitudes in the China Flight Level Allocation System (FLAS) are separated by exactly 1000 feet – that explains the odd metric altitudes. The feet equivalents to the above metric altitudes are: 29,100′, 30,100′, 31,100′, 32,100′, etc.

      Even more bizarre, when flying Chinese “metric” Flight Levels, aircraft (even Chinese and Russian) are not allowed to use a metric altimeter! When a controller clears a flight to 9,200 meters, the crew looks at a FLAS chart, finds the feet equivalent (30,100 feet) and flies 30,100 feet using a feet altimeter. Aircraft that do not have feet altimeters must have special approval before entering China RVSM airspace. Bottom line: even though Chinese controllers are issuing flight level altitudes in meters, the altitudes are really in feet, flown with feet altimeters, and aircraft are separated by the internationally agreed upon 1000 feet. Whew!

      If you’re interested, here’s a document that explains it… Have a bottle of aspirin standing by if you choose to read it!
      China Metric RVSM System

    • It’s been at least 3 years or more now. Pyongyang airspace extends pretty far east into the Sea of Japan. Depending on the winds, cutting across that airspace often provided a nice shortcut on our Anchorage-Incheon route. The controllers were cordial and spoke excellent English. For the last few years, western carriers have avoided the airspace for obvious reasons.

      Pyongyang Airspace

  7. Wind in m/s while airspeed and ground speed are measured in either knots or Km/h is foolish. If I’m traveling at an airspeed of 500km/h with a 13m/s headwind what is my ground speed?
    So many metric system advocates don’t seem to understand fractions, metrology, or the whys and what-fors of industrial standards.
    While coherence is certainly beneficial in a set of units, coherence is all the metric system has to offer; the base units are as arbitrary as anything and base ten is a horrible numeral system. (12 and 16 are far superior) personally.
    Nautical miles are really most beneficial when using their proper definition as an integer fraction of circumference, on boats needing long range lat-long position and using traditional astronomical measurements and trigonometry. The cool thing is this technique can be used to find precise lat-long on every stable body in the solar system, with no changes beyond knowing the length of a local day and year.
    Personally I would immediately toss out statute miles and kilometers per hour as they don’t fit with anything. Feet seem to be the convenient units for FL and sorting E-W traffic. But beyond these two items it doesn’t seem to matter, (like you said) as long as everybody just uses one.

    • Interesting comments, Mike.

      I think we’re stuck with feet because 1000′ vertical separation has become the international standard. I agree that using m/s for wind is for the birds. It’s just one more conversion I have to do in my head on short final. Nautical miles are fine for no other reason than I’m used to using them. But we should definitely standardize surface visibility and speed units to nautical and knots for consistency.

      I’m quite happy with the base-10 numbering system. Since I have 10 fingers, it’s makes basic math REALLY easy for me! 🙂

      Thanks for the comments!

    • I find it ironic that you complain about base ten, and then use base ten to describe “better” systems. All of our numbers are base 10. Heck, if we were using base 12 for our numbers, we’d say “base 10” then too. It would go 8, 9, A, B, 10. (Where A and B would be new, single-digit numbers representing the old 10 and 11 respectively).

      There’s this hoary old chestnut in the computing world that there are 10 types of people that understand binary: those that do, and those that don’t.

      Metric/SI was designed around the fact that we use base-10 Arabic numerals. How many miles is 86,354,941 inches? Here’s a hint, there’s 63360 inches in a mile. 86,354,941 meters is ~86,355km or ~86Mm.

      Having m/s and km/h in the same context is kinda dumb though, I agree. m/s is a hardcore, true SI measurement, whereas km/h uses hours, which .. isn’t really SI. They shouldn’t mix and match like that. Either everything should be km/h, or m/s. I could see advantages re: altitude, ascent/descent rates, speed etc in m/s…but only if everything was.

  8. I’m fine with measuring altitude in meters, but let’s keep speed and distances in knots and nautical miles.

  9. Flying in New Zealand, when operating the B1900D turbo prop, we burnt Fuel Lbs, ordered in Litres and calculated Loadsheets using Kg…… smh

  10. Hectopascals should also be chucked out the window along with the rest of the imperial/US customary lot for kilopascals like the rest of the world uses for every other kind of pressure. Using centi-, deci-, deca-, and hecto- anything leads to clunky units that are more trouble than they’re worth. Canada seems to have the right idea with this, and uses kPa for meteorological reporting.

    • I know I’m necro’ing an old thread, but the article said nothing about pounds vs. kilograms in the calculation of fuel upload. Does anybody have insight to offer here?

  11. I read the 65 Biggles books. I was a Flying Officer in the University Squadron. I write books for kids and introduce a lot of aviation stuff as part of each plot. This is where I land for authenticity and a clever and engaging slant on things aviation and the comments are first class. Kudos!

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