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!



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

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.]

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

When 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. 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! 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

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