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!
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.
As luck would have it, I often fly over Chinese, Russian, and North Korean airspace. The altimeters in our older aircraft, like the 767, 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! The good news is that newer generation airplanes have a switch that can change the aircraft’s native altimetry display from imperial to metric units. When crossing into metric airspace, the crew hits the switch and the airplane understands the metric system. This reduces crew workload considerably.
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.