A reader asks, “I heard Chinese and Russian air space uses the metric system while most other countries use the imperial system of measurement. Is it alot of trouble converting the different system of measurement? I remember a Air Canada 767 having to make an emergency landing due to running out of fuel from wrong metric conversion. That won’t happen with Cathay?” — Tomcat1

Two great questions that I wanted to answer in my blog, one at a time. The first issue deals with metric flight levels and the second issue deals with metric fuel volume. I’ll answer the flight level question below, and the fuel question in the next blog entry here.

Most of the time, you and I hear the captain come over the P.A. and say that we will be cruising along at 37,000 feet. Because he told us a value in feet, it makes sense. This is because ICAO, or International Civil Aviation Organization, the world standard on aviation issues, has deemed that altitudes and flight levels will be measured in feet. There are some nations though, that use metric altitudes and flight levels and don’t follow ICAO rules and suggestions. CIS countries (former Soviet countries dissolved in 1991 that have names that end in “stan”, like Uzbekistan) and Russia and China assign altitudes in meters instead of feet. The captain on a flight traveling from Moscow to Beijing might say we that we will be traveling at 10,600 meters. Huh? How high is that?

As a side note, there are altitudes and there are flight levels. In North America, below 18,000 feet is called an altitude, and 18,000 feet and above is called a flight level.**

The issue then becomes, when transiting the airspace of a country that uses meters, there has to be a transition from feet to meters for our assigned altitude. This occurs all the time and happened on my last flight from Anchorage to Hong Kong, as we passed through Russian airspace. Before we crossed into Russian airspace, we were flying at 38,000 feet, or flight level 380 (three eight zero). The American controller asked us what our requested flight level was in meters and we told him 11,600 meters. He then passed that on to his Russian counterpart, and when we crossed into Russian airspace, the Russian controller told us to climb and maintain 11,600 meters. This metric altitude is equivalent to 38,100 feet, so we climbed 100 feet.

Often times, this transition is done outside of radar contact, meaning we are not on anyone’s radar scope. The onus is on us to make the proper correction so we don’t hit an oncoming aircraft going the other direction. To make sure this is done properly, we have a small chart that we always pull out, even though we know what it says, and it tells us the metric values and their equivalents in feet. Another nice little handy dandy thing in the 747 is a meters button on the instrument panel. When it is pressed, our digital altimeters show both feet and meters together, making this procedure a no brainer. However, we use the chart, even though we have a meters button, because we always set our altitude in feet, even in a metric environment. Why? Easy: The autopilot only takes inputs as feet, in 100 foot increments. In some situations, the metric equivalent could be right in between these 100 foot increments, at say 39,450 feet. If that is the case, do you set 39,400 or 39,500? So that every Cathay pilot is on the same page, we set the altitudes given us by the conversion chart, and keep meters selected on our altimeter to double check our altitude in meters. That way, the chart does the rounding for us and it’s even less of a no brainer.

The great thing about no brainers? They are hard to mess up, so the procedure is safe and effective. Now, has the other pilot, in his 747, screaming along at 600 knots, coming right toward us in the opposite direction, has he set his altimeter correctly? Ha ha! To protect for that, there’s TCAS.

~ Toby

** A flight level is an altitude where altimeters are set to a standard barometric setting, 29.92 inches. In the U.S. and Canada, this is done at 18,000 feet and higher, so that below 18,000 is called an altitude and 18,000 feet and above is called a flight level. Other countries have different transition levels, where the pilots adjust to the standard setting (in Hong Kong, it’s 9,000 feet) but the principle is the same. Closer to the ground, we want our altimeters adjusted for local barometric pressure, as this can affect what the altimeter reads. With the local pressure put into the altimeter, the more accurate it is at telling us our height above the ground. Air traffic control lets us know what this baro pressure is. Up away from the ground, to help everyone to be separated by perfectly equal amounts of accurate airspace, we set a standard of 29.92, keeping everyone’s altimeter reading the same value, no matter what the local pressure is. Ground contact isn’t a factor, and altimeters give us an accurate reading, not above the ground, but a standard plane, at 29.92 inches of mercury. This allows tight vertical spacing of aircraft, because with everyone’s altimeter reading the same altitude above the same level plane, aircraft can safely fly more closely together.