NerdGuy Friday #8: Flying on the Level

NerdGuy Fridays: Dispatches from a Writer's Brain - M. L. Buchman

English vs. Metric

Before we talk flight levels, we need to talk about one of the great peculiarities of flying. This is true worldwide and for a couple of bizarre reasons.

As I often rail against in my stories, the US is the very last country in the world to use the English standards of measurement. One major reason for this is that long ago, just as metrication began, the car lobby convinced Congress to pass a law. To avoid having to recalibrate so many things on American automobiles, the law states that federal funds may not be used to change highway signs from English to metric nor can states be forced to do so. (Federal Highway Act of 1976)

This effectively halted a planned multi-year roll-out through areas of our lives in the US. This grinding halt has now left us with 2 liters of Coke, 750ml of whiskey, but a 12-ounce beer. And forty years on, there are few other signs of the planned change.

English vs. Metric Flight

Some few countries have converted to metric flight over their territories. China, North Korea, and five countries in SW Asia. One of which, Turkmenistan, uses meters near the surface, but English-based flight levels above 21,000′. For a while Russia flew in metric, but they switched back in 2011.

This original plan, I’m sure implemented by the US and the British Empire in the early days of flight, was adopted worldwide. And to get away from it, we really need to all change at once.

Why? Safety!

Flying Low

Flight below 18,000 feet (in most countries) is very different from flight above that.

At lower altitudes, pressure rules. There is a little dial on an airplane’s altimeter (lower left in the diagram below), that allows the pilot to dial in the pressure (little window on the right in inches of mercury), and read their altitude above sea level (even if they’re flying over Switzerland–that way everyone is calibrated the same).

Three point altimeter
The altitude shown is 10,180′ above sea level. 10,000 on the thin arm with the triangle, 0 thousands on the short fat hand, and 180′ on the long hand. This is at a standard pressure of 29.92″ of mercury.

The little pressure dial is critical because a high- or low-pressure weather system can change your displayed altitude drastically. So everyone must stay in sync.

Flight Levels

Naming a flight level is easy; knock off the last two zeros as irrelevant. 15,000′ becomes FL150 (read as Flight Level One-five-zero). In metric, they always have to add the word “meters” to avoid confusion. However, just to be more awkward, they have to read out the two zeros (or say something like “four thousand five hundred meters”) because no one say Flight Level Four-five hectometers.

Above 18,000′ everyone uses the same pressure setting of 29.92′ of mercury (or the metric equivalent pressure of 1013.25 hPa [hectoPascals]). This makes it so that they’re all flying on a common level, once they’re high enough that they don’t really care about much other than the Himalayas. So FL210 may not be at 21,000′, but it will be exactly the same altitude for everyone flying at that altitude.

Directional Flight Levels

This is the fun bit you can use to understand the pilot’s announcements a little better on your next plane flight: Flight Levels have directions.

Eastbound flights fly at an odd number of thousands of feet. Westbound flights at an even number. This provides a thousand feet of vertical separation for any planes going in generally opposite directions. So the next time a pilot announces, “We’ve been cleared to climb to our cruising altitude of 36,000 feet”, you’ll know that you’re headed west.

Actually, you’ll never hear that, unless your pilot is drunk.

Why? Because above 31,000′ it was decided that 1,000′ wasn’t enough separation, so they jump to 2,000′ separations. The levels go like this:

  • 27,000′ E
  • 28,000′ W
  • 30,000′ E
  • 31,000′ W
  • 33,000′ E
  • 35,000′ W
  • 37,000′ E

Just to add to the fun, countries with predominantly N/S routes will actually twist this to be North-South rather than East-West: New Zealand, France, Italy, and Portugal. (And you thought flying was easy. This is all just about how high to fly!)

In my upcoming book Condor, it creates conversations like this one:

Jon looked around for how to explain himself to the others. “Miranda, stand up and hold out your hand. It’s now an airplane flying at thirty-nine thousand feet.”

“Which is my nose?”

“Your fingertips.”

She twisted around and faced the other end of the parked airplane’s narrow cabin as she held up her hand, palm down.

“What are you doing?”

“Flight level three-nine-zero is for westbound traffic. I’m pointing the nose of my hypothetical aircraft west.”

He noted the direction of the late afternoon sun slanting through the windows.

She was.

“Okay, flight level four-three-zero.”

She glanced over her shoulder at him. “Lizzy had said that the Russians would most likely be using an AN-124 Condor for transport.”

“So you mentioned. Your point?”

Then he knew what she was going to say. They ended up speaking in unison.

“Service ceiling of thirty-nine thousand feet.”

“Flight level three-seven-thousand feet, please.”

She turned around and aimed her fingertips at him with her palm still down, lowering her hand slightly to represent the change in altitude as she made the turn. She made him want to laugh aloud. But with the way Holly was watching him… He decided that cautious professionalism was a far better choice.

Hope you join me for nerding out on March 10th with Condor!

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Author: Matt

writer, project manic, world cyclist

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