Transition talk

Recently, I spotted a copy of Now You’re Talking, an amateur radio text published by the American Radio Relay League. Flipping through it, dust motes flying, I caught a passage on Q-codes. These codes were established in the late nineteenth century, when radiotelegraphy and Morse code began their golden age.

These three-letter codes served as a universal language for conveying short, important questions and answers. It was the text messaging of the day and ruled until voice communications became the norm. Whether it was travel by ship or train, Q-codes kept up steady flows of vital information. They played a part in aviation history, too. Up through the World War II years, transport-category aircrews carried radio operators who tapped out messages picked up by military headquarters, airports, and airline offices.

They aren’t exactly intuitive, so they had to be memorized. For example, QBH asks “are you flying below cloud?” QCE asks “when can I expect approach clearance?” QEK asks “are you ready for immediate takeoff?”

This isn’t just a mini-disquisition on radio arcana. They may seem antiquated now, and they aren’t even mentioned in the Aeronautical Information Manual, but a few Q-codes still live on. You don’t much see them in daily use under VFR operations in the United States, but they do crop up in IFR documents and reports—especially if you fly internationally. After all, their use is formalized in International Civil Aviation Organization (ICAO) procedural documents. Three of them relate to altimetry: QFE, QNE, and QNH.

We’re most familiar with QNH. This uses a weather-reporting airport’s barometric pressure. For QNH, the local atmospheric pressure is measured, corrected to sea level pressure, and reported as an altimeter setting. It’s the correction that adjusts for the effects of terrain elevation and references and standardizes all altimeter settings to a single datum—mean sea level. Without the correction to sea level, altimeter settings would be skewed to pressures at various elevations instead of a common reference pressure.

Up through the World War II years, transport-category aircrews carried radio operators who tapped out messages  picked up by military headquarters, airports, and airline offices.So, when we see a METAR, or a tower gives us an altimeter setting, we’re getting QNH, whether it’s given in inches of mercury (inHg), millibars (mb), or hectopascals (hPa). (Millibars and hectopascals use the same values, just under different names.) We dial that setting into our altimeters and now everyone flying in the general area is flying along the same reference plane and at the same indicated altitude. Station pressures change with fronts and other weather patterns, so as we travel, we’re given updated altimeter settings that reflect the pressure changes happening at the surface.

This works well, until we climb above 18,000 feet msl. The moment we pass through 18,000 feet, we enter Class A airspace and transition to the use of QNE. That means changing our altimeter setting to a different reference pressure—standard pressure—which is 29.92 inHg, or 1013.2 mb/hPa. Once leveled off, we’re flying at a flight level—a level of constant pressure. Instead of reporting our altitude in feet, we tell ATC that we’re at, say, Flight Level 250 (FL250).

That switch from QNH to QNE, or feet to flight levels, is an important one. It brings up a term—transition altitude—that hasn’t seen much use in the United States until recently. We all know that altimeter settings go to 29.92 inHg/1013.2 mb at 18,000 feet msl, but now approach charts publish a transition altitude to conform to international charting standards. Our transition altitude is 18,000 feet msl, but in other countries it’s often lower, and can vary from nation to nation and airport to airport depending on nearby terrain and/or procedural standards. Some transition altitudes can be as low as 3,000 or 5,000 feet msl.

Now let’s say you’re descending out of FL250 (not 25,000 feet!) and approach FL180 on your way to an altitude clearance of 7,000 feet. FL180 is your transition level, and the moment you descend through it you change your altimeter setting from 29.92 inHg/1013.2 mb to a local QNH of, say, 30.15 inHg or 1021 mb/hPa.

Pilots sometimes confuse the terms transition altitude with transition level, or think they have the same meaning. But transition altitudes are for climbs to the flight levels, whereas transition levels are designated for descents altitudes. If it helps, think of the “A” in altitude as pointing the way up, and the “V” in flight levels as pointing down.

Again, in the United States transition altitudes and transition levels are usually the same: 18,000 feet msl and FL180—unless altimeter settings below 29.92 render FL180 unusable. But in foreign nations and airports, the two can differ. To add to any confusion, ATC—in the United States and elsewhere—may assign you a transition level on its own initiative—simply by clearing you to a lower altitude and adding an altimeter setting. If you’re at FL190 and hear, for example, “Cleared to 7,000 feet, altimeter 30.15,” you can consider your current altitude as your transition level, and descend to 7,000 feet using your new altimeter setting—even though you’re above the published transition level. You’ll have advance notice of this sort of clearance if you see that the charted transition level is given as “By ATC.”

Airshow and other aerobatic pilots use QFE because it gives a quick, accurate picture of their proximity to terra firma.Then there’s QFE. This is a condition where the altimeter is set to read an altitude of zero feet when the airplane is on an airport surface—typically, a runway threshold. Think of the “FE” as “field elevation.” Airshow and other aerobatic pilots use QFE because it gives a quick, accurate picture of their proximity to terra firma.

Some airlines use QFE as a crosscheck when flying instrument approaches. Often, two of the panel’s altimeters are set to show conventional, QNH altitudes. Another is set to read QFE so that the crew can have an instant “countdown” altitude to the runway. Another big value is that decision heights and MDAs can be bugged.

Well, it’s getting late and I’m sitting outside on my deck. It’s a warm night, the wind is calm, and all is quiet. Then a distant, haunting sound floats through the trees. It’s a train whistle, slowly sounding out Morse code… dah…dah…dit…dah.

It’s a mandatory Q code—literally, Morse for “Q.” It means the train is closing in on a road crossing.

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