Altimeter Setting Procedure — Page 221, Lesson 300

Let’s start with the big picture, because this whole procedure is about one simple question: what number do you put in the altimeter window, and when do you change it? After take-off, you’re climbing away from the aerodrome. At some point you’ll change the altimeter setting from QNH to SPS. And on the way down to land, you’ll change it back from SPS to QNH at some point. The process that lets this happen safely, and at a logical, predictable point, is called transition. Now, two terms you must know cold. The transition altitude is the altitude above the aerodrome at which you change from QNH to SPS during a climb. The transition level is the level at which you change from SPS back to QNH during a descent. Notice the direction: QNH to SPS is the transition altitude; SPS to QNH is the transition level. Why does this matter? Because of how we express vertical position. When you’re flying below the transition altitude, you’re flying at altitudes — that means your vertical position is referenced to sea level pressure, which is QNH. But above the transition altitude, you stop talking about altitudes and start talking about flight levels. So the whole point of transition is to give you a clean, specified boundary where you switch from one reference to the other. Let me walk you through the climb. You take off with QNH set. You climb, and upon reaching the transition altitude, you set SPS and continue climbing to your desired flight level. In the descent, it’s the mirror image: you’re coming down on SPS, and upon reaching the transition level, you set QNH and continue down to your desired altitude. There’s one exception you need to remember. On descent, if the pilot is passed the QNH whilst still above the transition level, then the pilot would refer to vertical position as an altitude — even though you’re technically above the transition level. So the rule is: if you’ve been given QNH early, you treat your vertical position as an altitude. Now let’s define a flight level, because that’s the unit you use above the transition altitude. A flight level, abbreviated FL, is the vertical displacement of the aircraft above a constant level of barometric pressure related to 1013 hPa. So it’s not referenced to sea level pressure — it’s referenced to a fixed pressure datum of 1013 hectopascals. Flight Level Zero, or FL0, is located at the atmospheric pressure level of 1013 hPa. That’s your zero point. Subsequent flight levels are separated by a pressure interval corresponding to 500 ft in the standard atmosphere. So each flight level is 500 feet apart in pressure terms. And here’s how they’re numbered: FL30; FL35; FL40; FL45 — and so on — and then FL100; FL105; FL110 — and so on. So you count in 500-foot steps: FL30 is 3,000 feet on the 1013 datum, FL35 is 3,500, FL40 is 4,000, and so forth. Now back to the transition altitude itself. It’s the altitude — with QNH set — above the aerodrome at which the altimeter subscale is reset to SPS, and vertical position above that is then reported as a flight level. So it’s the boundary I mentioned: below it, QNH and altitudes; at it, you reset to SPS; above it, flight levels. Who decides this number? The transition altitude is specified for every aerodrome by the Authority of the State in which the aerodrome is located. So it’s not a pilot’s choice — it’s published. There’s a rule about how low it can be. The transition altitude shall be as low as possible, but normally not less than 3000 ft. So you won’t normally see a transition altitude below 3,000 feet. Where do you find it? Transition altitudes are published in the AIP — that’s the Aeronautical Information Publication — and shown on charts and instrument plates. So before you fly into an aerodrome, you check the published transition altitude. One more flexibility: a state may specify a general transition altitude for the whole country. The classic example is the USA, which uses 18,000 ft as a single transition altitude nationwide. So to tie it together: transition is the safe, logical switch between QNH-based altitudes and 1013-based flight levels. The transition altitude is where you go from QNH to SPS on climb, it’s set by the State, normally not below 3,000 ft, and it’s published in the AIP and on charts. Above it, you live in flight levels, each 500 ft apart on the 1013 datum, starting at FL0. And on descent, you come back through the transition level to QNH — unless you’ve already been given QNH early, in which case you’re back to altitudes.