Altimeter Setting Procedure — Page 221, Lesson 302

Let's pick up with the altimeter setting procedure, and I want to walk you through two closely related concepts: the transition level and the transition layer. First, the transition level. This is the flight level at which the altimeter is reset to the aerodrome QNH, and subsequent flight is reported with reference to altitude. Let me unpack that. A flight level is a pressure altitude referenced to the standard pressure setting, which we call SPS — that's 1013.25 hectopascals. QNH is the local aerodrome pressure setting that, when set on the altimeter, makes it read altitude above mean sea level. So the transition level is the point in the descent where you stop flying on standard pressure and switch to the local QNH, and from that point down, your vertical position is expressed as an altitude, not a flight level. Now, the key thing about the transition level is that it changes with the QNH. It's not a fixed level. It's calculated by the Approach Controller at regular intervals, and also whenever the QNH changes. And here's the precise definition: the transition level is the first available flight level above the transition altitude. That's a "rounding up" from what the altimeter is reading at the transition level with SPS set. So if the transition altitude gives you a reading that falls between flight levels, you round up to the next available flight level. I should note that the calculation of the transition level itself is not required by the learning objectives — you need to understand the concept, not do the arithmetic. Now, the flight crew must be provided with the transition level in three situations. First, prior to reaching the level during the descent. Second, in the approach clearance. And third, when requested by the pilot. So it's given to you before you get there, it's part of the approach clearance, and you can always ask for it. There's a figure here, Figure 11.3, that shows the transition level — let me bring that up for you. Now the second concept, the transition layer. This is the airspace between the transition altitude with SPS set and the transition level. So it's the vertical band sandwiched between those two. It's usually insignificant — a thin layer — but some states require a minimum depth to the transition layer. So in some jurisdictions, that layer has to be at least a certain thickness. And here's the reporting rule that ties it together. When you're ascending through the transition layer with SPS set, your vertical position is reported as a flight level. When you're descending through the layer with QNH set, your vertical position is reported as an altitude. So the direction of travel determines which reference you use and how you report your vertical position. Let me make sure the whole picture is clear. You have the transition altitude — that's the altitude at which you change from QNH to standard pressure when climbing. Above that, you're on SPS and you report flight levels. Then you have the transition level — the first available flight level above that altitude, which is where you change back from standard pressure to QNH when descending, and you report altitude. And the transition layer is the airspace in between those two. The transition level shifts with QNH, and the controller keeps it current and gives it to you in the descent, in the approach clearance, or on request. There's another figure on the same page that illustrates this — let me show you. That's the transition level and transition layer in full.