Airways — Page 153, Lesson 150

I want to walk you through the formal definitions that underpin airway navigation and altimeter procedures. These are terms you will use every single flight, so let’s get them precise from the start. First, Flight Level, or FL. A flight level is a surface of constant atmospheric pressure that is related to a specific pressure datum: 1013.2 hPa. It is separated from other such surfaces by specific pressure levels. In plain language, when we set our altimeter subscale to 1013.2 hPa, the altitude we read is no longer an altitude above sea level — it becomes a flight level. That’s why you hear “Flight Level 350” instead of “35,000 feet.” The key point: flight levels are all referenced to the same standard pressure, so every aircraft on the same flight level is on the same pressure surface, regardless of local weather. Next, Height. Height is the vertical distance of a level, a point, or an object considered as a point, measured from a specified datum. Notice the difference from altitude: altitude is usually measured from mean sea level, while height is measured from a specified datum — often the ground directly below, or the aerodrome elevation. So when we say “height above the runway,” that’s height. Now, Minimum Descent Altitude/Height, abbreviated MDA or MDH. This is a specified altitude or height in a non-precision approach or a circling approach below which descent may not be made without visual reference. In other words, if you are flying an approach that does not give you vertical guidance (like an ILS glideslope), you have a hard floor. You cannot descend below that MDA or MDH unless you have the runway or approach lights in sight. If you don’t see them, you must go around. Threshold. The threshold is the beginning of that portion of the runway usable for landing. It’s the start of the landing surface — often marked by a line of white stripes. When we talk about distance to threshold, that’s the point we aim for. Transition Altitude. This is the altitude at or below which the vertical position of the aircraft is controlled by reference to altitudes. Below this altitude, you set your altimeter to the local QNH (which we’ll define shortly) and read altitude above mean sea level. Above it, you switch to the standard 1013.2 hPa setting and read flight levels. Transition Layer. This is the airspace between the transition altitude and the transition level. It’s the buffer zone where you are changing from altitude control to flight level control. You must not level off in this layer unless it’s part of a procedure. Transition Level. This is the lowest flight level available for use above the transition altitude. Once you climb through the transition layer, the first usable flight level is the transition level. It ensures separation from aircraft still using altitude settings below. Now, the glossary also gives us the key ICAO abbreviations for pressure settings. These are critical for altimeter procedures. QFE. This is the atmospheric pressure at aerodrome elevation, or at runway threshold. When you set QFE on your altimeter subscale, the altimeter reads height above that aerodrome or threshold. So on the ground, it reads zero. In the air, it reads your height above the field. QNH. This is the altimeter subscale reading to obtain elevation when on the ground. When you set QNH, your altimeter reads altitude above mean sea level. So on the ground at the aerodrome, it reads the aerodrome elevation. This is the setting used for en-route flight below the transition altitude. QNE. This one is a bit more nuanced. QNE is the height indicated on landing at an aerodrome when the altimeter subscale is set to 1013.2 hPa. The excerpt gives a fuller definition: “When 1013.2 hPa is set on the subscale, the height indicated on the pressure altimeter with the aircraft on the ground is known as the QNE value.” This setting is used for some high-level aerodromes where the QFE would be so low as to be outside the limits of the subscale setting. The excerpt also notes that the setting of 1013.2 hPa is sometimes referred to, incorrectly, as the QNE — the correct term is that the indicated height is the QNE value, not the setting itself. The Jeppesen definition simply says: Altimeter setting 29.92”Hg or 1013.2 hPa. Let me tie this together with the figures we have available. shows the relationship between these pressure settings and the altimeter. illustrates what happens when 1013.25 hPa is set and the local QNH is higher — you gain less height than indicated. And shows the opposite: when 1013.25 hPa is lower than QNH, you gain more height, and it also introduces the effect of temperature difference from ISA at the cruising pressure level. So, to summarise: Flight Levels are based on 1013.2 hPa. Height is from a specified datum. MDA/MDH is your minimum on a non-precision approach. Threshold is the start of the landing runway. Transition Altitude, Layer, and Level define where you switch between altitude and flight level control. And QFE, QNH, and QNE are the three pressure settings you must understand to set your altimeter correctly for every phase of flight.