Altimetry — Page 135, Lesson 111

I want to walk you through the altimeter settings that are absolutely critical for your professional flying. Let's start with QFE. QFE is the pressure measured at the aerodrome datum. Think of the aerodrome datum as the reference point for that specific airfield. When you set QFE on your altimeter, the instrument will read zero when the aircraft is on the surface of the aerodrome. Once you're airborne with QFE set, the altimeter reads the approximate height above the aerodrome. There's an important practical rule here: QFE is always rounded down to the nearest hectopascal. So if the measured pressure is 1013.7 hPa, you'd set 1013 hPa. Now let's move to QNH. QNH is QFE converted to mean sea level using the ISA — the International Standard Atmosphere. With QNH set, the altimeter will read the aerodrome elevation when you're on the surface of the aerodrome. When airborne, it reads the approximate altitude of the aircraft. Note that QNH is always rounded down to the nearest integer — that means to the nearest whole hectopascal, not just the nearest tenth. Next we have Forecast QNH. This is the lowest forecast QNH within an area, forecast for one hour ahead. The key safety principle here is that because you're setting the lowest forecast pressure, the actual pressure will always be higher, or at least equal to the forecast QNH. What does that mean for your altimeter? It will read low — or safe — or the correct altitude. Reading low means you're actually higher than the instrument shows, which keeps you clear of terrain. That's the safety buffer built into this system. Now let's look at SPS — the Standard Pressure Setting. If you set the standard pressure of 1013 hPa on the altimeter, the instrument will read what is known as pressure altitude — height in the Standard Atmosphere. This is the altimeter setting used when flying above the transition altitude. That's the altitude where you switch from local pressure settings to the standard 1013 hPa for en-route flying. Let's lock in the terminology because these words have very specific meanings in aviation: - Altitude — vertical distance above mean sea level. - Height — vertical distance of a level or point measured from a specific datum, for example above the aerodrome surface. - Elevation — vertical distance of a fixed object above mean sea level, such as an aerodrome or an obstacle. - Flight Level — a surface of constant atmospheric pressure measured from the 1013 hPa datum, used for vertical separation by specified pressure intervals, usually 500 or 1000 feet. Flight Level is measured in hundreds of feet. For example, FL350 equals 35,000 feet. So to summarise the key distinction: QFE gives you height above the aerodrome, QNH gives you altitude above mean sea level, and 1013 hPa gives you pressure altitude for flight levels. Each setting has its proper use and its own rounding rule.