Pressure — Page 26, Lesson 25

I want to walk you through the different types of pressure that we use in aviation — QFE, QNH, and QFF. These are three distinct pressure references, and each one serves a different operational or meteorological purpose. Let's start with QFE. QFE is the atmospheric pressure measured at the aerodrome reference point. That's the actual pressure at the airfield itself, at its specific elevation. When you set QFE on your altimeter, the altimeter will read zero feet when the aircraft is on the aerodrome. So if you're sitting on the runway with QFE set, your altimeter shows zero — it's referenced to the ground you're sitting on. Now, QNH. QNH is the barometric pressure at the airfield — that's QFE — converted to mean sea level, or MSL. But here's the key: the conversion uses the ISA temperature at the airfield and the ISA pressure lapse rate. ISA stands for International Standard Atmosphere, which gives us a standard temperature and pressure profile. So QNH takes the actual pressure at the airfield and reduces it to sea level using the standard temperature and the standard rate at which pressure decreases with height. This means QNH does not account for any temperature deviation away from ISA. The correction made to the surface pressure depends solely upon the height of the airfield above mean sea level — we write that as AMSL. QNH is always a whole number without any decimal places, and it is always rounded down. So you'll never see a QNH like 1020.3 — it will be 1020 or 1019, always a whole number, always rounded down. When you are on the aerodrome with QNH set, the altimeter will read the aerodrome elevation. So if the field is at 270 feet AMSL, your altimeter shows 270 feet when you're on the ground. Now, QFF. Because temperature affects the change of pressure over height, QNH is not a true mean sea level pressure — unless ISA conditions exist. The forecaster needs to know the true mean sea level pressure in order to construct accurate analysis charts and to help with forecasting future changes. So the meteorological offices convert QFE to MSL using the actual temperature, and they assume isothermal conditions between the aerodrome and MSL. Isothermal means constant temperature — so they treat the layer from the airfield down to sea level as having a single, uniform temperature, which is the actual temperature measured at the airfield. This pressure is known as QFF. Because of the differential rate of change of pressure over height at different temperatures, QFF may differ from QNH. Let me explain that differential. We can determine, from the formula that relates pressure change to height change, that at temperatures below ISA we have a relatively small height change per 1 hPa change in pressure. And at temperatures above ISA, we have a relatively large height change per 1 hPa change in pressure. So if it's colder than standard, pressure changes more slowly with height — you need a bigger height change to see a 1 hPa change. If it's warmer than standard, pressure changes more quickly with height — a smaller height change gives you a 1 hPa change. That difference is exactly why QFF and QNH can give different numbers. Let me give you an example to tie this together. Example 1 asks: what is the relationship between QFF and QNH at Oxford, which is 270 feet AMSL, if the QNH is 1020 hPa and the temperature is ISA plus 10 degrees? So the actual temperature is 10 degrees warmer than the standard atmosphere at that level. Because the temperature is above ISA, the pressure change per height is relatively large. That means when you convert QFE to MSL using the actual warmer temperature — which is what QFF does — you get a different result than when you use the standard ISA temperature, which is what QNH does. The figure illustrates that the change in pressure is greater for the calculation of QFF than for QNH when the temperature is above ISA. So to summarise: QFE is the pressure at the aerodrome — altimeter reads zero on the ground. QNH is QFE reduced to sea level using ISA temperature and ISA lapse rate — altimeter reads field elevation on the ground, and it's always a whole number rounded down. QFF is QFE reduced to sea level using the actual temperature, assuming isothermal conditions — it gives the true mean sea level pressure that forecasters need, and it can differ from QNH when the temperature deviates from ISA.