The Pressure Altimeter — Page 69, Lesson 79

Let's start with the standard datum settings, because everything about the pressure altimeter hangs off these three settings: Standard Setting, QFE, and QNH. First, the Standard Setting. When you set 1013.25 hPa on the subscale — that's the little window on the altimeter face where you dial in the pressure — the reading you get is called Pressure Altitude. That's the formal definition: pressure altitude is simply what the altimeter reads when 1013.25 hPa is set. Now, why does that matter? Because when 1013.25 hPa is set, an aircraft would normally fly Flight Levels. A Flight Level is the aircraft's height above the 1013.25 hPa pressure surface, expressed in hundreds of feet. So if you're at 4,500 feet with 1013.25 set, that's FL45. At 36,000 feet, that's FL360. And note this: Flight Levels only occur at 500-foot intervals. So you'll see FL45, FL50, FL55 — never FL47. That's a hard rule. Next, QFE. This is aerodrome level pressure. When you set QFE on the subscale, an aircraft sitting on the ground at that aerodrome will read zero — assuming no instrument error. In flight, with QFE set, the altimeter indicates height above the aerodrome QFE reference datum, provided ISA conditions exist between aerodrome level and the aircraft, and there are no other altimeter errors. In practice, QFE is used mainly for circuit-flying, because it gives a good indication of height above the aerodrome, and any errors involved are only small. Then QNH. This is the equivalent MSL pressure — mean sea level pressure — calculated by Air Traffic Control from the aerodrome level pressure, assuming ISA conditions prevail between aerodrome level and MSL. With QNH set, an aircraft on the aerodrome indicates aerodrome elevation, that is, its height AMSL — above mean sea level — again assuming no instrument error. In flight, the altimeter will indicate altitude, but here's the catch: it will only be the true altitude if the mean temperature in the column of air beneath the aircraft is the same as in ISA conditions. If conditions differ from standard, the indicated altitude — sometimes called QNH altitude — may deviate considerably from true altitude. The navigational computer can be used to make an approximate correction for this temperature error. Finally, Regional QNH, more correctly called 'lowest forecast QNH'. This is forecast by the Met Office and is used to ensure safe terrain clearance. It's the value below which QNH is forecast not to fall in a given period and area. The key point: the value should be lower than the actual QNH anywhere in the area. So if you set regional QNH on the subscale, it will cause the altimeter to under-read — meaning it shows the aircraft to be lower than it actually is. That errs on the safe side, because you'd rather think you're lower than you are when you're near terrain. Let me show you the datum diagram so you can see how these three settings relate to the pressure surfaces. So to tie it together: Standard Setting gives you pressure altitude and Flight Levels, QFE gives you height above the aerodrome, QNH gives you altitude above mean sea level, and Regional QNH is the safety setting that deliberately under-reads to protect terrain clearance. Each one is a different reference datum, and the subscale is how you select which one you're working from.