The Pressure Altimeter — Page 69, Lesson 79

All right, let's get into the Pressure Altimeter. We've already covered how the instrument physically works, so now we're moving into the real-world operational side: the datum settings. This is where we take the raw pressure reading and turn it into something meaningful for flight. Let's start with the Standard Setting. When you set 1013.25 hPa on the subscale, the altimeter reading is called Pressure Altitude. That's the formal definition. Now, why do we use this? If 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 100s of feet. So, if you're at 4,500 feet, that's FL45. If you're at 36,000 feet, that's FL360. Notice that Flight Levels only occur at 500 ft intervals—so you'll see FL45, FL50, FL55, but never FL47. This is the standard for high-altitude cruising, ensuring everyone is measuring from the same pressure datum, regardless of local weather. Now, let's look at QFE. This is aerodrome level pressure. When you set QFE on the subscale, the altimeter of an aircraft on the ground will read zero, assuming there is no instrument error. In flight, with QFE set, the altimeter will indicate height above the aerodrome QFE reference datum. But there's a condition: this is only accurate 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. It gives a good indication of height above the aerodrome, and any errors involved are only small. So, think of QFE as your "height above the field" setting for local flying. Next is QNH. This is the equivalent MSL pressure—that's Mean Sea Level—calculated by Air Traffic Control from the aerodrome level pressure, assuming ISA conditions prevail between aerodrome level and MSL. With QNH set on the subscale, the altimeter of an aircraft on the aerodrome indicates aerodrome elevation, that is, the height AMSL—Above Mean Sea Level—if there is 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 are different 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. So QNH gives you altitude above sea level, but it's only exact when the air is standard. Finally, we have Regional QNH. More correctly called 'lowest forecast QNH', this setting is forecast by the Met. Office and is used to ensure safe terrain clearance. It is 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. If you set regional QNH on the subscale, it will cause the altimeter to under-read. That means it errs on the safe side—the altimeter shows the aircraft to be lower than it actually is. This is a safety buffer for terrain clearance. Let me tie these together. We have three main datum settings: QFE for height above the aerodrome, QNH for altitude above mean sea level, and Regional QNH for terrain clearance with a built-in safety margin. And above the transition altitude, we use the standard 1013.25 hPa setting to fly Flight Levels. Each one is a different reference, and knowing which one to use and when is fundamental to safe operations.