Let’s pick this up right where the idea of a standard separation comes from — Page 221, Lesson 298

Let’s pick this up right where the idea of a standard separation comes from. I want you to hold onto that phrase: “have at least one altimeter referenced to the same subscale setting so that a standard separation can be applied regardless of the sea level pressure.” That is the whole reason we bother with altimeter setting procedures at all. If every aircraft in the same airspace sets its altimeter to the same pressure reference, then the vertical distances between them are consistent, no matter what the actual sea level pressure happens to be that day. Now, the baseline for that standard reference. According to the ICAO International Standard Atmosphere, the average barometric pressure at sea level is 1013.25 hectopascals. When we round that down to 1013 hPa, we define it as the Standard Pressure Setting, abbreviated SPS. So SPS is not just “some number” — it is the rounded-down ICAO ISA mean sea level pressure, and it is the reference we use when we want everyone on the same scale. Let me give you the two main objectives of altimeter setting procedures, because everything else in this chapter serves these. First, to provide adequate terrain clearance during all phases of flight, especially departure and arrival. Second, to provide adequate vertical separation between aircraft. So one objective is about keeping you off the ground and obstacles; the other is about keeping you apart from other traffic. Both depend on getting the subscale set correctly. Now, the subscale itself. There are three altimeter subscale settings that can be applied at any aerodrome. Let’s take them one by one, because each one answers a different question. First, QNH. This is the observed barometric pressure at an aerodrome, adjusted in accordance with the ISA pressure lapse rate, to indicate the pressure that would be observed if the observation were carried out at sea level. Let me unpack that. You measure the actual pressure at the aerodrome, then you correct it back down to sea level using the standard lapse rate, so you get what the pressure would be at sea level. If you set QNH on the altimeter subscale, the altimeter will read aerodrome elevation at touchdown. So with QNH set, when you land, your altimeter shows the field elevation above mean sea level. Second, QFE. This is the observed barometric pressure at an aerodrome which, if set on the altimeter subscale, would result in the altimeter reading zero at touchdown. So QFE is the actual pressure at the aerodrome, and with it set, your altimeter reads zero when you touch down. That is your height above the aerodrome, not your height above sea level. Third, QNE. This one is the trickiest, and I want to be very careful here because there is a popular misconception. QNE is not 1013 hPa. Let me walk you through the situation where QNE arises. A situation can occur where the QNH is below the lowest altimeter subscale setting. For instance, if the altimeter subscale will not read below 940 hPa and the QNH is 935 hPa, it would appear that the altimeter is useless. But it is not useless. If you set the subscale to a standard setting, say 1013 hPa, then you can calculate what the altimeter would read at touchdown. Let me do that calculation with you, because the numbers matter. Suppose the QNH is 930 hPa and the subscale is set to 1013 hPa. Assume the aerodrome elevation is 100 feet AMSL. First, find the amount of pressure wound on: 1013 minus 930 equals 83 hPa. Then multiply that by the ISA interval, which is 27 feet per hPa. So 83 times 27 gives you 2241 feet. Then add the aerodrome elevation of 100 feet. That gives you 2341 feet. In this case, 2341 feet is the QNE. So QNE is what the altimeter will read at touchdown with the standard pressure setting set. In practice, the air traffic control officer would instruct the pilot something like: “G-CD set 1013 land with QNE 2340.” Notice they round it slightly. And here is the key point to clear up the misconception: when 1013 hPa is used as a reference, as opposed to a QNH, it is defined as the standard pressure setting, SPS. So QNE is not 1013 hPa. QNE is the reading you get at touchdown when SPS is set. The 1013 is the setting; the QNE is the resulting indication. So to tie it all together: QNH gives you altitude above mean sea level, QFE gives you height above the aerodrome, and QNE gives you a standard reference reading when the local pressure is too low for the subscale to handle. Each one has its place, and the objectives of terrain clearance and vertical separation are what drive which one you use when.