Altimeter Setting Procedure — Page 231, Lesson 304

Let's start with the heart of this: the altimeter setting procedure. I want you to think of the altimeter as an instrument that tells you your height, but only if you tell it what your reference point is. That reference point is set on the subscale, and that's what we call the altimeter setting. Before you even start the flight, you and the operator have to plan the route. You select an appropriate IFR or VFR flight level, and you must comply with the rules of the state and the general flight rules. When you pick those flight levels, three things must be true. First, they should ensure adequate terrain clearance at all points along the route. Second, they should satisfy ATC requirements. And third, they should be compatible with the table of cruising levels in Chapter 6. Now, before you commence the flight, you must confirm the serviceability and accuracy of the altimeter. Here's the procedure. With knowledge of the aerodrome elevation, you set the altimeter to either QNH or QFE. Then you vibrate the instrument — and I want to stress this — you vibrate it, avoiding tapping the glass. The reason is to make sure the instrument has actually reacted to the mechanical adjustment of setting the subscale. A serviceable altimeter will then indicate one of two things. If you've set QFE, it will indicate the height of the altimeter above the reference point. If you've set QNH, it will indicate the elevation of the position of the aeroplane plus the height of the altimeter above the ground. Let me make sure that distinction is crystal clear. QFE gives you height above the aerodrome reference point. QNH gives you elevation above mean sea level — that's the elevation of the aeroplane's position plus the height of the altimeter above the ground. Now, altimeter accuracy. There are tolerances you must check against. Plus or minus 60 feet, which is 20 metres, for a test range between 0 and 30,000 feet. Or plus or minus 80 feet, which is 28 metres, for a test range between 0 and 50,000 feet. Now let's move to approach and landing, because this is where the procedure really comes together. Before you commence an approach to an aerodrome, you must obtain the transition level. And before you descend below that transition level, you must obtain the latest QNH for the aerodrome. Now, this does not preclude you from using QFE for terrain clearance purposes during the final approach to a runway. ATC may clear an aircraft to be operated using QNH when above the transition level, if it's required for the purpose of descent in accordance with a prescribed procedure — and I want to stress, that's not for level flight. Here's a key point about obstacle clearance. When an aircraft has been given clearance as number one to land, and is using QFE to complete the approach, the OCH — that's the Obstacle Clearance Height — is to be established with reference to height above the aerodrome datum for that portion of the flight. And there's a standard format you'll see on approach plates. All vertical displacement is shown as both AMSL and AGL. AMSL means Above Mean Sea Level, and AGL means Above Ground Level. They're shown in this form: 2000 (1485). The AMSL figure is in bold type, and the AGL figure is in parenthesis. This is a standard format used in all publications. Finally, minimum flight altitudes. These are determined and promulgated by the State for each ATS route and Control Area — that's CTA — over its territory. They're published in the national Aeronautical Information Publication, which we call the AIP. Let me tie this together for you. The whole procedure is about knowing what your altimeter is referencing. QFE for height above the aerodrome, QNH for elevation above sea level. You check accuracy within those tolerances, you get the transition level before you descend, you get the latest QNH, and you understand how OCH is established when using QFE. And on the plates, remember the format: bold for AMSL, parenthesis for AGL.