Altimeter Setting Procedure — Page 231, Lesson 303

Let’s pick up right where the transition layer leaves off and talk about what you actually set on that altimeter, and when. This is the heart of the Altimeter Setting Procedure. First, the big one: QNH. The QNH should be passed to an aircraft in the taxi clearance prior to take-off. So before you even leave the stand, the clearance you receive on the ground includes the QNH for that departure aerodrome. Now, what does QNH do for you? When you set QNH, your altimeter reads altitude — that is, your vertical position above mean sea level. So while you are flying en route below the transition altitude, your vertical position is reported as altitude, with QNH set. That is the key relationship: QNH set equals altitude reported. But here is the catch — QNH is only valid for the aerodrome it came from. When you fly outside the ‘vicinity’ of the departure aerodrome, that QNH is no longer good enough. You will need the QNH from another, closer aerodrome, and you will set that instead. Now, different countries handle this differently, and I want you to see the contrast. In the UK, we have a system of regional pressure settings, abbreviated RPS. That system covers the en route case without you having to reference each en route aerodrome’s QNH individually. In the USA, it is different: when flying cross-country, the pilot will need to contact ATC at ‘local’ aerodromes and update the altimeter setting regularly. That regular updating is what allows you to determine terrain clearance with an acceptable degree of accuracy. So the purpose of all this QNH updating is terrain clearance — knowing how far you are above the ground and obstacles. Now, what happens when you climb above the transition altitude? When en route and above the transition altitude, the aircraft level is reported as a flight level. So the reporting standard flips: below transition altitude, altitude with QNH; above transition altitude, flight level. Now let’s look at the approach and landing side, because this is where it gets operationally important. When approaching an aerodrome to land, the QNH will be passed to aircraft in clearances to enter the traffic circuit. Normally, vertical position is reported as a flight level until you reach the transition level in the descent. But — and this is a critical ‘however’ — after an approach clearance has been issued, reference should then be made in terms of altitude, with the QNH set. So the sequence is: you descend on flight levels down to the transition level, then once you get the approach clearance, you switch your reference to altitude with QNH set. This is intended to apply primarily to turbine aircraft, for which an uninterrupted descent from high altitude is desirable. That is the reason for the rule — turbine aircraft want a continuous, unbroken descent, so you don’t want to be fiddling with setting changes mid-approach. Now, one more setting: QFE. If a pilot decides to remain in the aerodrome traffic circuit — the visual circuit — throughout the flight, then the altimeter may be set to QFE. QFE is the setting that reads height above the aerodrome. So the contrast is: QNH gives you altitude above mean sea level, QFE gives you height above the aerodrome, and QFE is only used when you stay in the visual circuit the whole time. Let me tie the whole picture together. Below transition altitude en route, you report altitude with QNH. Above transition altitude, you report flight level. On descent, you stay on flight level until the transition level, then after approach clearance you switch to altitude with QNH. And if you never leave the visual circuit, you can use QFE throughout. That is the complete setting procedure.