Altimeter Setting Procedure — Page 221, Lesson 296

I want to walk you through the start of the Altimeter Setting Procedure chapter. This is foundational material for your ATPL, and it's one of those areas where a small misunderstanding can have fatal consequences, so we're going to take it step by step. Let me begin with the big picture. The entire purpose of this chapter is to make sure you understand how to set your altimeter subscale correctly. The introduction makes this crystal clear: to ensure separation between aircraft, and to make sure that when you're flying an instrument procedure, your aircraft is actually at or above the procedure design minimum altitude, it is essential that the altimeter subscale is correctly set to the appropriate reference pressure. Let me unpack that. Your altimeter measures pressure, not height directly. The subscale is the little window on the altimeter where you set a reference pressure — the QNH, the QFE, or the standard setting. What you set there determines what the altimeter reads. If you set it wrong, the instrument will lie to you about how high you are. And the book makes a sobering point: history is littered with accidents caused by incorrect altimeter setting. Even with the best intentions of Air Traffic Controllers, the basic responsibility remains with the pilot to ensure that whatever he or she does with an aircraft, it must be safe. That responsibility is yours, not the controller's. Now let's look at the first core concept, which is Terrain Avoidance. During instrument departure or arrival procedures, the aircraft must be flown according to the published flight profile. Until the aircraft is at or above a 'safe in all cases' altitude, the altimeter must be referenced to mean sea level, so that the pilot knows exactly how high the aircraft is. Why mean sea level? Because all obstacles shown on approach and departure plates are referenced to sea level, and likewise, all altitudes required by the procedures are also referenced to sea level. So if the plate says you must be at 3,000 feet over that obstacle, and your altimeter is referenced to sea level, then 3,000 feet on your instrument means 3,000 feet above sea level, which clears the obstacle. Now here's the key nuance. Sea level pressure — which we call QNH — varies geographically. And the terrain avoidance problem is geographic in nature. That means the reference setting must be a local QNH. You can't use the QNH from London when you're flying into an airport in the Alps, because the pressure at sea level under that air mass is different, and your altimeter would be off. So for terrain avoidance, you set the local QNH. Next, we have a definition you need to know precisely: the Lowest Useable Flight Level. This is the flight level that corresponds to, or is immediately above, the established minimum flight altitude. Let me explain that. A flight level is a pressure altitude referenced to the standard setting of 1013.25 hectopascals. The established minimum flight altitude is the lowest altitude you're allowed to fly in that area, usually set by terrain or obstacles. The lowest useable flight level is the flight level that matches that minimum altitude, or if the minimum altitude falls between two flight levels, it's the flight level immediately above it. So you always round up to the next flight level, never down. Finally, we come to ATC Separation. Once you're above the 'safe in all cases' altitude, the problem ceases to be terrain avoidance and becomes avoidance of other air traffic. This is a fundamental shift in your thinking. Below that altitude, you're worried about hitting the ground. Above it, you're worried about hitting another aircraft. In this case, it is essential that all aircraft 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's the key idea: if everyone is on the same reference pressure, then the vertical separation between aircraft is consistent, no matter what the actual sea level pressure is. If one aircraft set QNH and another set a different QNH, their altimeters would disagree, and separation could be compromised. So let me tie this together. You have two distinct problems. Below the safe altitude, you set local QNH to avoid terrain. Above it, you set a common reference so that ATC can separate you from other traffic. The transition between these two regimes is what the next part of the chapter covers, but for now, hold onto these three pillars: the altimeter subscale must be set correctly, terrain avoidance requires local QNH referenced to mean sea level, and ATC separation requires a common subscale setting across aircraft.