Topographical Chart — Page 139, Lesson 138

I want to walk you through how we choose cruising levels and then how altimeter errors come into play when we set those levels. This is a core part of flight planning, so let’s take it step by step. First, the choice of a safe Flight Level depends on three things: the aircraft’s planned Magnetic track (not heading — track is the path over the ground, heading is where the nose points), the stated obstacle clearance allowance, and the regional forecast route QNH. For VFR flight in Single-Engine Piston and Multi-Engine Piston aircraft, we use the BELOW FL290 VFR diagram — that’s the top-right part of Figure 9.1. The rule is simple: for a magnetic track between 0° and 179°, you fly odd levels plus 500 feet. For a track between 180° and 359°, you fly even levels plus 500 feet. So if your track is 090°M, you’d be at, say, FL 75 (which is 7,500 feet) or FL 95 — odd plus 500. If your track is 270°M, you’d be at FL 80 or FL 100 — even plus 500. That’s the ICAO semi-circular cruising level system. For the Medium Range Jet Transport aircraft — the MRJT — when flying IFR, you use the IFR BELOW FL290 and AT AND ABOVE FL290 diagrams as your reference. So the VFR diagram is for SEP and MEP; the IFR diagrams are for the jet. Now, let’s move to altimeter errors and corrections. An altimeter is calibrated to the International Standard Atmosphere, or ISA. That standard assumes a mean sea level pressure of 1013.25 hPa, a temperature of +15°C, and a temperature lapse rate of 2°C per 1,000 feet — actually 1.98°C per 1,000 feet — up to 36,090 feet, where the temperature stays constant at -56.5°C. So when we calculate the height gain or loss in feet from an airfield up to a Flight Level — which is pressure altitude — we have to account for the differences between actual conditions and these ISA standards. Let’s talk about pressure difference. At take-off, you set the QNH on the altimeter subscale. That makes the altimeter indicate the aircraft’s altitude above mean sea level — so at the airfield, it shows the airfield elevation, ignoring instrument error. Then, when you pass the transition altitude, you set the standard pressure setting of 1013.25 hPa on the subscale. The difference between that 1013.25 hPa and the airfield QNH creates a barometric error — a height loss or gain — of about 30 feet per hectopascal. So if the QNH is lower than 1013.25, you lose height relative to the standard; if it’s higher, you gain height. Figure 9.2 shows the case where 1013.25 hPa is greater than QNH, meaning you get less height gained — you’re effectively lower than your indicated Flight Level would suggest. Let me show you the diagram for that. So in summary: you choose your cruising level based on magnetic track and the appropriate VFR or IFR diagram, and you must correct for the pressure difference between QNH and the standard 1013.25 hPa setting to know your true height above the ground.