Given: Economy @ FL80 OAT +20°C — Page 68, Lesson 72

I want to walk you through a worked example from the Multi-engine Piston Aeroplane, or MEP, section of the performance charts. This is the Descent Fuel, Time and Distance table, which comes from CAP 697 Figure 3.6. First, let me explain how this table works. It operates in exactly the same way as the climb table you've already seen. You make a single entry into the table, and that one entry gives you the fuel, time, and distance required to descend from a given pressure altitude, expressed as a flight level or FL, all the way down to Mean Sea Level, or MSL. Now, here's the important twist. If your destination airfield is not at MSL — which is almost always the case — you cannot simply use that one entry. You need to make a second entry. That second entry extracts data for a hypothetical descent from the airfield's own pressure altitude, expressed as a flight level, down to MSL. You then subtract that second set of data from the first set. The result is your actual descent fuel, time, and distance from your starting flight level down to the airfield elevation. Let's apply this to the example given. We are asked to descend from FL120, with an Outside Air Temperature of minus 20 degrees Celsius, to an airfield that is at 4,000 feet, with an OAT of plus 0 degrees Celsius. We need to find the fuel, time, and distance covered in the descent. We start with the first entry. For FL120 at minus 20°C, the table gives us: Fuel 4, Time 12, Distance 32. I'll read those as 4 units of fuel, 12 minutes of time, and 32 nautical miles of distance. Then we make the second entry. For a hypothetical descent from the airfield's pressure altitude of 4,000 feet down to MSL, at the airfield's OAT of plus 0°C, the table gives us: Fuel 2, Time 4, Distance 10. Now we subtract the second set from the first. The difference in fuel is 4 minus 2, which equals 2. The difference in time is 12 minus 4, which equals 8 minutes. The difference in distance is 32 minus 10, which equals 22 nautical miles. So the actual descent from FL120 to the airfield at 4,000 feet requires 2 units of fuel, takes 8 minutes, and covers a distance of 22 nautical miles through the air. The example then asks: with a 20-knot headwind, what is the ground distance covered in the descent? The air distance we just calculated is 22 nautical miles. With a headwind, your ground distance will be less than your air distance. You would need to apply the wind correction to that 22 nautical mile air distance using the time of 8 minutes and the wind speed of 20 knots to find the ground distance. The worked answer on the page doesn't show that calculation explicitly, but that is the principle you would follow. Let me show you the chart itself so you can see the table layout. `` And here are the worked answers for that descent example. ``