With a 40 kt headwind; what is the ground distance covered in the climb? — Page 68, Lesson 70

I want to walk you through a climb-performance problem for a Multi-engine Piston Aeroplane — an MEP — and I'll show you exactly how to find the ground distance covered in the climb when you have a 40-knot headwind. Let's start with the problem statement. We have an airfield at Mean Sea Level — MSL — with an outside air temperature of +20°C. We're climbing to Flight Level 120, which is 12,000 feet on the standard pressure setting, and the temperature up there is -10°C. The question asks for fuel used, time taken, and distance covered in the climb. The answers given are: Fuel 10 (units are gallons, typically), Time 19 minutes, and Distance 34 nautical air miles — that's the still-air distance from the climb chart. Now, the specific question you're looking at is: "With a 40 kt headwind; what is the ground distance covered in the climb?" The answer provided is 17 NGM — that's 17 nautical ground miles. Let me explain how we get that. The climb chart in CAP 697 Figure 3.1 gives you the air distance — the distance the aeroplane travels through the air mass. That's 34 NAM in this case. But ground distance depends on wind. With a headwind, your groundspeed is slower than your true airspeed, so you cover less distance over the ground for the same time in the climb. The formula you use is: NAM divided by NGM equals TAS divided by GS. Rearranged: NGM equals NAM multiplied by GS divided by TAS. But in this specific climb example, the chart already gives you the still-air distance, and the wind correction is applied directly. With a 40-knot headwind, the ground distance comes out to 17 nautical ground miles — exactly half the still-air distance of 34, because the headwind is significant relative to the climb speed. Now, let's look at the second example on the page. This one starts from an airfield at 4,000 feet with OAT 0°C, climbing to FL140 where the temperature is -20°C. The climb is done in two segments: from 4,000 feet to FL140. The fuel, time, and distance for the full climb to FL140 are: Fuel 12, Time 22 minutes, Distance 39 NAM. For the segment from 4,000 feet to the destination level, you subtract the values from the surface-to-4,000-foot segment: Fuel 4, Time 6 minutes, Distance 11 NAM. The difference gives you Fuel 8, Time 16 minutes, Distance 28 NAM for the climb from 4,000 to FL140. Then the question: "With a 40 kt headwind; what is the ground distance covered in the climb?" The answer is 17 NGM. That's the ground distance for the climb from 4,000 feet to FL140 with that headwind. So the key takeaway: the climb chart gives you still-air distance. When you have a headwind, you reduce that distance to get ground distance. With a 40-knot headwind in this MEP climb, the ground distance is 17 nautical miles. Now, moving on to the next section — this is about range at standard temperatures, using CAP 697 Figure 3.2. This graph gives you two values of air range: one with 45 minutes reserve fuel at 45% power on the left-hand side, and one with no reserve on the right-hand side. On the top right of the graph, there's a power phrase to percentage translation — for example, economy power is 65% power. You also need to correct the air range for temperature. The notes at the top left of the graph tell you: add 1 nautical mile for every degree Celsius above ISA, and subtract 1 nautical mile for every degree below ISA. ISA is the International Standard Atmosphere — at sea level, that's 15°C, and it decreases by about 2°C per 1,000 feet. And remember: range is affected by wind. So you might have to convert NAM — nautical air miles — into NGM — nautical ground miles. If you need to do that, you'll be given a True Airspeed and a Wind Component, and you apply the formula: NAM divided by NGM equals TAS divided by GS. That's the core of how you handle climb distance and range corrections for wind and temperature in a Multi-engine Piston Aeroplane.