What is the ground range ? — Page 58, Lesson 66

I want to walk you through the concept of ground range and how we calculate it from the data we extract from performance graphs. This is a fundamental skill for flight planning. Let's start with the core idea. The ground range is the actual distance the aeroplane travels over the ground. It's what your navigation log cares about. In the excerpt, we see a worked example: we extract 799 NAM from a graph. NAM stands for Nautical Air Miles — that's the distance the aeroplane travels through the air mass, not over the ground. We also extract a TAS of 169 knots from the same graph. TAS is True Airspeed — the actual speed of the aeroplane through the air. Now, to convert that air distance into ground distance, we need the GS, or Ground Speed. The excerpt shows us the relationship. We have TAS of 169 knots, and a GS of 199 knots. The difference is the wind component — in this case, a tailwind is adding 30 knots to the ground speed. The calculation is a simple proportion. We know the NAM (799) and the TAS (169). We want the NGM, which stands for Nautical Ground Miles — that's the ground range. The formula is set up as a ratio: NAM over TAS equals NGM over GS. So, 799 NAM divided by 169 knots TAS equals x NGM divided by 199 knots GS. Solving for x, we get x equals 941 NGM. So the ground range is 941 Nautical Ground Miles. Let me be clear on the terms: NAM is the distance through the air, unaffected by wind. NGM is the distance over the ground, which includes the effect of wind. The relationship is: NGM equals NAM multiplied by the ratio of GS to TAS. Now, the excerpt also introduces the concept of endurance for a Single-engine Piston Aeroplane (SEP). Endurance is how long the aircraft can remain airborne. The key point here is that endurance is not necessarily the time to achieve the maximum range — it's simply the total time you can stay up. The excerpt also tells us that endurance is NOT affected by wind. That makes sense because endurance is about fuel flow and time, not distance over the ground. There's a critical detail about the endurance you extract from the graphs. Because this endurance includes a 45 minute reserve, the value you get is called a safe endurance. That means the time shown already accounts for the legal reserve fuel. The excerpt then gives an example of extracting endurance from CAP 697 Figure 2.5. For a given power setting and flight level, you read the endurance directly. For example, at FL100 with FT (Full Throttle) at 2500 rpm, the endurance is 5.1 hours, which is 5 hours and 6 minutes. At the same flight level but with 2100 rpm, it's 6.35 hours, or 6 hours and 21 minutes. At FL70 with 23 inches of manifold pressure at 2300 rpm, it's 5.2 hours, or 5 hours and 12 minutes. Finally, the excerpt sets up SEP Exercise 1. This is a practical flight planning exercise. You have an airfield at Mean Sea Level elevation, a departure mass of 3400 lb, and you plan to cruise at 23 inches and 2300 rpm. The temperature is +20°C. You use SEP Fig 2.1 and Fig 2.2 to complete a fuel log. You need to calculate the trip fuel, time en route, and ETA at destination "D". The aircraft takes off at 10:00 and climbs from airfield "A" to FL80, with the temperature at the Top of Climb being 0°C. The wind is light and variable. The table then gives you the legs from TOC to B, B to C, and C to D, with specific flight levels, ISA deviations, wind vectors, TAS, and NGM distances for each leg. You would use these to calculate headings, ground speeds, times, fuel flows, and fuel required. So, to summarise the key takeaway: ground range in NGM is calculated by taking the air distance in NAM and scaling it by the ratio of ground speed to true airspeed. Endurance gives you the time you can stay airborne, including a 45-minute reserve, and it is not affected by wind.