Let’s pick this up right where the idea of visual checkpoints leaves off — Page 229, Lesson 208

Let’s pick this up right where the idea of visual checkpoints leaves off. I want to walk you through the key principle first: visual checkpoints do not need to be exactly on track. A good feature close to track is better than a poor feature on track. That’s a really important mindset shift. You’re not hunting for something dead ahead on your planned line; you’re looking for something you can positively identify that sits near your route. A strong, recognisable feature slightly off your track is far more useful than a weak, ambiguous feature that happens to lie exactly on it. Now, the selection criteria. The ideal visual checkpoint will be recognisable from some distance away, and it should have three main features: it should be large, unique, and have vertical extent. Let me take each one in turn, because each has its own trap. First, large. A large feature will be recognisable from some distance, but here’s the catch — it may be too large to use for precise navigation. If you pick something enormous, you can see it coming from miles away, but you can’t pinpoint your position against it accurately. The good principle here is to use part of a large feature as your checkpoint. So, for example, if you’re using Birmingham as a checkpoint, you will see Birmingham from some distance, but you wouldn’t use the whole city as your accurate fix. Instead, you choose a large motorway junction within Birmingham as your accurate checkpoint. That gives you the recognisability of the big feature with the precision of a specific point. Second, unique. You must be sure the checkpoint you choose could not be mistaken for a similar checkpoint in the same area. This is where the trap really bites. Disused airfields make good checkpoints, but in East Anglia there are so many disused airfields — left over from WW2 USAF operations — that it is easy to make mistakes. So a feature that looks great on the chart might be one of a dozen identical ones around you. Uniqueness is what protects you from misidentifying where you actually are. Third, vertical extent. Features which have significant vertical extent — masts, mountains, that sort of thing — should always be identified for safety reasons, but they also make good visual checkpoints, particularly because they can be identified, in good visibility, at some distance. Let me give you the worked example from the text, because it shows exactly why this matters. Suppose you can see a radio mast 20 miles away, and you know you need to fly 1 mile to the right of the mast. You’re flying at 120 knots. You have just solved the navigation problem for the next 10 minutes. Let me show you why that arithmetic works. At 120 knots, you cover 2 miles per minute. So 20 miles takes you 10 minutes. That means for the next ten minutes, you don’t need to re-fix your position — you know exactly where you are relative to that mast, and you can spend your time checking aircraft systems, keeping a good lookout, and enjoying your flight. That’s the real value of a distant, vertically prominent checkpoint: it buys you time. But there’s a limitation I want you to hold onto. The value of features with good vertical extent alone may diminish if you fly higher. At 5000 feet AGL — that’s above ground level — you may not easily see a 400-foot-high mast. Think about that geometry. The higher you are, the smaller that mast’s vertical extent appears relative to your altitude, and the harder it is to pick out against the ground. So vertical extent is a great asset at low level, but it’s not a guarantee once you climb. So, to tie it all together: when you’re selecting a visual checkpoint, you want something large enough to see from a distance but with a precise part you can use for accuracy, something unique so you can’t confuse it with its neighbours, and something with vertical extent so you can spot it early — while remembering that vertical extent loses its edge at higher altitudes. And always remember the opening rule: it doesn’t have to be exactly on track, just close to it and good.