ATPL · Navigation · General Navigation Atpl Ground Training… lesson — Page 229, Lesson 213

I want to walk you through some important visual navigation considerations that build on what we've already covered about height above the surface. Let's start with features you can use as visual references. Power stations are often located along rivers, and you may find several power stations along one stretch of river — so you can't assume you're looking at a specific one just because you see a power station near water. Airfields stand out well by day from heights above 1000 feet AGL — that's Above Ground Level. But you always need to be aware whether the airfield you're looking at is active or not. At night, airfield identification beacons are readily visible, which helps you confirm which airfield you're seeing. Quarries stand out particularly well as visual features, but you must check NOTAMs — Notices to Airmen — for any blasting activity in the area, because that could affect your safety. Monuments, follies, and chalk cuttings are precise features — meaning they show up clearly on a map and are easy to identify — but they are rarely lit, so they're only useful during daylight hours. Lighthouses are excellent features both by day and by night. However, they are not unique unless you identify them in relation to coastal features during the day, or by their correct light transmission characteristics at night — each lighthouse has a specific flash pattern and colour sequence that you can use to positively identify it. Now let's move to some visual navigation hazards — these are meteorological or geographic circumstances that can increase flight safety risks when you're flying or navigating visually, because they can cause visual illusions. These are covered in more detail in the Human Performance syllabus, but I'll summarise them here. In less developed areas such as deserts, jungles, tundra, or snow fields, usable features may be widely separated or even non-existent. Another problem is that topographical maps of these areas may be incomplete, so you can't rely on them for precise navigation. In areas of snow and ice that extend from horizon to horizon, and where the sky consists of a uniform layer of clouds so that no shadows are cast, the horizon disappears — Earth and sky blend together. In this situation, and in what are called 'white-outs' caused by blowing snow, your ability to assess distance and height above ground visually becomes virtually impossible. In conditions of poor contrast, particularly over snow-covered ground, it is possible to focus on high ground well ahead of the aircraft and fly the aircraft to cross that ground with the appropriate clearance. But here's the danger — the poor contrast may mask intervening high ground over which you're actually flying, meaning you could be much closer to the ground than you think, without realising it until it's too late.