One moment it would think you had 5,000 feet of clear air beneath you, the next it was asking the poor, confused AI to climb to avoid terrain. The terrain avoidance systems were getting two very different readings depending on both exact timing and position because they were just clipping the edge of the MSA squares and one of those squares covered high ridges which finished at sea level just inside the square’s boundary. If you went wide on the terrain checking, you’d never be allowed to land at any airport in a valley or just generally mountainous terrain. Sample points giving a segment MSAĬhecking a little to either side of the route can make this problem more or less go away, and that’s done in some places, but it can’t be used during approach because you’re actively trying to get close to the ground. A few hundred meters difference in location when the terrain samples were taken were enough to change the MSA between about 50 feet and 6,200 feet. This flight had a couple of steep ridges going from sea-level to about 6,200 feet which ended just inside one MSA square, and the flight was going just along the top of it at about 90 degrees to the ridges. MSAs departing to the north MSAs departing to the south The terrain elevation data is reduced so that a single pixel gives the MSA for whatever area that pixel covers, and this makes it relatively quick and easy to check. It’s simple, it works, and X-Plane does pretty much exactly the same. If you fly through that square, you’ll be guaranteed not to hit the ground if you stay above that altitude. Basically the chart is divided into large squares and, in each square, there’s an MSA given. If you’re familiar with aviation charts, you’ll know the acronym MSA – Minimum Safe Altitude. Where that falls down is where you’re deliberately trying to get close to the ground like, say, during approach and landing. On the face of it, terrain avoidance should also be simple: If hill then climb. The next problem soon comes to light: terrain avoidance. Re-test.Īs expected, much the same as before total indecision on the part of the controller. So, we’ve got one inappropriate climb instruction sorted. X-Plane 12 is wise to this though, so it shouldn’t have been a problem. The Cessna 172 climbs slowly, so it was approaching the whole “descend, approach, land” sequence from below. If you think about it, this completely breaks the simple altitude model already. This was kicking in because, when the AI was handed off from tower to the next level controller, it was already close enough to the destination airfield to be seen as an incoming arrival. Before X-Plane 12, the ATC system was pretty much exclusively written for airliners and an old bit of code had a problem where, if you were below a normal cruising altitude for an airliner, you might be told to climb slightly before starting the descent. Descend to 7000, then descend to 5500, then climb to 7000, then an endless loop of climb/descend to what looked like random choices of 5000, 6000, 70 feet.Īll of the “normal” – and a lot of the unusual – situations for approach and landing are covered in automated tests and they’d last passed the previous day, so I knew that this wasn’t a common problem but it would still be damned annoying if it happened to you.ĭigging in a bit, the first thing I found was a good old-fashioned bug. Checking the map, okay, there was a mountain ridge in the way so that makes sense. Takeoff was fine, and then it was told to climb to 8000 feet – above the requested cruise altitude. Happily the person reporting had included the correct logfile (not everyone does, we get loads where the log shows the sim sitting at the main menu!) and given the flightplan details nothing complicated, a simple direct flight between two airports about 35 miles apart, cruising at 6000 feet. That’s something that has had a lot of attention given to it for X-Plane 12, so I jumped on it and reproduced the flight. Why, I’m glad you asked! Make yourselves comfortable, this might take some time.Ī bug report arrived from someone saying they’d been vectored into a hillside on approach. Deciding what altitude a particular flight segment needs shouldn’t be tricky, right? You take off from somewhere near ground level, you climb to whatever you chose for cruise altitude, you start to descend when you get near the destination and eventually end up on the ground again.
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