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If my (enthusiast-level) understanding is correct, on instrument approach there are specific glideslope paths a plane is expected to follow. On ILS this is self-evident, but (again, if I read approach plates correctly), VOR DME approaches also entail a certain altitude that must be crossed at given DME distances. I assume, crews estimate their rate of descent based on their ground speed etc.

Now, when visually, apart from PAPI lights, is there any other way of estimating the correct glideslope? With C172s and 55 knots, I expect it's easier to simply crawl to the runway and adjust accordingly. But how does it work when you have a B737 or A320 landing on a 6000ft/1800mt runway without PAPI lights? I expect a certain kind of precision is required, but does it come merely with experience, or is there some other way?

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    $\begingroup$ Practise makes perfect. Basically experience let's you estimate your glideslope pretty accurately (and it doesn't matter if it's a tiny bit off, you can make the runway anyway). $\endgroup$ – expeditedescent Jul 29 '16 at 6:22
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    $\begingroup$ I can't speak for today's pilots since I retired in 1999, but back in the previous century, if you came out of a general aviation environment where you had done literally thousands of landings on runways that had no ILS, PAPI or the like, you internalized how the runway should look, and that didn't really change with a transition to large aircraft. I spent my last 10 years on the 747, and in good weather there was no critical need for glidepath assistance other than just looking at the runway and landing. $\endgroup$ – Terry Jul 29 '16 at 6:23
  • $\begingroup$ It seems there is a consensus on the matter, so I'll mark this question as answered. Thanks for your replies. $\endgroup$ – Digital Dracula Jul 29 '16 at 9:13
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There's no instrument you need for a good approach angle in good weather, it's all done by eye. When you train as a pilot you develop a sight picture of what a good approach looks like, and you develop a sense of what a runway looks like when you get too low or too high. It comes from experience and practice combining with the human brain's excellent spatial processing capabilities.

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  • $\begingroup$ When in a stable descent, the point on the ground (runway) that is not moving away or toward you - is your touchdown point. $\endgroup$ – Mike Brass Jan 7 '18 at 20:05
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A landing approach in good weather can be judged visually in pretty much any kind of aircraft: The exact sight picture will vary (the view from the flight deck of an A380 is obviously different than from the cockpit of a Cessna 152 - you're higher up among other things), but the basic geometry of what you're looking at is similar.

The best illustrations I've found for this are below:

PAPI Lights (Spanish) PAPI Lights (Spanish)

Broadly we can break these images down into the 3 categories a VASI gives us (High, Correct, and Low):

  • High/Steep
    If you are high and your approach path is therefore steep The runway will look long, skinny, and relatively rectangular. Terrain/buildings near the airport will also look "flat" since you're looking down on them.

  • "Correct" (on altitude / on glide path)
    If you are at the correct altitude for your position on the approach and therefore are flying a normal descent angle to the runway everything will look "normal" -- The runway has a trapezoidal shape as it recedes into the distance, and the markings/terrain will look as you expect for a normal landing.
    Normal in this case is something you learn judge largely by experience and all the bad landings you made as a student, though there are some tricks to help you estimate the angle.

  • Low/Shallow
    If you are below the normal approach path and flying at a shallow angle the runway and terrain/buildings begin to resemble what they look like when you're on the ground: The runway is wide and flat, and the markings generally appear larger than you would expect on a normal approach. Buildings/terrain around the airport also start looking more and more like they do when you're sitting on the runway.
    Taken to its most extreme the sight picture for a low approach looks like you're standing on the ground at the runway threshold (or a few yards before it), but in good weather there are sufficient visual cues that a pilot shouldn't get into that situation.

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  • $\begingroup$ I know there are good pictures of High/Correct/Low approaches from the cockpit, but I wasn't able to dig them up -- Google is not my friend today! If I find them I'll add them. $\endgroup$ – voretaq7 Jul 29 '16 at 16:08
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    $\begingroup$ Note that while judging of an approach by the visual appearance of the runway, a number of visual illusions may cause the pilot to wrongly assume his position relative to the glideslope. Additionally, if approaching a runway at a constant viewing angle, the glideslope actually isn't linear, but circular, so the pilot might find himself under the planned glideslope. $\endgroup$ – Czechnology Aug 2 '16 at 0:39
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    $\begingroup$ @Czechnology Also note that this is why runway marking are standardized (see pages 14-16 of this helpful FAA publication), and why airports with non-standard runway markings have that fact noted in the airport/facility directory. The runway itself may look strange, but if you can see the markings they're an excellent reference. (I believe spacing of runway edge lights is also specified as an aid for night ops, but I'm not 100% certain on that) $\endgroup$ – voretaq7 Aug 2 '16 at 2:27
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Commercial carriers (questioner mentioned B737 or A320) generally use runways that have some form of instrument approach (ILS and/or VOR/DME and/or GPS RNAV). Those aircraft also have baro-VNAV, which essentially creates a glide slope without an ILS. Whatever form of guide is available, it can be used on an advisory basis when cleared for a visual approach operation.

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    $\begingroup$ While this is true, it doesn't really answer the original question $\endgroup$ – expeditedescent Jul 29 '16 at 19:13
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It should be done by sight picture and all pilots eventually get there.

On a standard glidepath of 3 degrees you will descend 319 ft/nm. Most pilots will round to 300 ft/nm though I have seen European approach charts with a SCDA at 320 ft/nm.

If you know your distance to the runway, do the math. Typically the FAF is 5nm from the runway so that is roughly 1500 ft agl. Try to descend so that you are 1200 ft agl at 4nm and so on.

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