How do pilots estimate glideslope visually without PAPIs?

Question

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?

Answer 1

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.

Answer 2

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:

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.

Answer 3

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.

Answer 4

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.