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I'm trying to use a simulator to learn to land properly. One part that I'm finding really tricky is picking a proper rate of descent. I know I'm supposed to do my best to keep it so I have two red lights and two white lights on the Precision Approach Path Indicator (PAPI), but at the moment I basically am just eyeballing it and adjusting my course whenever the lights aren't right.

I have to assume, though, that for any given airspeed there is a rate of descent I can maintain that will keep me right in the middle of the preferred glide slope for that airport. I just have no idea how to figure it out. Is there an application or formula that is preferred for this? Or is it time to learn Vector Calculus?

PS- I did check this question to see if it related. But that question is about starting the descent from cruise. I'm talking about rate of descent for final. I did also check this answer, but it's about using the PAPI/VASI lights. I'm looking for a formula to figure out the proper rate of descent during final for a given speed.

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Jae Carr
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  • The question is good and if I weren't on my phone I'd take a crack at the equations, but for practical purposes I'm willing to bet you'll have better luck eyeballing it than you will trying to fly that precise a rate of descent by instruments. Especially if the simulator isn't full-motion. – Steve V. Feb 09 '15 at 23:38
  • Are you approaching VFR or IFR? IFR, you can take a look at the approach chart, it will give the descent rate for the glide path depending on groundspeed. For VFR... well, it's called VFR.. you just aim for the runway. It also depends on aircraft used, a TMG will drop from higher altitude and approach with higher descent rate than a SEP. – SentryRaven Feb 09 '15 at 23:50
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    The formula really shouldn't change if you are accounting for speed, descent rate and optimal descent angle. – Jae Carr Feb 09 '15 at 23:53
  • You have a good answer, but I will add this: As a pilot, I only use the VSI as a cross-check in this situation. I change the pitch to stay on the PAPI and adjust the power setting to keep the airspeed that I want. The VSI takes care of itself. – Lnafziger May 08 '15 at 21:28

3 Answers3

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Assuming you want a 3 degree glide slope, just take 5 times your ground speed in knots as a starting point.

As an example, approaching at 90kt ground speed in a light aircraft, go for 450feet per minute. at 140kt in a jet, try 700fpm. If you're flying a jet outside of 4 miles, and the controller wants you to keep the speed up at 200kt then descend 1000fpm.

It's not perfect, but it works pretty well for a first guess, and you can adjust as necessary.

Of course, your ground speed is not the same as your air speed, and even for a constant IAS it will likely vary as you descend and the wind changes. Another way to think about this is that a 3 degree glideslope is almost exactly 300ft per nautical mile. If you're 2 miles out and 700 feet up, then you need to descend a bit faster, conversely, if you're at 500 feet, you need to descend slower.

A third related rule of thumb is that your final approach speed (in CAS, although IAS is good enough in most aircraft) is 1.3 times your stall speed in the landing configuration (Vso). So if you stall at 100 knots, you want to approach at 130KIAS.

IFR approaches are all about having an initial guess at these numbers in your head, trying them out, and then adjusting as the conditions require. If you're good at the guesses then the adjustments will be very small.

jwoolard
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For most purposes, if you're flying something with a jet engine, you can probably schwag -700fpm for a 3 degree glideslope and be fairly accurate. It will at least give you a starting point. You shouldn't be flying the approach solely off your VSI anyway, but it is useful to determine how large of a correction you are making, or if you're about to bust through the bottom.

Rhino Driver
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It helps to know the pitch and power settings for your jet. That way you're not chasing the airspeed, VVI, etc. So how do we figure out what pitch and power settings to use?

Well if we look at our ADI in level-flight at our final approach speed we see we are 2 degree nose-high for example. And we know that for each degree of pitch change it equates to 100 ft/NM. With an average ILS the glideslope is around 3 degrees. So we know that this is about 3 degrees low from our 2 degree level flight attitude. This leaves us with about 1 degree nose-low.

Fly the approach at 1 degree nose-low, and adjust your power to maintain your airspeed which in the end should result in a VV of about 5 * your groundspeed. Or you can take your groundspeed and convert it to NM/min and then multiply that by 300 ft/NM. If we approach at 160 KTS (2.67 NM/min), we end up with a VV of 800 ft/min which is the same as using our rule of thumb of 5 times your groundspeed.

I know this is probably a little more math than what you wanted, but setting a known pitch and power setting and making minor adjustments from that will help improve your approaches tremendously.