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Old January 22nd, 2007, 10:32 PM   #1
Erice Erice is offline
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Descent rate on non-precision approaches?

In my training on non-precision approaches, I've learned to descend to the minimum altitudes at step-down fixes, or to the DH after the FAF, as soon as practicable. The reason for this being to hopefully break out of the overcast sooner, and establish visual contact with the touchdown zone.

However, when I flew with an IR safety pilot the other day, he questioned why I was descending so quickly. He advocated to continue with a more gradual, steady descent, like you would fly on an ILS appraoch. His reasoning was that it would be easier on the passengers' ears, and it wouldn't tempt me to descend below DH if I could see the ground (directly below me), but not the touch-down zone.

How did you learn to descend once you reach the step-down fix? Or, how do you teach it to your Instrument students?

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Old January 22nd, 2007, 10:37 PM
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Re: Descent rate on non-precision approaches?

I'm not an IR pilot, but Barry Schiff (profficient pilot series) is a strong advocate of getting down as fast as possible to maximize the chances of seeing the runway environment. I see no disadvantages of doing so.

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Old January 22nd, 2007, 10:54 PM
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Re: Descent rate on non-precision approaches?

i was taught (and teach) at least 500 fpm in typical trainers on a non precision approach. faster down for faster airplanes. yes you can spend countless hours trying to figure out the exact descent rate to get to MDA at the MAP, or you can just get down, level off, and fly to MDA.
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Old January 22nd, 2007, 11:05 PM
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Re: Descent rate on non-precision approaches?

The theory behind the slower constant rate descent is that the approach is flown like the ILS, and the approach is stable. This works when the approach is a straight in aligned with the runway, provided you have the discipline to treat it like an ILS - look up at MDA and land or go missed. It's also a standard practice for large airplanes when flying non-precision approaches, because they are so big and have so much inertia that it's better to be stable and consistent.

For approaches not aligned with the runway, I believe it's better in our more manueverable aircraft to follow the "dive and drive" philosophy, and get down to MDA as soon as possible, giving more time to look for the airport so you can make the visual approach using normal manuevers.
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Old January 22nd, 2007, 11:07 PM
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Re: Descent rate on non-precision approaches?

I use 1000fpm descents on NP approaches. Unless you figure out the exact descent rate needed to hit the next stepdown or MAP at exactly the right time (which in itself makes you work harder than necessary), you run the risk of not making the MDA before you reach the MAP.

If you descend at a rate only slightly higher than the exact descent rate needed to hit the next fix/altitude simultaneously, you'll end up needing to level off and then have to start descending again almost right away. Too much going on, too easy to screw up.

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Old January 22nd, 2007, 11:23 PM   #6
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Re: Descent rate on non-precision approaches?

Quote:
Originally Posted by TMetzinger View Post
The theory behind the slower constant rate descent is that the approach is flown like the ILS, and the approach is stable.
Tim,

Just wondering how you'd respond to my last post, given the above?

I guess the real trick is being able to quickly determine the correct rate of descent, but I'm not yet a good enough IFR pilot to take [(feet here-feet there)/(nm here to there)]*1.5 that would give you the correct rate of descent at 90 knots in my head while I'm flying in the muck. And I'm better at doing math in my head than anyone I know, although I bet Lance Fisher could give me a run for my money.

Looking at the most popular NP approach used at the home drome (VOR/DME 32), the proper rate of descent would be 495fpm, and it took me 1:48 to calculate this WITHOUT flying the plane. I figured the 1220 foot descent in my head, but I did the 1220/3.7 on paper for speed and accuracy purposes, and then the *1.5 in my head again.

Now, how about an NP approach without a GPS on board where there's no DME at the navaid (as in, no groundspeed indications)? In that situation, you pretty much have to dive and drive.

IMHO, without some sort of flight management system to do this for you, unless you've done the calculations in advance and know the proper descent rates, the dive-and-drive technique would work a lot better.
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Old January 23rd, 2007, 12:01 AM   #7
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Re: Descent rate on non-precision approaches?

Dive and drive.

If you keep a reasonable pitch attitude for speed, and smoothly take out power for descent rate of 800-1000fpm, it's still comfortable to the pax.

And they haven't thrown-up yet!
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Old January 23rd, 2007, 07:51 AM
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Re: Descent rate on non-precision approaches?

PIC teaches using a descent rate on NP approaches about twice that of precision approaches. This usually works out to about 800 fpm or so for the average light single flying the approach at 75-90 KIAS. The reasoning is that on NP approaches, you aren't allowed to go one inch below MDA, and if you try the steady descent system, you have to make your decision before MDA or risk busting it. That can cause you to go missed when you could have gotten in. Alternatively, you have to transition from descent to level near the point where you would like to be looking outside for the runway -- a bad division of attention that is likely either to cause you to miss seeing the airport or miss seeing the plane getting off MDA (high = missed, low = bust).

Note that on precision approaches, you are allowed to sink below DH during the transition from descent to climb once you start your go-around. Thus, you can keep looking all the way to DH, hit the power, and not be considered busting mins if you settle 20 or 30 feet below DH (or a lot more in something like a 747) during the transition.
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Old January 23rd, 2007, 07:53 AM
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Re: Descent rate on non-precision approaches?

Quote:
Originally Posted by Erice View Post
How did you learn to descend once you reach the step-down fix? Or, how do you teach it to your Instrument students?
I was taught (in general) 500fpm for the intermediate stepdowns, and 1000fpm for the final stepdown to MDA. Of course, you need to brief your plate, look at distances and approach speeds, as you may need to decend faster than these if necessary.
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Old January 23rd, 2007, 08:09 AM
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Re: Descent rate on non-precision approaches?

On NP approaches I use between 750 and 1000 fpm. The idea is as has been said before to get down quick. This helps one to break out sooner and start looking for the airport. This was brought home to me recently on a night approach to my home airport where the ceiling was only 300 feet above the MDA. It wa solid and I was happy to get below and then set up for a nice approach to landing onc eI found the airport.
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Old January 23rd, 2007, 11:49 AM   #11
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Re: Descent rate on non-precision approaches?

I was taught to shoot for around 800 fpm as an average on NP approaches, as advocated by Ron Levy above.

But, I firmly believe in "dive and drive" on the step downs, for one reason: It gives you more time down at your MDA for each stage, to look at conditions, etc. You get down as soon as you can, the better to see the airport sooner. The last thing you want is to not get down until near the MAP, only to break out right over or too near the runway. Remember, with a NP approach, you often don't have great minimums. You get there, you see the airport, but you STILL gotta lose altitude to find pavement.

YMMV of course.

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Old January 23rd, 2007, 12:54 PM   #12
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Re: Descent rate on non-precision approaches?

A friend of mine has a WAAS IFR GPS. The computer figures out an equivalent glidepath which he can follow by flying the glideslope needle. I've seen him fly approaches with it before and it seems to work out well.

I'm not equipped with one of those, so for me it's "dive and drive" at 1000fpm.
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Old January 23rd, 2007, 03:26 PM
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Re: Descent rate on non-precision approaches?

My instructor taught me to fly stabilized approaches (shoot for 3 degree glideslope), but make sure you get down to DH a little before the MAF. I always used 2 rules of thumb, and adjusted descent rate accordingly.

1. Figure 3 miles per 1000' descent (I know 3 mi per 1000' is actually about 3.5 degrees)
2. 5 Times Groundspeed will give you an approximation of the descent rate needed for a 3 degree glideslope

That is pretty simple math that can be done quickly. If you don't have the distance increase descent rate. this way you can set up a stabilized approach without too much mental acrobatics.

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Old January 23rd, 2007, 05:34 PM   #14
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Re: Descent rate on non-precision approaches?

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Originally Posted by JustinPinnix View Post
A friend of mine has a WAAS IFR GPS. The computer figures out an equivalent glidepath which he can follow by flying the glideslope needle. I've seen him fly approaches with it before and it seems to work out well.

I'm not equipped with one of those, so for me it's "dive and drive" at 1000fpm.

My GPS does the same thing on most GPS approaches and I can even couple it to the autopilot. But I'm pretty sure that feature doesn't work on VOR or LOC approaches where the GPS is supplemental. For those or when I'm flying something that doesn't have a GPS like mine, I use 800-1000 FPM on all descents on the approach leading up to the MDA. For me it's not so much the idea of breaking out early to increase the chances of completing the approach, as it is for eliminating the potential to bust the MDA because I'm looking for a runway. If I let the autopilot fly the plane down a GPS generated glideslope it's pretty easy to keep track of the altitude while looking for the runway, but hand flying I want to keep the two (levelling off at MDA and looking for a runway) separated in time as much as possible.

And for those who said they want to get down to the next level off as quickly as possible, I assume you mean with a reasonable descent rate. Anything beyond 1000 FPM in a piston airplane is asking for an altitude bust IMO and totally unnecessary unless you have a heck of a tailwind (in which case the best thing to do is slow down rather than descend faster).
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Old January 23rd, 2007, 05:51 PM   #15
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Re: Descent rate on non-precision approaches?

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My instructor taught me to fly stabilized approaches (shoot for 3 degree glideslope), but make sure you get down to DH a little before the MAF. I always used 2 rules of thumb, and adjusted descent rate accordingly.

1. Figure 3 miles per 1000' descent (I know 3 mi per 1000' is actually about 3.5 degrees)
2. 5 Times Groundspeed will give you an approximation of the descent rate needed for a 3 degree glideslope

That is pretty simple math that can be done quickly. If you don't have the distance increase descent rate. this way you can set up a stabilized approach without too much mental acrobatics.

Pete
That's all true except that:

1> On most approaches you need to get to the MDA (BTW it's not a DH) way before not a little before the MAP. With a 600 AGL MDA (not uncommon on non-precision approaches) you need to hit the MDA no later than 2 miles from the touchdown point for your 3 degree slope to work.

2> The math is easy, confirming your progress is quite a bit more difficult unless you have distance to threshold in your scan and can do math in your head very quickly. If you have a GPS that gives vertical guidance then staying on top of your required descent rate is pretty simple.

3> If you aim for a 3 degree slope to the touchdown zone, chances are you will either be well above or below that profile when you break out of the clouds (assuming you're not continuously updating your descent rate and/or groundspeed during the descent). As a result IMO you are more not less likely to end up with a stabilized approach this way than if you had leveled off at the MDA a mile early and started a 3 degree slope from the point where it intersects the MDA.

4> When you are busy calculating adjustments to your descent rate while still in IMC and peering out the windscreen for a glimpse of the runway and/or lights you are way to likely to miss the MDA level off. Enough things are going on in your head that you are unlikely to give the altimeter the attention it deserves IME.

IOW, unless you have electronic guidance for that 3 degree slope in the clouds I strongly believe that you are much better off hitting the MDA early and flying level to the miss or final descent.


I was initially taught dive/drive and subsequently learned about using a calculated virtual glideslope which I tried for a while and discarded for the reasons listed above. I only recently went back to a continuous descent when I found how easy it was when you have ILS like guidance from a GPS. I just dont use the "calculated rate of descent" method when there's no electronic vertical guidance.
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Old January 23rd, 2007, 05:53 PM   #16
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Re: Descent rate on non-precision approaches?

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And for those who said they want to get down to the next level off as quickly as possible, I assume you mean with a reasonable descent rate. Anything beyond 1000 FPM in a piston airplane is asking for an altitude bust IMO and totally unnecessary unless you have a heck of a tailwind (in which case the best thing to do is slow down rather than descend faster).
I'm a "chop and drop" guy. I don't like to mosey down to the next altitude unless I have plenty of distance to cover. But I agree, I've not seen a situation where a rate in excess of 1,000 fpm is required. Then again, I haven't flown 'em all.
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Old January 23rd, 2007, 06:08 PM   #17
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Re: Descent rate on non-precision approaches?

Being that I did my IR training with Tony, I am a member of the "dive and drive" school.

This mentality definitely came into play on my first "instructor-less" approach on my first IR flight in December: Center Controller gave us a descent to IAP for GPS approach ~50nm out. During the descent, it got VERY bumpy and my wife (new to this whole flying thing) became VERY unhappy. I continued the approach as 'standard', but I definitely got down as rapidly as practical. From the FAF, I once again did the 'dive and drive' and once I popped out from the clouds and saw that I had a clear shot to the runway, I went back up to highest allowable power (blue-line) to try and get on the ground as fast as possible to try and salvage any hope I had of ever getting my wife back into the airplane with me. If I had held to a 'standard descent rate', we would have stayed bumping around in the moderate-severe turb for quite a while longer considering we were flying into a 20+kt headwind.

Just my .02.
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Old January 23rd, 2007, 06:31 PM   #18
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Re: Descent rate on non-precision approaches?

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That's all true except that:

1> On most approaches you need to get to the MDA (BTW it's not a DH) way before not a little before the MAP. With a 600 AGL MDA (not uncommon on non-precision approaches) you need to hit the MDA no later than 2 miles from the touchdown point for your 3 degree slope to work.

2> The math is easy, confirming your progress is quite a bit more difficult unless you have distance to threshold in your scan and can do math in your head very quickly. If you have a GPS that gives vertical guidance then staying on top of your required descent rate is pretty simple.

3> If you aim for a 3 degree slope to the touchdown zone, chances are you will either be well above or below that profile when you break out of the clouds (assuming you're not continuously updating your descent rate and/or groundspeed during the descent). As a result IMO you are more not less likely to end up with a stabilized approach this way than if you had leveled off at the MDA a mile early and started a 3 degree slope from the point where it intersects the MDA.

4> When you are busy calculating adjustments to your descent rate while still in IMC and peering out the windscreen for a glimpse of the runway and/or lights you are way to likely to miss the MDA level off. Enough things are going on in your head that you are unlikely to give the altimeter the attention it deserves IME.

IOW, unless you have electronic guidance for that 3 degree slope in the clouds I strongly believe that you are much better off hitting the MDA early and flying level to the miss or final descent.

Well said, Lance. I'm just an IR wannabe, but it would seem to me that it might be useful to calculate the 3 degree glideslope (while still in cruise if possible) and then use that as a "I gotta be down to MDA before then" instead of "this is the point where I want to be down to MDA". Then you have some idea of how fast you need to get down for a certain approach, especially with a tailwind or something. That's the method advocated by Barry Schiff.

Do yo think it's worth doing that kind of calculation, or do you just prefer to dive at 1000 fpm in all cases?

Chris
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Old January 23rd, 2007, 06:55 PM   #19
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Re: Descent rate on non-precision approaches?

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Well said, Lance. I'm just an IR wannabe, but it would seem to me that it might be useful to calculate the 3 degree glideslope (while still in cruise if possible) and then use that as a "I gotta be down to MDA before then" instead of "this is the point where I want to be down to MDA". Then you have some idea of how fast you need to get down for a certain approach, especially with a tailwind or something. That's the method advocated by Barry Schiff.

Do yo think it's worth doing that kind of calculation, or do you just prefer to dive at 1000 fpm in all cases?

Chris

I do eyeball approaches for steep descent requirements taking into account any anticipated tailwind, but I've never found one that requires more than 1000 FPM at 100 KIAS. Of course you can put yourself into this bind on an approach that begins at a navaid on the airport by not flying out far enough before turning around.
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Old January 23rd, 2007, 10:30 PM   #20
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Re: Descent rate on non-precision approaches?

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Tim,

Just wondering how you'd respond to my last post, given the above?

I guess the real trick is being able to quickly determine the correct rate of descent, but I'm not yet a good enough IFR pilot to take [(feet here-feet there)/(nm here to there)]*1.5 that would give you the correct rate of descent at 90 knots in my head while I'm flying in the muck. And I'm better at doing math in my head than anyone I know, although I bet Lance Fisher could give me a run for my money.

Looking at the most popular NP approach used at the home drome (VOR/DME 32), the proper rate of descent would be 495fpm, and it took me 1:48 to calculate this WITHOUT flying the plane. I figured the 1220 foot descent in my head, but I did the 1220/3.7 on paper for speed and accuracy purposes, and then the *1.5 in my head again.

Now, how about an NP approach without a GPS on board where there's no DME at the navaid (as in, no groundspeed indications)? In that situation, you pretty much have to dive and drive.

IMHO, without some sort of flight management system to do this for you, unless you've done the calculations in advance and know the proper descent rates, the dive-and-drive technique would work a lot better.
I agree, that unless you have good instrumentation, and by that I mean at least a GPS groundspeed readout and a chart that maps GS to VS, you're better off doing the dive and drive.

However, go watch a VNAV approach sometime in an airplane with a 480 coupled to an autopilot. Even when it's not an LPV approach, it's something magical to watch. So if you're equipped, you've got options, and a good pilot always considers all his options and then picks the one that works best for him. A not so good pilot doesn't consider all options.
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Old January 23rd, 2007, 10:41 PM
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Re: Descent rate on non-precision approaches?

the VNAV on the 480 is pretty sweet. thats what we have in the 421.
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Old January 23rd, 2007, 11:05 PM   #22
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Re: Descent rate on non-precision approaches?

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I agree, that unless you have good instrumentation, and by that I mean at least a GPS groundspeed readout and a chart that maps GS to VS, you're better off doing the dive and drive.

However, go watch a VNAV approach sometime in an airplane with a 480 coupled to an autopilot. ...
But if you're using a VNAV descent right on glidepath, you'd have to pull up before hitting MDA so you don't get too low, right? With a DH you can wait to pull up until you're at DH because there's a buffer built in, but for an MDA you can't go below it at all so you have to start checking your descent earlier.

So wouldn't it still be better to descend to MDA, level out, then look out the window for the runway? That way you're not trying to look for the runway at the same time you're trying to not bust the MDA.

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Old January 24th, 2007, 01:11 AM
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Re: Descent rate on non-precision approaches?

Was taught & also teach to use about 1000 fpm ASAP & level off, no probems yet.
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Old January 24th, 2007, 01:15 AM
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Re: Descent rate on non-precision approaches?

also a good method for descending through a cloud layer you suspect, or know, contains ice. spend minimum time in bad conditions.
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Old January 24th, 2007, 06:49 AM   #25
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Location: Salisbury, MD
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Re: Descent rate on non-precision approaches?

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Originally Posted by cwyckham View Post
But if you're using a VNAV descent right on glidepath, you'd have to pull up before hitting MDA so you don't get too low, right?
Unlike the MDA's on LNAV GPS approaches, the LPV and VNAV/LNAV minima are DH's (shown as DA(H) on Jepp), not MDA's, and are treated just like DH's on ILS approaches, not like MDA's on nonprecision approaches.
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