Constant-rate descents on non-precision approaches

[C150J]

I'm 99.9% sure that this is correct, but please forgive my idiotic question:

Say you're shooting a LOC/DME approach and are at 3,000 feet. Your next INTERMEDIATE step-down fix dictates a descent to 1,700 feet. However, you have determined that, by waiting a couple miles PAST said fix, you can keep a constant rate going all the way to the MDA. Is there any guidance preventing you from delaying a descent?

Thanks!

 

[poser765]

Why would you not be able to begin a constant descent AT the fix? It would just be shallower than if you waited till after.

 

[ftyflyboy]

If you are shooting the approach in IMC, the whole idea is to get as low as possible, as soon as possible. You want to clear the clouds and get a visual on the runway to ensure a landing.

 

[CaptBill]

Say you're shooting a LOC/DME approach and are at 3,000 feet. Your next INTERMEDIATE step-down fix dictates a descent to 1,700 feet. However, you have determined that, by waiting a couple miles PAST said fix, you can keep a constant rate going all the way to the MDA. Is there any guidance preventing you from delaying a descent?

The answer is NO, there is nothing keeping you from delaying your descent. Most intermediate altitudes are "at or above" altitudes so if you determine you can accomplish a constant rate descent without going below the prescribed altitude(s) on any intermediate fix(s), go for it. Be aware though that some approaches require a hard altitude over a fix and you would therefore have to comply with that requirement.

Something you should be aware of though is that the MDA still applies. If you are on a constant rate descent and decide at MDA that you can't see the runway, you will likely go below MDA executing the missed approach. At Continental, we bug 50 feet high on these constant rate approaches to give us that additional altitude to safely execute the missed approach without going below MDA. The bad thing about using this technique is that you have to decide 50 feet earlier if you are going to land or not. If the ceilings are very marginal, you may be better off descending to your MDA earlier (dive and drive) to give yourself the best possible chance to see the runway.

Great question...:clap:

 

[minitour]

If you are shooting the approach in IMC, the whole idea is to get as low as possible, as soon as possible. You want to clear the clouds and get a visual on the runway to ensure a landing.

Yuck. Dive and drive sucks, IMO.

I used to plan the descent to arrive at MDA at least a mile before the MAP. That gives me time to look around and see what's going on and then I'm out. Dive and drive...lots of time down low with the man made and natural obstructions.

-mini

 

[MikeD]

Yuck. Dive and drive sucks, IMO.

I used to plan the descent to arrive at MDA at least a mile before the MAP. That gives me time to look around and see what's going on and then I'm out. Dive and drive...lots of time down low with the man made and natural obstructions.

-mini

That's why there's an MDA, so you don't have to worry bout it. Don't go below it prior to VDP, and there's little chance of hitting anything......unless the TERPs guys screwed up. . Past VDP, both techniques no longer matter.

But truthfully, arguing which technique is better falls under the same realm as the FBO vs Academy argument. Both techniques have their respective merits.

 

[MidlifeFlyer]

Why would you not be able to begin a constant descent AT the fix? It would just be shallower than if you waited till after.

I thought of that question. But he has an intermediate stepdown so it might not always work. A constant descent from the fix to the MDA =could= mean going below altitude at the intermediate. I figured he was looking at something like that when he asked the question.

Of course, the other piece is that you probably want to reach MDA at or before a good VDP (whether charted or calculated), which can make the question a bit more complicated.

 

[///AMG]

I'm sure it is aircraft dependant, but that said, there are some non-precision approaches out there that spit you out at a MAP that would make any pilot struggle for a safe, controlled approach to landing. Like has been said, you want to be at MDA prior to the VDP (or equivalent if one isn't depicted) in order to make a smooth transition to a visual approach. Chopping the power and diving for the runway when you break out at the MAP is not good technique.

 

[C150J]

I'm sure it is aircraft dependant, but that said, there are some non-precision approaches out there that spit you out at a MAP that would make any pilot struggle for a safe, controlled approach to landing. Like has been said, you want to be at MDA prior to the VDP (or equivalent if one isn't depicted) in order to make a smooth transition to a visual approach. Chopping the power and diving for the runway when you break out at the MAP is not good technique.

Absolutely, and in those situations, descending ASAP is prudent. However, there are a good deal of approaches out there that put you at the MDA well before your VDP... fully-configured...engines spooled...basically shooting Jet-A out the back

Totally see your point... the LDA/DME 19 in DCA being a perfect example.

 

[shdw]

Why would you not be able to begin a constant descent AT the fix? It would just be shallower than if you waited till after.

I know how much you love math, so here.

Here is the approach, I chose a random one just to explain: http://www.bollar.org/travel/essays/ifrwest00/images/hawthornplate.jpg

The step down is 1200' in 6 miles and from the FAF to MDA is 3.7 miles and 1100 feet. (Is DME 2.2 the missed?) Let us make the math easy and say we are going 120 knots so 2 miles per minute.

Start by figuring out the descent rate necessary from the step down, 6 miles / 2 miles in a minute = 3 minutes to loose 1200'. Now take 1200 feet / 3 minutes = 400 FPM.

Next we have 3.7 miles / 2 miles in a minute = 1.85 minutes to loose 1100'. Now take 1100 feet / 1.85 minutes = 595 FPM.

As you can clearly see, the descent rates cannot be held constant if you start your descent at the beginning of the first step down. A descent from the start would require 400FPM then an increase to about 600FPM. Instead if you waited till 4 miles left from the step down then you would have 1200 feet to loose in 2 minutes which is 600 FPM. That way you can keep a close to constant descent rate throughout the entire descent.

Make sense?

 

[FlyChicaga]

Just like calcapt, our company has us doing non-precision approaches as constant-rate descents, rather than dive and drive. We have to add 75 feet to the MDA, however. This is due to the fact the airplane will "dip" below MDA up to 75 feet during a go-around while on autopilot. We also always use a VDP. If we cannot see the runway at the VDP, then we will go-around.

If the weather is near minimums, we'll still use the dive-and-drive method, just so we can get that extra 75 feet all the way to MDA. It's almost counter intuitive, because we're descending quickly in the soup. However, the purpose of the continuous descent approach is to help us be stabilized for landing, not because of obstructions.

I wish I could post instrument approach charts. I could show examples of charts that actually have a continuous descent angle drawn on them. Many of them have you stay at the initial altitude longer than normal... For instance, you'd normally start stepping down at 11.2 DME, but now you'll start a 3.0 degree descent at 7.0 DME.

 

[minitour]

That's why there's an MDA, so you don't have to worry bout it.

Assuming you're on course...

-mini

 

[minitour]

...the purpose of the continuous descent approach is to help us be stabilized for landing, not because of obstructions.

Another good point there. Depending on your company profiles, you may not be able to conduct dive and drive approaches. At the last place, that's all we did...and I hated it. Then again, most companies teach their pilots that stabilized means constant speed, constant rate, fully configured from 10 miles out...which is just not true.

-mini

 

[poser765]

I know how much you love math, so here.

Here is the approach, I chose a random one just to explain: http://www.bollar.org/travel/essays/ifrwest00/images/hawthornplate.jpg

The step down is 1200' in 6 miles and from the FAF to MDA is 3.7 miles and 1100 feet. (Is DME 2.2 the missed?) Let us make the math easy and say we are going 120 knots so 2 miles per minute.

Start by figuring out the descent rate necessary from the step down, 6 miles / 2 miles in a minute = 3 minutes to loose 1200'. Now take 1200 feet / 3 minutes = 400 FPM.

Next we have 3.7 miles / 2 miles in a minute = 1.85 minutes to loose 1100'. Now take 1100 feet / 1.85 minutes = 595 FPM.

As you can clearly see, the descent rates cannot be held constant if you start your descent at the beginning of the first step down. A descent from the start would require 400FPM then an increase to about 600FPM. Instead if you waited till 4 miles left from the step down then you would have 1200 feet to loose in 2 minutes which is 600 FPM. That way you can keep a close to constant descent rate throughout the entire descent.

Make sense?

yeah makes sense...not sure why I didn't consider that scenario when I asked my question. It does bring up a better question though...why do all that math...what's wrong with dive and drive?

 

[Orange Anchor]

The argument of D&D versus CROD is an old one. A study done a number of years ago by a fellow named Ratan Khatwa found that the CRODs had a higher rate of success in seeing the field and landing than D&Ds. Also, IF one is strict with the definition of a stable approach, driving at MDA with gear and flaps and at 400-500ft and then yanking off wads of power, changing flaps and retrimming.. that is not a stable approach. Not to say it can't be done. But there is a large gulf between what CAN be done and what SHOULD be done, statistics aside.

At my old house ALL non-precisions were done on autopilot and 2) all MDAs were rounded up to the next 100ft increment. It works quite well and avoids dishing out at MDA. The first thing to go was circling approaches and the next thing was NDBs. We saw nothing in the way of having to divert from losing those approaches.

So, it is not really a matter of one being good and one being bad but which has the higher success rate in airline ops. Gen Av.. maybe a different story with 80-90 approach speeds.

 

[poser765]

Yeah I guess it is all operation specific. If I am tooling around in a 172 i'm going to dive and drive. Much easier FOR ME. Now if you are in a transport category airplane with vnav a constant rate I would assume would be easier.

to me, the mental workload in GA environment is just wasting mental function at a time when you are already mentaly taxed. but that is just my OPINION.

 

[minitour]

to me, the mental workload in GA environment is just wasting mental function at a time when you are already mentaly taxed. but that is just my OPINION.

With all due respect, if you can't come up with (real quick in your head) speed (miles per minute), minutes from FAF to MAP (or PDP/VDP) and altitude to lose, and solve for X (rate of descent), you probably shouldn't be flying the instrument approach.

All of my instrument students are capable of doing it prior to checkride time. IMO, students should be required to show proficiency in constant rate NPAs at least one time on a checkride.

-mini

 

[shdw]

yeah makes sense...not sure why I didn't consider that scenario when I asked my question. It does bring up a better question though...why do all that math...what's wrong with dive and drive?

I personally always did the dive and drive method and as I am sure you have noticed I am a big pusher for mathematical understanding. The only reason I see for the math here is to be able to set up an autopilot descent.

On another note, on any LOC approach I have flown after the FAF I got to DA asap and flew till my time ran out or DME (with the GPS) if it was part of the approach. I don't know why you anyone would aim to get to DA at the expired time, I prefer to have 10-15 seconds or more to look for my airport.

Note: I do not teach this yet so my opinions here will likely change when I decide to get my II.

 

[shdw]

With all due respect, if you can't come up with (real quick in your head) speed (miles per minute), minutes from FAF to MAP (or PDP/VDP) and altitude to lose, and solve for X (rate of descent), you probably shouldn't be flying the instrument approach.

I bet you teach them to brief the approach before hand and not figure it out as they pass the fix though. I think poser forgot that key step, do it early, not while flying the approach.

One last thing on the D&D, in a 172 or mostly any small piston aircraft, you won't have configuration changes for the driving portion. *Insert flame shield* If you use power for altitude then a simple increase in throttle will yield level flight and then decrease back to where it was to continue descent when the field is in sight. There is no need to do all that re-trimming for 30 seconds you can apply a small amount of forward pressure, remember centerline thrust results in more authority on the elevator and will tend to over pitch if not held under control.

The argument of D&D versus CROD is an old one. A study done a number of years ago by a fellow named Ratan Khatwa found that the CRODs had a higher rate of success in seeing the field and landing than D&Ds.

That is really interesting, do you have the study. I have googled for it and discovered this, which seems interesting but I am unsure if it is what you are referring to.

http://www.flightsafety.org/alar/alar_bn7-2-nonprecision.pdf

 

[deadstick]

If you are shooting the approach in IMC, the whole idea is to get as low as possible, as soon as possible. You want to clear the clouds and get a visual on the runway to ensure a landing.

FWIW, the FAA doesn't like the "dive/drop and drive" method for 121 and 135. Ref: SAFO 09011

 

[MikeD]

Yeah I guess it is all operation specific. If I am tooling around in a 172 i'm going to dive and drive. Much easier FOR ME. Now if you are in a transport category airplane with vnav a constant rate I would assume would be easier.

to me, the mental workload in GA environment is just wasting mental function at a time when you are already mentaly taxed. but that is just my OPINION.

I do dive and drive and constant descent for light GA, cargo, fighter jet....you name it. Just to clarify, so people don't take it literally, "dive and drive" to me means at each stepdown, assuming about a 1000-1500 fpm descent to level out and stabilize at some point prior to the VDP, so I can begin looking for the runway and not have to worry about the plane descending or trying to descend. "Dive and drive" isn't some crazy maneuver that it literally sounds like.

Constant descent is what I do going to the FAF, then so-called "dive and drive" from then on. And even with constant descent, I'm not trying to make the altitude numbers at the particular fixes since they're usually at or above (unless they're hard alts), so close enough is fine.

Again, the two methods are techniques. Both acceptable techniques with their respective limitations and/or merits. Arguing which is better is again, like arguing whether coke or pepsi is better.