Max descent gradient???

[flyboydk]

I was told by a DPE that there is a maximum descent gradient on non-precision approaches. He said it was 500 ft/nm before the FAF and if I remember right 300 ft/nm after the FAF. This came up after I told him I was going to descend at 1000 fpm to our MDA and he asked me if this would be greater than our max descent gradient. Where would I find this at? I didn't see anything in the AIM or FARs. I didn't think to ask him more about it because I was too happy that I passed my checkride to question him in depth.

 

[Timebuilder]

I don't have a conversion table for ft/min to ft/nm handy, but I was taught to use 750 fpm for non-precision approaches to get down to the MDA.

 

[Flying Illini]

First off, congrats on passing your checkride!
Second, I have never heard about a max descent gradient before. If i find anything in the books tonight, I will post it. Anybody else have any ideas?

 

[enigma]

I was told by a DPE that there is a maximum descent gradient on non-precision approaches. He said it was 500 ft/nm before the FAF and if I remember right 300 ft/nm after the FAF. This came up after I told him I was going to descend at 1000 fpm to our MDA and he asked me if this would be greater than our max descent gradient. Where would I find this at? I didn't see anything in the AIM or FARs. I didn't think to ask him more about it because I was too happy that I passed my checkride to question him in depth.

I'd guess that it is in terps. The last time I looked, Wally Roberts' terps website was out of business, so I have no quick way of checking, but those gradients seem about right. He was just being an examiner and trying to find something that you didn't know.

regards,
enigma

 

[A Squared]

First off let's recall, the TERPS are instructions for people who *design* approaches. The TERPS are not instructions for people who *fly* approaches.

It is true that familarity with the TERPS can give us as pilots much useful insight, but it is not true that statements can be lifted verbatim from the TERPS and used as rules for flying.

This appears to be a case of exactly that. I believe that the examiner is completely wrong on this issue. I'll revise that if someone can show be a regulatory reference other than the TERPS which supports his position.

The maximum descent gradients which the TERPS specifies for *designing* approaches are: 500 ft/nm for the initial segment, 300 ft/nm for the intermediate segment, and 400 ft/nm for the final approach segment.

Now what if we used these as minimum descent gradients for flying an approach? Suppose we had an approach which had maximum descent gradients in all segments. (it could happen, I know of approachs which have segments very close to the TERPS maximums) Now suppose that we arrived at the initial approach fix at 1000 ft above the design altitude (perhaps because of an off airway routing which kept us above the MOCA) So, being a little behind the curve in our descent, but abiding by Mr examiner's rules, we fly the entire approach at exactly those maximum descent gradients. we're going to arrive at the missed approach point exactly 1000 feet above the MDA. It's gonna be tough to make the runway from that altitude over the threshold.

 

[mar]

Similar concepts but not the same

Well I guess I have a little to add as well.

Congrats on the instrument ticket.

Your vertical speed is an aid in helping you meet descent gradients but it's going to vary with your groundspeed.

Look at like this. If a segment has a gradient of, say, 250'/nm that will never change. So let's say in calm conditions you need 750 fpm to meet the requirement.

If you were shooting the same segment at the same IAS with a 30 knot tailwind you might need to come down 1000 or 1200 fpm to make the same gradient.

On the other hand, a 30 knot head wind might only require 300 to 500 fpm.

These are all just real generic figures but I hope you get the point. The same theory applies on departure procedures. If you subscribe to Jepps (and I think NOS supplies it now too) in one of the front tabs there is a handy chart that will convert gradients to vertical speed.

But as it's been stated, for your stage of the game, there is no restriction on vertical speed when shooting an approach.

When you fly transport category airplanes under 121 you'll have to meet stablized approach criteria. Usually if you're seeing descents in excess of 1000 fpm inside of the FAF then you're required to go around.

 

[Singlecoil]

Re: Similar concepts but not the same

The same theory applies on departure procedures. If you subscribe to Jepps (and I think NOS supplies it now too) in one of the front tabs there is a handy chart that will convert gradients to vertical speed.

...and if you actually look at the climb gradient on the departure procedure.....(drum roll please)....you obviously fly a DC-6.

 

[hydroflyer]

Is there any chance the examiner was confused and quoting the criteria for a straight-in non-precision approach as opposed to a -A (such as a VOR-A)? 300 ft/nm sure seems to match the standard glide path angle.

 

[AAsRedHeadedbro]

flyboydk,

I will only add to the comments already made by stating in clear English, when flying an approach, there is NO MAXIMUM DESCENT GRADIANT that must be observed between step-down fixes (as pertains to basic FAR's; airline OPS SPECS limit descent rates during approaches for safety's sake). Once a pilot passes a step-down fix, he is legally allowed to be at the next minimum altitude for the following segment.

As for your examiner, he seems to have mistaken the requirements for BUILDING an approach vs. FLYING an approach. The TERPS limit the maximum descent gradient between fixes of an approach so that the pilots flying the approach won't be required to descend at dangerously high descent rates in order to simply complete the approach. The descent gradient limitations basically prevent the designer from building an insane approach.

 

[avbug]

Also, though it's been aluded to but not actually said, if vectored to the IAF at a considerably higher altitude than the initial fix, your descent rate may be considerably higher than the TERPs approach-building criteria. In mountainous terrain in a non-radar environment, it's not uncommon to get vectored to an IAF at 13,000' or higher, when the published altitude over the IAF is say, 7,500'. Passing that fix, you then descend to 7,500', and then down to say 5,500' on the inbound leg.

Once down to the 7,500' initial altitude, you can assume a normal descent profile...but prior to that you may need to hustle down to get properly established. Unless you're prepared and slowed down before that IAF, then you've got additional speed to contend with, necessitating an even higher descent rate to meet the published altitudes for any given approach segment.

 

[wickedpilot]

Congrats on your instrument!

I've never seen (in print) any minimum decent gradients on non-precision appchs. I teach 750-1000FPM depending on tailwinds.

I'm surprised no one has chimed in with a story about "slam dunk" approaches. Do you 135/121 guys ever get dunked on non-precisions?

 

[avbug]

If you read carefully, you'll find I just described one.

 

[labbats]

I just read in Rod Machado's Instrument book that there is indeed a maximum descent rate. Where he got the information, I don't know. But let's realize that 500 feet per mile is nowhere near 500 feet per minute.
Even with those minimums, you could fly at well over your pre-planned 1000 feet per minute and be well clear of it, even at an approach speed of 90 KTS.

 

[wickedpilot]

If you read carefully, you'll find I just described one.

Pardon my lack of reading each post, never thought of a slam dunk occurring that far out on the approach

 

[A Squared]

I just read in Rod Machado's Instrument book that there is indeed a maximum descent rate. Where he got the information, I don't know. But let's realize that 500 feet per mile is nowhere near 500 feet per minute.

Unless Rod has some official reference, I wouldn't put too much stock in an unsupported statement like that.

The discussion has been about descent *gradients*, you mention a maximum descent *rate*, which is Rod referring to? Either way, I think he's on pretty shakey ground claiming there is a legal maximum descent rate or gradient.

Even with those minimums, you could fly at well over your pre-planned 1000 feet per minute and be well clear of it, even at an approach speed of 90 KTS. [/B]

I'm not quite sure what you're saying here, but if you're suggesting that a 1000 fpm descent at 90 knots is less than a 500 ft/nm gradient that is incorrect. At 90 knots, 1000 fpm gives you 667 ft/nm

 

[80/20]

I agree a with A Squared, avbug and AAsRedHeadedbro don’t confuse procedure design with how to fly an approach.

It is easy to get confused when we discuss descent rate and gradient.
Descent rate (Ft/min) is controlled by your speed and descent profile management.
Descent gradient or angle (Ft/Nm) is only controlled by how you manage the descent profile and it is restricted by the minimum profile on the approach chart (Procedure design – TERPS):.


I can't remember seeing any specific checkride requirement for max feet/min standards of acceptable performance during approaches. Various FAA orders and bulletins discuss stable speed, descent rate, vertical flightpaths, configuration and give examples like: "An ATP applicant would be expected to be able to maintain 180 knots, configure the aircraft, and establish a stabilized approach before descending below 1,000 feet above ground level (AGL)".

Back to the question - is there a specific limit for ft/nm? The TERPS have set limits but there are many approaches with steeper angles, which simply required special approvals.

How should a designee evaluate this? In my opinion the applicant should demonstrate advance planning and avoid starting too high or too fast. If you are fully established and configured on speed at minimum altitude over FAF and fly a constant angle down to VDP then you have done what you can and will have to accept whatever you VSI indicates. Finally and most important if you end up unstable below 1000 feet just go missed and try again.

 

[80/20]

Oh…. by the way I can’t recall many approaches that required more than 1000 fpm even with published angles of 4 degrees. The exception would be airports with straight in approaches to only one runway which had a strong tailwind. This is the most common cause for circling approaches on the line and they could involve a lot of tailwind which could give an extra high rate of descent.