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frankbell View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote frankbell Quote  Post ReplyReply Direct Link To This Post Topic: localizer DME
    Posted: 17 Mar 2016 at 9:59pm
Apparently there is no DME available when tuned to a localizer. This is not a problem, except for the occasional localizer approach where there's an unnamed fix on the approach, identified only by a DME distance. Example is the LOC RWY approach to KFQD, where the final stepdown is only identified by DME (and the MAP is also identified by DME).

Am I missing something? Is there a way to get DME on a localizer? Is this a possibility in the future?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rolfe_tessem Quote  Post ReplyReply Direct Link To This Post Posted: 18 Mar 2016 at 9:53am
My understanding has always been that when the box is in VLOC mode, it can't act as a DME. Since it has to be in VLOC from FAF to MAP, for that portion, DME is unavailable. I think is more of a regulatory issue than a technology one...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brou0040 Quote  Post ReplyReply Direct Link To This Post Posted: 18 Mar 2016 at 11:23am
The GPS does not stop functioning because you put it in VLOC mode.  The GPS is a suitable alternate for DME for all phases of flight.  There is no reason you can't be using VLOC guidance while still being supplied with DME type information from the GPS simultaneously.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AviSimpson Quote  Post ReplyReply Direct Link To This Post Posted: 18 Mar 2016 at 11:57am

The DME location is in the database.  I included the IFD screenshot and the approach plate below.

Are you looking for a DME display on an external control head?  There are plenty of options.

- Assuming your on the localizer, you could look for crossing the 095 radial from SUG using a second nav radio.

- Look at the flight plan for distance to DASCA when that leg is active.

- Use the active waypoint datablock when DASCA leg is active




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Post Options Post Options   Thanks (0) Thanks(0)   Quote frankbell Quote  Post ReplyReply Direct Link To This Post Posted: 18 Mar 2016 at 1:13pm
In this case, finding DASCA isn’t a problem… it shows up like any other waypoint on an approach.

Finding the unnamed 2nm DME fix is the problem… it doesn’t show up as a waypoint, there’s no crossing radial listed for it, no separate leg for it. Distance to the runway would be different from distance to the localizer transmitter. True, you could probably use the “1.1” miles from said fix to the runway, though that probably wouldn't pass muster if you had to defend doing it.

Maybe I’m reading this approach incorrectly, but I believe that you have to cross DASCA at 1600’, and the minimums at the 2 nm fix would be 1440 (1440 is noted as “DASCA minimums”, but I don’t think you can actually go that low at DASCA…??). The line sloping down from DASCA to the 2 nm fix would imply continuing descent after DASCA, based on DME.

This one is near me, but I assume there’d be other approaches like it.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote DavidBunin Quote  Post ReplyReply Direct Link To This Post Posted: 18 Mar 2016 at 3:13pm
The V at 2 miles is not a waypoint and it is not a missed approach point either.  It is just the visual descent point, the place where a normal visual descent would take you to the runway touchdown zone.

You are safe (from an obstacle standpoint) to dive-bomb down to 1440 the moment you pass DASCA.  There is no more descending allowed on instruments after 2.0 miles than there is prior to 2.0 miles.

At least that's how I read it.

David Bunin
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PA20Pacer Quote  Post ReplyReply Direct Link To This Post Posted: 18 Mar 2016 at 3:16pm
Hello Frankbell-

The 2 nm DME fix is a VDP (Visual Descent Point). It has no significance with respect to the MDA. After DASCA, you can descend to the DASCA MDA, 1440 in the case of a straight-in. If you do wish to identify the VDP, you can do so via the distance to the charted MAP (1.1 nm), as you have suggested. I would be happy to defend this at the hearing, but I cannot think of any reason why a hearing would come up over this issue.

Regards,

Bob
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Post Options Post Options   Thanks (0) Thanks(0)   Quote pburger Quote  Post ReplyReply Direct Link To This Post Posted: 18 Mar 2016 at 4:05pm
[EDIT - I started typing this prior to the posts from David and Bob.  I got sidetracked and came back and posted it.  If I'd seen their posts, I wouldn't have bothered posting.  But it is nice to see that we're all thinking the same.]

I'm no expert, but I think I'm right about the following:

This is a non-precision approach.  Once past DASCA you can DIVE and DRIVE to 1440 immediately. The sloped line is the visual descent angle which is suggested at 3°.  The 2 nm DME fix would be where the 3° vertical descent angle would bring you to 1440, at which point you would level off.  But you could already be at 1440 if you wanted to.  You don't need that 2 DME  at all.  You just need to make sure and cross DASCA at 1600 and then stay at or above 1440 until the MAP.  You can certainly use the 1.1 nm from the MAP to identify that unnamed fix, but since you don't need that fix anyway, what's the point?

Edited by pburger - 18 Mar 2016 at 4:07pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rolfe_tessem Quote  Post ReplyReply Direct Link To This Post Posted: 18 Mar 2016 at 4:14pm
If it is just a DME fix that is not in the database, I believe that paragraph 9 (b) 2 would apply.

http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_90-108_Chg_1.pdf

That doesn't seem to be the case with the approach depicted above.

Rolfe
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brou0040 Quote  Post ReplyReply Direct Link To This Post Posted: 18 Mar 2016 at 9:04pm
Frankbell,

You can see a similar discussion here.

There is some bad information regarding GPS usage, but the bottom line is that I requested a feature that would provide radial and DME to a user selected waypoint, which does not need to be part of the flight plan.  I think this feature would be helpful in many situations including yours.  You could select the localizer as your waypoint and it would be totally legal to use that GPS information as DME for any purpose in any phase of your approach.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rolfe_tessem Quote  Post ReplyReply Direct Link To This Post Posted: 18 Mar 2016 at 9:51pm
Originally posted by brou0040 brou0040 wrote:

Frankbell,

You can see a similar discussion here.

There is some bad information regarding GPS usage, but the bottom line is that I requested a feature that would provide radial and DME to a user selected waypoint, which does not need to be part of the flight plan.  I think this feature would be helpful in many situations including yours.  You could select the localizer as your waypoint and it would be totally legal to use that GPS information as DME for any purpose in any phase of your approach.

AC_90-108 would seem to specifically preclude this strategy. 

As I read it, and it is incredibly verbose, it seems to say that either the box is a VHF receiver or it is a GPS, but it can't be both at the same time. GPS can substitute for DME under all circumstances, but if it is busy being a VHF receiver, it is not supposed to also be a GPS. Obviously, from a technological standpoint it can, but this is a regulatory issue. If you have a second GPS box or a second VOR receiver that can be setup to identify a cross radial, this is a non-issue. 

From an operational standpoint, I can't imagine anyone getting violated for ignoring this "problem". I would be happy to find that I'm wrong, but that is my reading of the circular.

Rolfe

Edit: To be clear, I'm talking about DME fixes that are not in the database as part of the approach.


Edited by rolfe_tessem - 18 Mar 2016 at 10:08pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PA20Pacer Quote  Post ReplyReply Direct Link To This Post Posted: 19 Mar 2016 at 9:04am
Originally posted by rolfe_tessem rolfe_tessem wrote:

...

AC_90-108 would seem to specifically preclude this strategy. 

As I read it, and it is incredibly verbose, it seems to say that either the box is a VHF receiver or it is a GPS, but it can't be both at the same time. GPS can substitute for DME under all circumstances, but if it is busy being a VHF receiver, it is not supposed to also be a GPS. Obviously, from a technological standpoint it can, but this is a regulatory issue.
...

Rolfe

Edit: To be clear, I'm talking about DME fixes that are not in the database as part of the approach.

Hi Rolfe-

Could you reference the section of AC 90-108 that leads to your conclusion above? I can see that section 9.d.(1) may require the installation of both GPS and VHF equipment for part 91K, 121, 125, 129, and 135 operators, but I am not sure if that is what you are saying, and it does not apply to part 91 in any case.

I agree with your comment on the verbosity of the AC, and I may have missed something.

Regards,

Bob
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rolfe_tessem Quote  Post ReplyReply Direct Link To This Post Posted: 19 Mar 2016 at 9:23am
Bob,

So what we're talking about are DME fixes NOT in the database, such as those on a LOC approach where there is a VOR off to one side and stepdown fixes are established by various radials. Typically, these are also defined by DME distances on the LOC. These kinds of DME fixes are not usually in the database unless they happen to serve some purpose on another approach at the same airport.

Here are the relevant sections of the AC, paragraph 9:

(2) Pilots must extract waypoints, NAVAIDs, and fixes by name from the onboard navigation database and comply with the charted procedure or route. Heading-based legs associated with procedures may be flown using manual technique (based on indicated magnetic heading) or, if available, extracted from the aircraft database and flown using RNAV system guidance. c. Operating Requirements. (1) For the purposes described in this AC, pilots may not manually enter published procedure or route waypoints via latitude/longitude, place/bearing, or place/bearing/distance into the aircraft system.

This is also covered in the Instrument Flying Handbook in the section about GPS/DME substitution.

Whoever wrote that AC should get the obfuscation award of the year...


Rolfe 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AzAv8r Quote  Post ReplyReply Direct Link To This Post Posted: 19 Mar 2016 at 9:52am
I see no conflict in AC 90-108.

Those unnamed "fixes" aren't really fixes.  They are the "Visual Descent Point" and the "Missed Approach Point", note the absence of "Fix", unlike IAF, Intermediate Fix, and FAF.   The DME values on the plate are advisory or supplemental in nature - if you have DME, you get to substitute them for Time from the FAF.    I think the procedure is pretty clear in that regard.  The last FIX is DASCA, and it is optional, with additional constraints on equippage if it is to be used.

Of course, I got my instrument rating before GPS was common, and the planes I rented rarely had DME.  If they did, my instructor usually shut it off so I'd learn to use my timer.

Now, if I had a distance to I-QFD or distance from DASCA data block (or from JAKIB, since technically the MAP is defined relative to JAKIB), I'd probably use it to double-check my timer...
But if the MAP is in the database, as part of the procedure, then why would it not be legal?

If you simply want to identify the VDP (for your knowledge, since passing it without seeing the runway environment pretty much ensures you have to do the missed since you are below circling minimums) use the distance to the MAP which will show on flight plan and in the next waypoint datablock.  You're not breaking any rules - it's supplemental information, and not part of the approach.  You just have enhanced situational awareness.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote rolfe_tessem Quote  Post ReplyReply Direct Link To This Post Posted: 19 Mar 2016 at 10:54am
Just to be clear, I wasn't talking about the approach above. I was talking about the more general case with regard to unnamed DME fixes.

Rolfe

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Post Options Post Options   Thanks (0) Thanks(0)   Quote BobsV35B Quote  Post ReplyReply Direct Link To This Post Posted: 19 Mar 2016 at 12:44pm
Good Morning Rolfe,

I believe this whole argument is due to some folks reading into the FARs something that is not meant to be.  

You can use the GPS in lieu of DME any time the distance from the DME transceiver location associated with the localizer in question is noted on the chart. Those DME transceivers are often located near the localizer transmitter, but not always! 

In Europe and at a few USA airports (KMSP for one), the localizer associated DME transceivers are located near the touch down zone.  A runway that has a localizer to each end of the runway will almost always use the same DME transceiver for both approaches and the mileage from the actual location will be shown on the chart even though the identifier will change. (Note: KRFD, Rwy 1/19)

I was involved in the writing of the early "use of GPS in lieu of DME" back in the mid to late nineties, and the language did get a bit complicated. The language has been improved, but the intent is still the same, though often confusing. Definitely partly my fault!!

Happy Skies,

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Post Options Post Options   Thanks (0) Thanks(0)   Quote rolfe_tessem Quote  Post ReplyReply Direct Link To This Post Posted: 19 Mar 2016 at 1:14pm
Originally posted by BobsV35B BobsV35B wrote:

Good Morning Rolfe,

I believe this whole argument is due to some folks reading into the FARs something that is not meant to be.  



So it's your fault!

I won't belabor this much further as this is an Avidyne specific forum, but the Instrument Flying Handbook seems to make a clear distinction between named and unnamed DME fixes.

---

To Determine Aircraft Position Over a DME Fix: 
1. Verify aircraft GPS system integrity monitoring is functioning properly and indicates satisfactory integrity. 
2. If the fix is identified by a five-letter name that is contained in the GPS airborne database, select either the named fix as the active GPS WP or the facility establishing the DME fix as the active GPS WP. When using a facility as the active WP, the only acceptable facility is the DME facility that is charted as the one used to establish the DME fix. If this facility is not in the airborne database, it is not authorized for use. 
3. If the fix is identified by a five-letter name that is not contained in the GPS airborne database, or if the fix is not named, select the facility establishing the DME fix or another named DME fix as the active GPS WP

---

Is it not the case that when on an ILS or LOC approach, after the FAF the active waypoint is the airport? If so, how can the above criteria possibly be met?

Rolfe
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brou0040 Quote  Post ReplyReply Direct Link To This Post Posted: 19 Mar 2016 at 8:59pm
Originally posted by rolfe_tessem rolfe_tessem wrote:

Is it not the case that when on an ILS or LOC approach, after the FAF the active waypoint is the airport? If so, how can the above criteria possibly be met?

Rolfe

When you are flying a LOC or ILS approach (or VOR), there is no need for any active waypoints at all!  In practice, you will have the airport in the flight plan and the approach loaded with it's associated fixes.  However, there is no reason why you can't just dial in the NAV frequencies and fly with no GPS fixes.  The fact that the IFDs contain GPS, it does not take away their NAV/COM capability.

In my PMD discussion, it would have been legal for to not have an approach loaded, dialed in the LOC NAV frequency with the IFD in VLOC mode, and hit Direct PMD as my only waypoint in order to identify SKKEE as my stepdown.  This just isn't the way these units are meant to be used, but legally there is no issue.

If you were flying in an area of GPS jamming testing, above is exactly how you'd be flying (except you'd lose the DME capability).  The IFD can still be used as an information database in addition to NAV/COM, but you'll lose all of the GPS and nearest capabilities.


Edited by brou0040 - 19 Mar 2016 at 9:03pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DavidBunin Quote  Post ReplyReply Direct Link To This Post Posted: 20 Mar 2016 at 1:17pm
Originally posted by AzAv8r AzAv8r wrote:

passing [the VDP] without seeing the runway environment pretty much ensures you have to do the missed since you are below circling minimums


Maybe in a jet, but most GA airplanes are capable of landing in less than 1500' while most runways with an instrument approach are significantly longer than that.

Flying past the VDP (in my 172) wouldn't give me any heartburn if I had 6000' (or more) of runway in front of me.  I would give it very little thought at a 4000' runway, but I would know that for every moment that I fly past the VDP, my chances of making a landing out of the approach diminish.

David Bunin

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Post Options Post Options   Thanks (0) Thanks(0)   Quote BobsV35B Quote  Post ReplyReply Direct Link To This Post Posted: 20 Mar 2016 at 1:52pm
Excellent point David.  

We part 91 folks are better off getting down to the MDA early and getting stabilized long before the VDP. Once we pass the VDP we can be sure we will need a higher rate of descent than three degrees, but any light airplane from a J3 Cub to a Beech King Air can comfortably execute a six degree glide path. On top of that, we are NOT required to touch down in the "landing zone". Every bit of the available runway is there for our use. Any time I can touch down (at the proper speed <G>) with at least two thousand feet in which to stop, it is a perfectly safe and legal operation for my old Bonanza.

Sorry for turning this into an approach seminar. but there have been a lot of untrue statements on the forum recently.

Happy Skies,

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Post Options Post Options   Thanks (0) Thanks(0)   Quote ronl Quote  Post ReplyReply Direct Link To This Post Posted: 20 Mar 2016 at 9:16pm
Everyone here probably already knows this but just in case... When I see a VDP on an approach plate it means that I can not descend below that altitude before I reach that point even if I have the runway in sight. The usual reason for these are difficult to see obstructions like high line wires near the end of the runway. It's obviously important to be able to identify that point accurately but it has nothing to do with touchdown zone, descent angle, length of runway, category of aircraft, etc.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BobsV35B Quote  Post ReplyReply Direct Link To This Post Posted: 20 Mar 2016 at 10:53pm
I am afraid I may be sorry I posted this BUT!

The following is the U.S. FAA's official definition of VDP: "A defined point on the final approach course of a non-precision straight-in approach procedure from which normal descent from the MDA to the runway touchdown point may be commenced, provided the approach threshold of that runway, or approach lights, or other markings identifiable with the approach end of that runway are clearly visible to the pilot."
 
The view of at least one of the required visual cues is still required before descending below the MDA. If we reach the MAP before acquiring  a required visual cue, we must execute the miss.  Three degrees is normally used to determine the maximum distance from the Rwy threshold, but we are legal to descend at a steeper angle (closer to the runway) if it suits our operation as long as we have acquired the required visual cues. 

Don't place more emphasis on the VDP than the FEDs intended!

Old Bob 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ronl Quote  Post ReplyReply Direct Link To This Post Posted: 21 Mar 2016 at 12:37am
Bob,

"from which normal descent from the MDA to the runway touchdown point may be commenced"

The definition you posted clearly states that we are not authorized to descend below the MDA until we have reached the VDP. It's a limitation that has been placed on the approach for a reason. 

When flying a non-precision approach that does not have a VDP, we are authorized to descend below MDA as soon as the runway is in sight.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oskrypuch Quote  Post ReplyReply Direct Link To This Post Posted: 21 Mar 2016 at 7:25am
Originally posted by ronl ronl wrote:

Bob,

"from which normal descent from the MDA to the runway touchdown point may be commenced"

The definition you posted clearly states that we are not authorized to descend below the MDA until we have reached the VDP.

Ron,   

No, there is no such restriction. The definition says MAY not SHALL/MUST. These are carefully chosen regulatory words.

A charted VDP is an advisory only construct meant to assist the timing of the final approach, and when published also advises that continuing a CDAP below the MDA is likely to encounter no obstructions. A VDP marking on a plate can be entirely ignored otherwise, and is not regulatory.

If a charted VDP is missing, either the approach has not yet been updated, or more likely there are significant obstructions below the MDA, and a VDP could not be charted.

In all cases, regardless of a published VDP, once you meet the visibility requirements of the runway environment, and the mechanics of a safe final approach, then you may descend below the MDA.

Pilot's have been creating their own VDPs to assist in the final approach, it is just a bit of simple math. Published VDPs have been added by the FEDs to make this easier and these charted advisory markings also provide advice that the final CDAP is likely clear of obstructions.

* Orest



Edited by oskrypuch - 21 Mar 2016 at 7:43am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ronl Quote  Post ReplyReply Direct Link To This Post Posted: 21 Mar 2016 at 8:21am
Ok. I stand corrected. A VDP is not a required limitation. Sorry about the misinformation but I think I'll continue to treat them as important:

"Every year when the 
annual Nall report is released landing phase are among the highest accidents recorded.  The National Transportation Safety Board (NTSB) has recommended updating visual descent points or requiring Part 121 and 135 operators to use a VDP five times over the past 30 years.  One of the most famous accidents happened on September 8th, 1989 when a USAir flight collided with and severed four transmission cables.  The flight performed a missed approach and landed uneventfully right after that.  The crew did not understand the importance of a VDP, even though they had referred to the VDP several times in the approach brief."
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oskrypuch Quote  Post ReplyReply Direct Link To This Post Posted: 21 Mar 2016 at 3:43pm
Indeed, CDAP and the VDP itself, whether published or calculated, are important facets to flight operation and approach management.

* Orest

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Post Options Post Options   Thanks (0) Thanks(0)   Quote BobsV35B Quote  Post ReplyReply Direct Link To This Post Posted: 22 Mar 2016 at 9:59am
Forgive me Orest. The term CDAP is unfamiliar to me.  I assume it has to do with the Non Precision Approach.  Some sort of Constant Angle Non Precision Approach maybe? Do not recall seeing that reference in the US FARs. Would appreciate being informed <G>

Happy Skies,

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Post Options Post Options   Thanks (0) Thanks(0)   Quote chflyer Quote  Post ReplyReply Direct Link To This Post Posted: 22 Mar 2016 at 10:25am
Bob,

This is perhaps a bit off track, but there an approach profile called CDFA (Constant Descent Final Approach) which I believe has recently become mandatory during flight tests in Canada unless not practical, for example a need to rapidly descend due to ice or to circle.

There was an article on this written by the COPA CEO in 2014. Here is a link to it:

Perhaps Orest meant something different.




Edited by chflyer - 22 Mar 2016 at 10:26am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oskrypuch Quote  Post ReplyReply Direct Link To This Post Posted: 22 Mar 2016 at 1:25pm
CDAP = Constant Descent Approach Profile, an older term. CDFA (constant or continuous ...) is the newer term, there are a few different acronyms out there for the same thing.

And yes, all refer to maintaining a continuous rate descent profile on the final approach segment on a NPA. Can be a critical maneuver in large, high inertia aircraft, but it is becoming more favored in small aircraft as well, especially on flight tests! It is typically what I will fly.

That said, in my view a properly managed dive & drive approach still has operational merits in the proper circumstances, in the correct aircraft, and should be in one's quiver.

* Orest



Edited by oskrypuch - 22 Mar 2016 at 1:35pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BobsV35B Quote  Post ReplyReply Direct Link To This Post Posted: 22 Mar 2016 at 2:33pm
Thank You Orest,

After reading the Canuck version, I see where you are coming from! <G>

This will be my attempt to stop the off subject messages I seem to be making, but I really think you folks up north are doing a great disservice to the flying public by requiring a non standard procedure to become the standard.

The Dive and Drive definitely takes more training to execute for the various conditions under which it is best. Seem's to me that we should train and check for the rarely used stuff rather than the newer, simpler procedure.  

The real safety in a non precision approach is the level off at the MDA. The CANPA leads less experienced folks into descending before all of the required visual cues are available. that big open spot up ahead could be the Walmart parking lot rather than the runway. Why not stay up at that very safe MDA until we are sure of what we are seeing? If we spot the airport when we are too close for a safe approach and landing, we can very safely take the miss and come back for another shot with much better knowledge as to what conditions exist. I guess that is why they call me Old Bob!

By the way, I shot my first actual non precision approach off a low frequency four course radio range in 1950 and have usefully employed it in the old rope start Boeing 747.  Everything ancient is not necessarily bad.  

As Always, It All Depends.

Happy Skies,

Old Bob

PS Love my 540
Old Bob, Ancient Aviator
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oskrypuch Quote  Post ReplyReply Direct Link To This Post Posted: 22 Mar 2016 at 10:39pm
Quote By the way, I shot my first actual non precision approach off a low frequency four course radio range in 1950 and have usefully employed it in the old rope start Boeing 747.  

Now, that is cool!

* Orest

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