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Distance from RW threshold in RNAV approach

Dimme wrote:

The AIP plate depicts that you can fly all the way down to 1200’ and stay there until reaching SP402 before going missed. However, the Jeppesen plate depicts that you should go missed as soon as you are down at 1200’, even before reaching SP402.

Please correct me if I’m wrong.

The Jepp plate shows a CDFA where you are supposed to go missed once at the MDA which you treat as a DA.

EGTK Oxford

There is no mention of DME. This is a GPS approach. One would need to look elsewhere to see if DME=0 happens at the runway threshold.

On a CDFA you can descend to the next SDF at an unlimited VS and your obstacle clearance is assured, so you can descent to 1200ft QNH immediately after DEXIS. You have to climb up (go around) at 2.8nm before the runway i.e. at SP402. That is probably why the G1000 doesn’t show any other distances.

Administrator
Shoreham EGKA, United Kingdom

G1000 always shows the distances only to defined points. If MAPt equals THR that you get distances to THR. Plates (Jepp and AIP) always show distances to THR. In my opinion this is just poorly designed and adds unnecessary workload to cockpit. Effectively this puts LNAV approach at the same level as NDB approach.

LDZA LDVA, Croatia

Peter wrote:

There is no mention of DME. This is a GPS approach. One would need to look elsewhere to see if DME=0 happens at the runway threshold.

But I think the question was regarding how do you make use of the check altitudes based on THR05. The simplest is to use DME.

Peter wrote:

On a CDFA you can descend to the next SDF at an unlimited VS and your obstacle clearance is assured, so you can descent to 1200ft QNH immediately after DEXIS.

Well you can, but that is not a CDFA, it is dive and drive. If you are doing it as a CDFA you need the check altitudes to know if you are on the profile.

Emir wrote:

G1000 always shows the distances only to defined points. If MAPt equals THR that you get distances to THR. Plates (Jepp and AIP) always show distances to THR. In my opinion this is just poorly designed and adds unnecessary workload to cockpit. Effectively this puts LNAV approach at the same level as NDB approach.

I think you can set the flightplan to show cumulative distances which would allow the cum dist to RW05 to appear. But I agree, it would be far more helpful on these approaches to show check altitudes as a distsnce from the MAPt.

Last Edited by JasonC at 06 Dec 09:16
EGTK Oxford

This site explains it great.

Regular non-precision approach:

CDFA:

^ESM(E|S)$

JasonC wrote:

But I think the question was regarding how do you make use of the check altitudes based on THR05. The simplest is to use DME.

No. The question was how to check altitudes during such LNAV approach like we do when we have ILS or LOC/DME or VOR/DME approaches. In particular environment DME is very close to THR (but not exactly) so using it would be helpful although not completely correct (and of course it wouldn’t be legal). But what would you do if DME is not located nowhere near THR like in case below (Zagreb LDZA RW23).


Even if I wanted to use DME in LNAV approach it would again require recalculating distances to get distance from THR. However, in this particular case RW23 is GPS point and Jepp plate info corresponds to G1000 distances.

So let’s be precise: If this is LNAV approach then only GPS related values can be used (or whatever is written on official plate); we can’t invent something else and add navigation aids of our choice. Should we just accept this and amend plates manually or ask Jeppesen to align info on plates and G1000 database to refer to the same points? IMO in case I described it would be the most beneficial to provide distance to MAPt (SP402) with corresponding altitudes to Jeppesen plates i.e. to provide same info as shown on G1000 (and I assume on other navigation boxes). Or we can ask CroControl (the issuer of official AIP plate) to provide additional GPS point marking RW THR?

Last Edited by Emir at 06 Dec 16:46
LDZA LDVA, Croatia

Emir wrote:

So let’s be precise: If this is LNAV approach then only GPS related values can be used (or whatever is written on official plate); we can’t invent something else and add navigation aids of our choice.

No need for that aggressive tone. If we want to be precise then the check altitudes are only advisory and so long as you are flying using a GPS approach loaded from a database you can use anything you like to aid your flight. I was only trying to help you by providing some ideas. You seem to be on top of it so why bother asking??

EGTK Oxford

No need for that aggressive tone.

Precise is not same as aggressive, Jason. I see nothing aggressive about it.

Administrator
Shoreham EGKA, United Kingdom

JasonC wrote:

I think you can set the flightplan to show cumulative distances which would allow the cum dist to RW05 to appear.

Yes you can (and that’s what I do) but there’s no RW05 THR point in G1000 database. The last point in approach is SP402 followed by HUM (missed approach holding fix).

Last Edited by Emir at 06 Dec 16:58
LDZA LDVA, Croatia

Emir wrote:

So let’s be precise: If this is LNAV approach then only GPS related values can be used (or whatever is written on official plate); we can’t invent something else and add navigation aids of our choice.

For stepdown fixes, no. For advisory altitudes like the CDFA descent profile, yes you can.

ESKC (Uppsala/Sundbro), Sweden
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