Use DA or MDA in RNAV with Displaced Threshold?
Thread Starter
Joined: Jul 2009
Posts: 102
Likes: 0
From: Earth
Use DA or MDA in RNAV with Displaced Threshold?
Hello gals and gents,
This doubt has come up occasionally where we operate.
There is a 3000m runway that got one of the thresholds displaced 350m, so now it has only 2650m available. The current IFR procedure available to this runway is a RNAV with a 3 degree angle. Our equipment has VNAV PATH guidance and GP (Glide Path) capabilites to intercept and follow the CDA of 3º.
However, the doubt rose from the fact the glidepath leads you to the aiming point, 1000' mark. Since the threshold is displaced, now the glidepath leads us to the beginning of the displaced threshold.
Some argue we shouldn't use the DA minimums, and instead the MDA minimums (something like 80' more).
Others argue that we should not even use the Approach capability to intercept the glidepath, and instead do a dive-and-drive, considering it a non-precision approach (arguing that this is like a LOC only approach with an U/S GS) and reach MDA, level out until we reach the VDP.
I haven't really searched much, but in our regulations, manuals and SOP there isn't such situation described.
I have yet to search ICAO documents.
Any input from you would be greatly appreciated!
Thanks!
This doubt has come up occasionally where we operate.
There is a 3000m runway that got one of the thresholds displaced 350m, so now it has only 2650m available. The current IFR procedure available to this runway is a RNAV with a 3 degree angle. Our equipment has VNAV PATH guidance and GP (Glide Path) capabilites to intercept and follow the CDA of 3º.
However, the doubt rose from the fact the glidepath leads you to the aiming point, 1000' mark. Since the threshold is displaced, now the glidepath leads us to the beginning of the displaced threshold.
Some argue we shouldn't use the DA minimums, and instead the MDA minimums (something like 80' more).
Others argue that we should not even use the Approach capability to intercept the glidepath, and instead do a dive-and-drive, considering it a non-precision approach (arguing that this is like a LOC only approach with an U/S GS) and reach MDA, level out until we reach the VDP.
I haven't really searched much, but in our regulations, manuals and SOP there isn't such situation described.
I have yet to search ICAO documents.
Any input from you would be greatly appreciated!
Thanks!
Joined: Jun 2000
Posts: 4,507
Likes: 4
From: last time I looked I was still here.
However, the doubt rose from the fact the glidepath leads you to the aiming point, 1000' mark. Since the threshold is displaced, now the glidepath leads us to the beginning of the displaced threshold.
How do you know this? Is this a 'temporary displaced threshold'? If it permanent it might be the VNAV is taking you to the GPS coordinates of the RWY TDZ. It is something to ask your performance dept and they will pass it on to the FMC data base supplier.
If it is a temporary displacement then your Ops dept should issue crew guidance.
How do you know this? Is this a 'temporary displaced threshold'? If it permanent it might be the VNAV is taking you to the GPS coordinates of the RWY TDZ. It is something to ask your performance dept and they will pass it on to the FMC data base supplier.
If it is a temporary displacement then your Ops dept should issue crew guidance.
Joined: Jun 2013
Posts: 326
Likes: 1
From: Or-E-Gun, USA
Sorry, But...
This may not be the response you are looking for, but RAT5 mostly nailed it: One way or another, you must follow company SOP. If the displacement is temporary, company ops should issue a revision; If not, ask them! If the displacement is perm., ask company now and look for official guidance when revised plates are published. Sorry to say it, but you do not really have many other choices. Or, I did not understand your question.
Joined: Sep 2013
Posts: 47
Likes: 0
From: Canada
In FAA land at least, RNAV approaches are to be suspended by NOTAM when invalidated by displaced thresholds (see section 4-1-7 of this document).
Joined: Jan 2004
Posts: 3,247
Likes: 203
From: Here and there
Sydney is frequently like that during curfew hours. The minima is raised to the circling minima (by YSSY NOTAM) and you fly the standard profile down to the circling minima then fly level until on the PAPI.
Joined: Nov 2000
Posts: 624
Likes: 0
From: I wish I knew
Perhaps this current document regarding STN will help clear the fog: Ref 5.6 (b) 11
http://www.acl-uk.org/UserFiles/File...ing%202015.pdf
Clearly you can see the revised GPA according to the displacement, the revised GPA will itself derive new aiming points and hence minima.
http://www.acl-uk.org/UserFiles/File...ing%202015.pdf
Clearly you can see the revised GPA according to the displacement, the revised GPA will itself derive new aiming points and hence minima.
Joined: Mar 2002
Aviation Qualifications: ATPL
Posts: 9,226
Likes: 996
From: Seat 1A
Perhaps this current document regarding STN will help clear the fog
The OP, on the other hand, has a displaced threshold but the database approach takes him to the "normal" threshold ie 350m short.
In the big scheme of things though, 350m is equivalent to 60ft of profile error. If popping out at 3500m-odd from the runway at the MDA/DDA, that's going to put you only slightly low on the PAPI (if there is a PAPI at the displaced aim point). I'd be happy with that provided it is briefed.
Thread Starter
Joined: Jul 2009
Posts: 102
Likes: 0
From: Earth
Thanks for the input you all, and sorry for the delay...
As raised by others, this is a temporary notam due to work in progress near the threshold that is now displaced.
However the RNAV procedure has not been suspended by NOTAM's.
I'll get back to you as what is the current policy.
Capn Bloggs, I see it is not much of a deviation 60ft, and that is an interesting point, and being properly briefed all you need is to correct the deviation as usually happens.
As raised by others, this is a temporary notam due to work in progress near the threshold that is now displaced.
However the RNAV procedure has not been suspended by NOTAM's.
I'll get back to you as what is the current policy.
Capn Bloggs, I see it is not much of a deviation 60ft, and that is an interesting point, and being properly briefed all you need is to correct the deviation as usually happens.
Joined: Aug 2013
Posts: 30
Likes: 0
From: PA
Typically, this situation would be evaluated in procedure design, and the decision to issue/not issue a NOTAM would be based on the obstacle evaluation
As the displaced THR brings you in higher, the same DA/MDA would give you more protection back in the obstacle clearance area.
Human factors with the lights is a different issue.
As the displaced THR brings you in higher, the same DA/MDA would give you more protection back in the obstacle clearance area.
Human factors with the lights is a different issue.
