Question: Melbourne 16 NDB approach with two ADF's in aircraft.

[Centaurus]

Question re Melbourne 16 NDB instrument approach. The Air Services chart states both NDB are required for the approach. While that is a ground navaid requirement, one assumes an aircraft with a single ADF is still legal to conduct the approach by simply switching from tracking to BOL NDB and after passing BOL inbound tune ROC and track on that.

Aircraft that have two ADF's have the choice of having one ADF on BOL and the second ADF on ROC for the entire approach?

Another point of view operationally is that at pilot's discretion he he has the choice of tuning both ADF's for tracking to BOL and after leaving BOL inbound, tuning both ADF's to ROC.

As the only chart limitation is that both NDB ground navigation aids are required to be serviceable before commencement of approach, is there any other published operational reason why a pilot should not utilise both ADF's (if installed) on each NDB if desired? After all, an aircraft with a single ADF installation can legally fly the approach which requires switching from one NDB to the next NDB.

[conflict alert]

Not quite sure what your question is.

Although I am in Kiwiland, I have just looked at the approach plate in the ASA database. Over here it would be called an NDB/DME RWY 16 as the DME is required (no overhead NDB to start a timed approach).

Looking at the chart, it would seem that BOL is needed to be able to navigate to the commencement of the approach and ROC is needed to conduct the approach.

If you have 2 ADF's I imagine it would be personal choice on what you select for each. If I were flying that approach I would have one on BOL and the other on ROC main reason is that I wouldn't want to be pissing around after having crossed BOL changing freqs and identing, particularly single pilot ops, when at a critical stage of flight (configuring for descent, approach and landing). Each would have been tuned and idented before commencement of the approach (prior to BOL) so all I had to do after crossing BOL was change which dial I looked at.

NDB's are slowly being phased out over here. Where the airfield has a VOR, NDB - the NDB is being removed with the introduction of a GPS type approach (GNSS/RNAV etc etc). Enroute NDB's will slowly go as well as the system becomes more and more RNAV routes. It won't be long before the youth of tomorrow go "a non directional what or....coastal refraction - never heard of it or....what are all these round dials with needles or......gee, you mean back in the old days you could break the glass on a pressure instrument if the static tube blocked..wow.

I digress!

[kalavo]

Isn't this just a renamed twin locator approach?

I would say where the chart says "NAVAID RQ: DME, BOL, ROC" and "BOTH NDB RQ FOR PROC" it means you're required to have two ADFs and a DME.

The twin locator approaches allowed a lower MDA when both NDB's simultaneously tuned.

[ozblackbox]

Gosh, haven't done twin NDB/twin locator approach in a while.

I guess the only limitation, for single ADF equipped aircraft, would be the positive backbearing requirement when you cross BOL to commence your descent.

In the Notes: DME & BOTH NDB REQUIRED. Jepp YMML/MEL 26-1 (2 Apr 10)NDB RWY 16.

With 2 ADFs you have two methods of ensuring you are on track, a positive backbearing to BOL and and on track indication to ROC.

You could have both ADFs tuned to BOL, get a backbearing, then set up both to ROC once established. That really defeats the purpose of having 2 ADFs in this case doesn't it?

[A37575]

The twin locator approaches allowed a lower MDA when both NDB's simultaneously tuned.
Impossible with a Chieftain with only one ADF installed. In any case where is the specific regulation or document that states that?

[A37575]

You could have both ADFs tuned to BOL, get a backbearing, then set up both to ROC once established. That really defeats the purpose of having 2 ADFs in this case doesn't it?

Not really. Once you have commenced descent after leaving the first NDB the ADF bearing will gradually get less accurate with distance from that aid. Best to bring both ADF on to the NDB you are homing on, giving redundancy if one ADF fails. Personal choice as someone said earlier.

[Icarus53]

To look at it from a common sense perspective - would you commence an approach knowing that one of the navaids required for the approach was not tuned, identified and tested?

As I would personally answer "no", I can't see myself continuing past BOL without a second aid already tuned and identified.

Another way to look at it is that if you consider it adequate to switch both (or your only) ADF to ROK after becoming established on the BOL outbound bearing, wouldn't it be just as safe to get established on the inbound bearing to ROK and not worry about BOL at all? Clearly not the intended design of the approach.

The answers to these two questions strongly suggest to me that you need to commence the approach with the BOL and ROK beacons tuned and identified on two separate ADFs, and not do any switching of navaids mid-approach.:ok:

[Capt Casper]

The requirement is for the 2 NDB's (navaids) and DME or GPS.
To conduct the approach you require the equipment to utilise the navaid systems. One ADF and one DME or GPS receiver will do that.
Also ERSA indicates Bolinda has a night range of 30 nm and Rockdale a night range of 20 nm.
A navaid approach designer might have an opinion on whether the NDB range is a factor in the design of the approach and the requirement to use both NDB's.

[boofhead]

The twin NDB/Locator approach was a great tool, and was more accurate than a single NDB approach. If you had an RMI with both tuned, the needles would be superimposed, so that if they were one on top of the other, you were on track. If you were off track, you could see by how much and which direction to turn to get back on track. No matter the drift. You were at the intersection of the two needles, and the track you wanted was between the needle points. Easy, even for me.
Once past the second NDB you continued to keep the needles superimposed, and could get a coarse as well as a fine interpretation of your track, although the second NDB was usually located close to the missed approach point so it was not so important.
You needed to tune the No. 1 ADF to the first NDB and the No. 2 to the second (or vice versa, so long as you always did it the same way) to get an immediate orientation when past the last NDB. In the day, it was a superb approach, not only for it being easy to interpret, but I think the random errors of the two ADFs cancelled each other out.
I think the minimas were lower for the twin NDB approach than for a single (or tuning one at a time if you only had one ADF) because of the better accuracy.
Never seen the procedure except in Aus.