2 NDB.
 

As I recall it, what is listed on the approach page for nav aids, NDB, DME, VOR ILS etc must all be tuned and identified prior to commencing the approach.Hence a twin NDB approach can't be attempted with only one serviceable ADF on board.

Yes you need 2 ADFs as the previous poster said, if you try to keep up with two NDBs on one ADF it may end in grief
Edit though 2NDB approaches are very rare, it would be cute seeing two little trees on the plate...I call NDBs little trees or bushes.

Any criteria regarding unserviceability should be on the approach chart, I'd imagine. Dual NDB approaches still common in Russia (I note that's the OP's location).

Remember having to do a twin locator in anger once into YMEN RWY 17.

Four things required. Two NDB's and two ADF's. Simples.

halas

The Russians know how to use dual NDBs. With a dual needle RMI you make sure both needles are in lockstep. That yields the most accurate possible track for NDB navigation.
At Dubrovnik had Secretary Brown's USAF crew had dual NDB (and had they known how to use it "the Russian way") they all would still be alive today.

Problem is in many modern outfits a single ADF is available, making these approaches impossible to fly raw data, unless approval is gained to mitigate against the lack of a second ADF.
In years gone by, we were allowed to operate dual FMC aircraft only to fly a dual NDB approach using a single ADF installation. (Approval was gained for this prior to operating)

Yep, same in our Ex-Fleet (A300/310).

I find it unbelievable that pilots are still letting down in the goo with only 90 year old technology nav aids to line them up with runway.

Biggie old mate, Do you have any better ideas when that is all you have??

When you have two NDBs, that old technology can save the day, such as at Dubrovnik when the rebels had stolen the ILS ground components. The ILS minimums at Dubrovnik are 200 feet. The NDB minimums are 1781.

Here in the US, if equipped,with dual ADFs you can use one to fly the approach and the other to listen to AM radio... you will never see a dual NDB approach here AFAIK.

I'm joking of course!

Many thanx friends. It was useful:)

In my opinion it means both, except I would expect approval to be granted to use one ADF plus GPS in lieu of twin ADF. The problem with this type of question is what applies in general is not necessarily correct for each company or country.

That would be a GPS which will give far superior stable track guidance with a pseudo glide slope even on a non precision approach.The much increased accuracy makes for a vastly safer operation NDB approaches of any kind are basically done in North America and the NDB,s are being decommissioned at a great rate

The U.S. used to have many of them in the days where most, if not all ILS, approaches had both a LOM and LMM. However, as I recall an NDB approach could be made using only the LOM.

Yeah, could you imagine, no magenta line, no map display, no FMS , no coupled approach, no RNAV plus skill required.

NDBs went from Non-Directional Beacon to Nuisance Death Beacon :}

... if my recall is not yet totally deranged ... we used to be able to do a twin locator approach in Oz with one ADF. Start on the outer and then switch to the inner between the two. I don't know that any of us actually liked the idea at all but, with eyes wide, we managed. I guess it helped not having any real tiger country to contend with.

It is possible that, with the debilitating passage of time, we only trained these on the Link .. it was all a long time ago, now.

JT was that on the 727 or 737, and was the ADF needle on a RMI or fixed card? just curious