Originally Posted by
TelsBoy
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).
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.