Interesting subject. Makes a change to find something I don't recall being discussed before!
Although I can't argue with what you say, I'm not sure I'd like to rely on it, especially with airspace nearby. As you point out, you can't track directly towards the DME, only to a position which is somewhere abeam. Even when abeam, you can turn 90 degrees towards it, as you say, but it's trial and error which way you turn. Hardly the most efficient method of navigation.
Here's my thoughts; I wonder what others think:
VFR, you ought to be able to revert to your map+compass at any point on the flight if you experience a multiple navaid failure.
IFR in IMC, you should immediately tell ATC of the failure and ask for assistance. They will quite happily vector you in these circumstances, I'm sure.
IFR in VMC is a slightly more tricky one, where it might be reasonable to expect the flight to continue VFR by reference to map+compass in some cases, but not all. The type of aircraft, the recency of the crew's experience of VFR flight, whether a VFR chart is being carried and the type of airspace the failure occurs in and the weather conditions on the day are all factors which immediately spring to mind.
If the DME really is the only method you have of navigating, bear in mind that lots of DMEs are located on airfields with ATC, and lots of ATC units have VDF, in which case you can ask ATC for a QDR or a QTE and plot the bearing and range on your chart for position fixes. Another method of getting a position fix is to plot 3 different DME arcs and find where they intersect, although this is probably not for the faint-hearted if in tricky weather conditions, especially without an autopilot.
And finally (because if I don't say it, someone else will), you could just refer to the backup GPS you always carry!
Sounds like you were pretty switched-on when dealing with your multiple failures, though, so well done. I'm pretty sure I wouldn't have been able to come up with any of this under pressure in the air!
FFF
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