Question about an NDB approach

[ooizcalling]

In the SIM today we were to do a let down using an NDB approach, not NDB/DME or any other chart structure, just NDB. It was timed 3 minutes outbound descending to an intermediate altitude, a proceedure turn and then down to the MDA on the inbound leg. The Missed approach point was defined by a DME fix ! There were no timing V's G/S or any other cross check, just this DME fix.
Question 1 How can this chart be defined as just a NDB chart and not an NDB/DME letdown ?
The instructor then failed the DME halfway down the proceedure.
Question 2 Can you continue to the MDA or must you abandon the approach then ? There were no other published cross checks for the MAP even though we had a GPS which was probably more accurate than the DME (?).
Practical approach might be to use a conservative GPS position as an alternative, but whats the 'legal' answer to this question ?

[Av8rMarc]

Was "Radar Required" listed anywhere in the notes? A controller could advise your position over the fix. Don't think you can substitute GPS unless it specifically states it. Should maybe be named the NDB/FUBAR Rwy19.

[fatboy slim]

Was the DME on the field, or at another location?? If it was on the field i can't understand why its not used as part of the approach ie turn inbound at xDME and TOD at yDME.

And i think the only 'legal' answer is to miss the approach (if not visual obviously) when the DME fails otherwise you cannot 'find' the MAP.

Ask this in 'Safety and CRM' - there are some experts in this there...

[OzExpat]

The primary navaid used for the approach always appears in the approach title, but some States may not necessarily require title reference to any other navaids that may be required. Thus it's a matter of reading the plate in advance, as ooizcalling obviously did.

In this case, with a DME fix marking the MAPt., if the DME fails and you aren't visual, you need to execute a missed approach. You don't say which design criteria was used so, if it was TERPS, I may not be giving you the right answer. However, under Pans Ops, you need to maintain the altitude at which the DME failed (ie not below MDA) and track to the MAPt by DR, before following the missed approach.

This isn't especially critical if the missed approach goes straight ahead, but is very important if it involves a turn. How you find the MAPt by DR is up to you and, in an such an emergency I'd be inclined to use the GPS, if it had been running pretty close to the DME for distance info before the DME failed.

There's more options if ATC radar is available at the airport so I'd be telling ATC about the missed approach and requesting suitably safe vectors.

fatboy slim... a DME fix at the end of an outbound leg isn't always necessary and, in this event, the approach won't require it. If it IS necessary, a reference to DME is usually included in the procedure title.

I prefer things to be spelled out so that there's no chance of any misunderstandings. Thus, if I need to use DME in ANY part of an approach or missed approach, I'll stick "DME" into the title. Procedure designers in other parts of the world might have a different attitude though.