Hi all

Please help me with the "two chances" rule:

Part 135 MOS 11.09 (5): The navigation equipment ... must be such that, in the event of the failure of any navaids at any stage of flight, sufficient navaids remain to enable the aeroplane to navigate in accordance with ... the aeroplane's operational flight plan.

I'm struggling to apply this correctly in examples and test questions.

Example (paraphrased from the practice exams): an IFR flight with a duration of 90 minutes. For the first 60 minutes of flight, the aircraft will be over featureless desert at night, beyond the rated coverage of any ground-based navaid. The destination has an NDB (only) and published approaches NDB and GNSS. The aircraft has 1 x GNSS (TSO 146a) and 1 x ADF. Can this be planned as an IFR air transport flight?

Well... the equipment is enough for an air transport IFR flight according to Part 135 MOS 11.09(3). And the aircraft can maintain track using GNSS at all stages. So the only concern is, what happens if the aircraft's GNSS receiver fails while the aircraft is beyond the reach of ground-based navaids. The pilot will have to fall back on Dead Reckoning until the aeroplane flies into range of the destination NDB.

So my question is whether Dead Reckoning en route would satisfy the requirement to 'navigate in accordance with ... the aeroplane's operational flight plan'.

My answer was 'No' but apparently the correct answer is 'Yes'.

If the route is published, so it is RNP 2, does "navigation in accordance with the flight plan" mean that the Dead Reckoning would have to be accurate to within 2 nm?? Or do we just have to satisfy the basic requirements of IFR navigation, namely that we can get a positive fix every 2 hours or less?

I notice that Bob's IREX book, page 154, paraphrases the rule as ".. must be able to survive the failure of an airborne aid and still have a means of conducting an instrument approach." That would suggest the correct answer is 'Yes'.

Any advice would be welcome!