Hi Pilot Pete,
Another rule says: Enroute Alternate Selection Criteria
This is an alternate, which may be used in addition to a conventional Destination Alternate,
allowing reduced contingency fuel to be planned. The abeam point to the en-route alternate
must be in the latter half of the route. The shortest distance from the en-route alternate to the
aircraft must be no more than half the distance the aircraft is from destination. The airfield
must be approved for the aircraft type and promulgated as open. It is planned so that should
the calculated fuel remaining to destination fall below the minimum required then a diversion
to it could be initiated to uplift fuel.
That’s the case, when you would nominate an en route alternate due to commercial reasons. This would be the case if you need to reduce your contingency down to 3% or company minima (lets say 600 kg), basically its redispatch in flight. If you noticed there is a limitation where the en route alternate can be located, in particular somewhere around 75% of the route.
Due to that regulation I don’t see a reason why you would select another enroute alternate between your destination and destination alternate. If you would have to select a destination alternate with more than 60 minutes, due to political reasons or whatsoever reasons, then this part of the flight would become an ETOPS flight, with associated planning procedures.
I think, its not quite correct to call the aerodrome placed between your destination and destination alternate as an en route alternate.
That’s how I would interpret the definition, but I might be wrong.
Cheeers.