Sorry BOAC - 5 years since I stepped from that seat - I wouldn't have the confidence to say either way! What would you think?

Cough

ps The FMS does have RNP vs ANP alerting...

The LTMA wil eventually be RNP1, P-RNAV airspace. There really isn't any need for RNAV overlays in the first place as the present SID's work very well and are mainly flown by RNP1 capable aircraft accurately tracking through modern Flight Management Systems.

There is as yet no mandate within Europe for RNP1 within TMA's. The problem arises in the cost of conversion etc for aircraft that aren't currently compliant. Whilst Heathrow users may be, that doesn't necessarilly hold true for the whole of users of the LTMA airspace.

There was a stakeholders meeting in Brussels in Feb of this year, and indications are there will be no mandate before circa 2020.

In the mean time, overlays could be used, but the current system works, indeed it works well, why spend money on what will only be an interim change but still requires the ground navaid infrastructure to be maintained for non compliant types/operators?

That said, NATS would love to move away from the current infrastructure - VOR's, NDB's etc are both expensive to maintain and of course require staff to carry out that function....

Hope the above helps..

30W