Airbus FMGC PROG page HIGH/EPE real meaning.

[9 points]

Prior to the GPS days it was well understood that HIGH only meant your EPE was less than your required. HIGH did not mean you were on track because the EPE is just an estimate, nothing more.

Often I saw the aircraft do a big turn after a long overwater leg when it picked up a new ground based update for the first time in hours. The HIGH indication was clearly not to be relied on and we had to check nav accuracy against raw data before any managed RNAV approach.

These days the airlines / authorities seem to have much higher trust in HIGH. But does it actually mean something different to what it meant 20 years ago? As far as I can tell the logic of the HIGH/LOW presentation has not changed.

[vilas]

Without GPS whenever Low message is displayed or once every hour if high is displayed NAV accuracy check needs to be carried out. If GPS is primary no check is required.

[9 points]

Thanks for the refresher on procedures. Very illuminating!

Now would anyone care to offer any insight into the position of the regulatory authorities? As per my question I'm wondering how they decided to put such trust in the HIGH ACCURACY display. What sort of data gathering and analytical thought process etc?

[Goldenrivett]

Now would anyone care to offer any insight into the position of the regulatory authorities?

Fill your boots.
An explanation from 2009:
http://www.smartcockpit.com/docs/Getting_To_Grips_With_RNP-AR.pdf

[9 points]

Not really what I was hoping for but better than nothing. Thanks for the suggestion.