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Old 14th Mar 2015, 07:02
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Last technical stuff on RNP

For those that want to learn something here : just back from a meeting with some experts on RNP present . (i.e. EGNOS and Airbus ) I asked a few questions and now know a little bit more about RNP APPs and this is how it works ( on Airbus aircraft at least ) in simple terms.

The 0.3nm accuracy is only for the last part of the App segment not for the whole APP . 2 minutes before FAF , the RAIM box inside the aircraft will verify if all the satellites are aligned and if the position they received is correct and will be guaranteed for the next 5 minutes . The guaranteed precision you will get is still 0.3,NM either side of track , but in practice it is better . The correct definition is “ The system will guarantee that in the next 5 minutes your deviation will not be superior to your RNP value “

If the RAIM cannot guarantee the integrity it will issue a warning that should result in a go around (if IMC ),but this accuracy is only guaranteed for 95 % of the time.( although in practice it is better ) Only with an augmentation system ( such as WAAS or EGNOS ) you will get more than 99% . But Never 100%, mainly because of solar activity . 2014 had a very strong solar peak activity and the lowest was 99,4 . Better availability is expected in 2015.
EGNOS is already available today for regional’s and business jets , for large airliners the first aircraft fully factory equipped is the A350 . Retrofit for older types is expensive. But remember EGNOS is only for Europe .

What is more interesting is he obligation to follow certain rules to actually perform an RNAV-AR APP :you need certified aircraft, certified and trained crews , a certified airport and trained air traffic controllers . It is not because it is in the aircraft data base that you can use it . And there lies our first problem.
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The other is when an airport changes things in the ground or in the runway and do not tell the people that make the data bases . Remember that one .

Next : some airports have issued visual RNAV APP plates on their own and there are currently no official procedures for them , hence the (correct) IFALPA warning.

Finally all strongly ay that RNP is not and never will be a substitute for ILS,. It is still a non precision APP and should be treated as such.
Now I put my willy back where it belongs and leave the discussion over to the "real" experts here.
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