Originally Posted by
applecrumble
Hi everyone,
If you were flying a standard RNAV approach with a requirement RNP of 0.3NM. Let’s say the ANP is less than RNP but for argument sake the ANP is 0.29NM. I can continue the approach as the ANP<RNP of 0.3NM.
This could mean though that the aircraft’s true position is 0.29NM behind where it says it is. If this were true then if you started descent at the FAF you would be descending early given the aircraft’s TRUE position 0.29NM behind where it says it is.
0.29NM is about 100 feet altitude. Surely this could put you dangerously low down the approach and at MDA??
Hi applecrumble
Every fix, whether based on NDB/DME, DME/VOR etc. or RNP has a 'fix tolerance area'. In the case of an RNP 0.3 fix, this is given by a 'cross-track tolerance' (XTT) equal to the RNP value of 0.3nm and an 'along track tolerance' (ATT) equal to 0.8 x the RNP value i.e. 0.24nm. This produces a rectangle 0.3nm either side of the fix and 0.24nm before and after the fix.
The procedure designer will assume the aircraft starts a descent at the earliest point of the fix tolerance area and so will start the protection of the next leg of the procedure from 0.24nm prior to the actual fix position.
Cheers
TeeS