A320 radial equivalent

[che ci dò che ci dò!]

Guys,
A320 FCOM - Special Ops - RNP 10

"If manual entry of a required accuracy is desired, the flight crew must manually enter
For RNP-10, enter 10 nm or use the radial equivalent to 10 nm XTK accuracy that is 12.2 nm."

What does the last part mean? "radial equivalent to 10 nm XTK accuracy" is something I've never heard before..

[Natstrackalpha]

Radial. Radial Error Rate. The amount by which the INS is in error at the end of the flight at shutdown, measured or calculated by (a formula) within x nautical miles left or right, or North or south of its actual position, calculated over a period of time - the time being the time of the entire flight, in hours. The little picky you draw to work this out looks a bit like a right sided tri-angle.

Xtrk = Across the trk. As in XTRK Error.

EG. If you fly an offset TRK, say, then you are parallel TRK offset by lets just say 2 miles to the right, which you inserted intentionally in order to avoid a load of cumulonimbus, say. If, if, there was an error causing this, then the pilot(s) would refer to this as XTRK error, error across the TRK (geddit?).

So, the MNPS Minimum Navigation Procedures, have to contain accuracy of the equipment to within certain parameters - in your example 10 NM. In actual fact, this is usually only a buffer, albeit a huge one. 10 NM tolerance over an Ocean 10 NM is hardly acceptable in today`s accurate navigation environment. In that unlikey event though, you would be `contained` within the airway - assuming the airway is 10 NM wide assuming 5 NM either side of TRK.

Hence, .3 accuracy for approaches or .25nm.

Don`t forget, this is one piece of (very important kit) there is also back up of Radio Aids, VOR, VOR DME, DME/DME, NDB although, obviously not present over an ocean, and super duper GPS with more than 4 SATTs and RAIMS. Strangely, the system tends to bias from the FMS which has a combination of man and computers to enhance its beauty to Radio aids (see below), by the end of the flight - the whole system is almost entirely biased towards Radio Aid inputs as the updated data.

Hopefully, this has answered your question, a little bit.

Accuracy. Across, your, TRK. XTRK accuracy.

Fly to the right, you are not on TRK therefore there is a XTRK error of however much you are `not` on centreline of your TRK.

You could also go nuts and say that you are trking a radial of a VOR and are offset from that radial - here is a distinct example of xtrk error, but has nothing to do with radial error.

I just mentioned this last bit, to make you think a bit.