Hi Bob / Rich,

Chaps two questions if you be so kind:

1)
GEN 3.3.4.9 OPERATIONS with Area Nav. Systems
Refers to 30 nm each side of the nominal track (Non-GNSS), Question being if Navaids use 50nm "Off the nominal track", and GPS 7nm, where does the 30nm come into play?

2) When planning for alternates, as it relates to aids, we need to have a ground based (vor / ndb) aid. Having said that, would a DME Arrival Procedure coupled with a NDB or VOR be usable for planning purposes? I ask this because the procedure itself only provides CIRCLING MINIMA, and not ALTERNATE minima, thus suggesting it cant be used, yet it qualifies in so far as it is not a GPS.



With thanks.

Andy

G'day Andy.
To answer your question about LSALT, the 30nm buffer for Area Nav. systems (aka RNAV). The 30nm buffer applies to the Interial navigation system (INS) if it is installed in the aircraft. These are mostly airliners due to the size and weight of the equipment. The INS uses information from gyros, accelerometers and other air data inputs to derive the aircrafts position and updates itself with VOR/DME position fixes. That's why it comes under the Non-GNSS title and it has a large buffer of 30nm compared to the mordern GPS which is contantly updating it's position and therefore can allow the buffer to be smaller e.g. 7nm.

To answer your second question, I don't know why the alternate minima is not shown on the DME arrival chart! I have searched the AIP and I am yet to find the secret.
I think it might be a basic assumption that the pilot will use the alternate minima off a VOR/NDB DME approach plate in the planning stage since most places that have DME arrivals will also have Reversal type procedures as well.

Hope this makes things a bit clear for you.

Cheers,
Mister W.

Mr W,

Thanks mate that puts my mind at rest. I had fleetingly wondered about the INS, but didn't know they were still used at all.
Have a good weekend and may your teams win. ..unless they are playing the Bulldogs!