Trust the AIP to be confusing!
On reading through it all again, I think that this is the situation (charter op assumed):
- In order to fly IFR without a TSO145/6 GPS, you need to have at least one ADF/VOR (AIP Gen 1.5 para 2.1)
- If you are going to an aerodrome with an NDB or VOR, you can use that as one aid/approach and a TSO129 GPS as the other, as you do have 'an alternate instrument approach utilising ground based navigation aids' (AIP Gen 1.5 para 8.5.5.4) available (the NDB).
- If you have to nominate an alternate anyway due weather etc, your alternate must have a ground based approach available (which makes sense really, as otherwise that alternate would in itself require an alternate – don't know why they bothered with that note in 8.5.5.3 at all).
- You can't use two TSO129 receivers as your two independent radio navigation aids/receivers due to 8.5.5.4 above, however you can use two TSO145/6.
- The flow chart in the CAAP says you don't need an alternate if the wx is above the alternate minima for a ground based approach (as you need to provide for that as your second approach type per 8.5.5.4).
So I surmise that since you can't legally depart without a functioning ADF or VOR anyway, the only real limitations of sticking with TSO129 equipment are:
- if there is no ground based aid at the destination you're going to need an alternate.
- if the weather is below ground-based approach alternate minima but better than RNAV alternate minima you're still going to need an alternate (makes sense), but how often are the alternate minima very different for different approaches?
Does that seem right?
