If unsure about local habits, I always ask for a startup clearance.
That “direct connection” switch is often done, and those I have seen were all done “off the books”. To do it properly you have to protect the wire from the battery with a fuse, so you have a fuse, close to the battery, then a switch which powers on just the radio. There needs to be a diode, and another diode powering the radio from the avionics bus once the bus is being driven, and to prevent current flowing from the bus back into the battery! IOW, either the battery (via the switch) or the avionics bus can power the radio.
I don’t currently have this but next time I get any work done I will probably get it fitted.
The main reason is that at some big airports you have to be listening on the clearance delivery frequency, until they get around to calling you. At some disorganised place (Barcelona comes to mind) you can wait for half an hour. It’s OK to power one KX155A radio for half an hour but you don’t want to power anything else and certainly you don’t want to power a big GPS box.
Can the VHF radio in a GPS+radio box (e.g. GNS430 onwards) be used immediately the power is applied? The good thing about a separate radio is that it is usable instantly.
Yes, as Sebastian said, the COM part works immediately, bit you can’t see the set frequency.. so you have to set it before shutdown.
For now, I’ll keep on using my small YAESU handheld. … easiest
I did it with ground clearance, now with EMER.
If no handheld you have to turn on the avionics and do it then. In an SEP I can’t see why that matters. In a turbine battery voltage is key and it is very unpleasant to use it before start.
I usually ask when paying, they always know or know who to ask.
Flyer59 wrote:
Or do you not care and start the engine with the Avionics on?
This is never a good idea. Switch avionics off during start. Chances are small, avionics could be damaged or files can go corrupt (more common) when you do that.
Flyer59 wrote:
I wonder if such a switch could be installed …
Sure why not?
Peter wrote:
That “direct connection” switch is often done, and those I have seen were all done “off the books”. To do it properly you have to protect the wire from the battery with a fuse, so you have a fuse, close to the battery,
Fuse is a good point, which is often overlooked (also with small charger connections, many use these days, which is good, yet they don’t use a fuse as close to the terminal as possible.
Peter wrote:
There needs to be a diode, and another diode powering the radio from the avionics bus once the bus is being driven
Altough it happens this way, this is a poor installation IMHO. With these diodes you will add extra voltage drop, which isn’t such a problem for normal use, but it will disable your radio faster when you have a failed alternator. It is better to use a relay or SPDT switch with the radio at the common contact, the avionics bus at the normally closed contact and the fused ground clearance wire on normally open. This way you can select either normal operation (as other avionics) or ON for ground clearance.
I normally connect my handheld radio with a bnc cable to to antenna of the aircraft radio. It is only a small loop of wires to install to have the bnc connector in a good place. And it gives me also the advantage of having the handheld as real backup with good transmission in flight in case of radio failure. If you are interested I may do some photos on startup on my next flight and post it here.
If you really need a startup clearance, acquire the phone number of Ground control and call them on the phone?
eddsPeter wrote:
If you are interested I may do some photos on startup on my next flight and post it here.
Definitely interesting for me.
If you fly to places regularly, you know if they want you ask for startup clearance before starting up in a SEP or if you can call them on the radio afterwards. I think the whole startup clearance is designed primarily for larger aircraft. So, there are places where I just startup, then call them on the radio. I get the startup clearance and tell them that I am started up and ready for taxi. Never a problem. Otherwise I do have a handheld radio where I just switched from the ICOM radio to a new one (better channel spacing). This one also has the ILS as backup and a way better battery which lasts forever.
Altough it happens this way, this is a poor installation IMHO. With these diodes you will add extra voltage drop, which isn’t such a problem for normal use, but it will disable your radio faster when you have a failed alternator. It is better to use a relay or SPDT switch with the radio at the common contact, the avionics bus at the normally closed contact and the fused ground clearance wire on normally open. This way you can select either normal operation (as other avionics) or ON for ground clearance.
True, although a schottky diode would drop only 0.4V, and (would need to check this) there appears to be a trivial solution using two P-channel MOSFETs and then you get “zero” voltage drop. I don’t like relays much because the ones which get used in practice (which can be bought with a Form 1) are of low quality, not sealed and straight out of a car. If I used a relay I would buy a milspec hermetically sealed one off Ebay. I think one should do all electrical switching with sealed components, in case of a fuel leak (the Apollo fire issue). The extra cost is minimal. Presumably you would energise the relay coil from the avionics bus so if that has power the radio will run from the bus and no longer from the battery. And hope the brief interruption (break before make contacts on the relay) does not mess up the radio.
acquire the phone number of Ground control and call them on the phone?
It isn’t always Ground. It even isn’t always the Delivery frequency… they like to keep you guessing 
