Not long enough if you have a TB20, or most other IFR tourers for that matter 
Not long enough if you have a TB20, or most other IFR tourers for that matter.
Only if you are one of the select few pilots who plans to hand-fly IFR with no radios for more than four hours…
If the alternative is likely death, yes.
I have a battery powered radio, BTW.
IMHO a 4hr backup, after which you lose all gyro instruments, is of limited use.
While an hour sounds short, how much is really needed to divert to the closest instrument runway in such an event?
True, but then you end up with a plane sitting in some place you don’t want to be, which is probably very hard to reach unless you can wangle another GA pilot to fly you (with a replacement alternator, or whatever) there, and good luck with that, anywhere on mainland Europe other than Le Touquet 
Quite likely the place is a dump, too.
It will be the mother of logistical exercises. Your only option will be to fly back out of there on a nice VFR day, but you can’t do that (practically and definitely not legally) if the alternator has failed… but it might not be the alternator… it could be the voltage regulator… or…
Getting back home is priceless.
Cobalt wrote:
While an hour sounds short, how much is really needed to divert to the closest instrument runway in such an event?
Yes 30min is more than enough in Europe to be on the ground, if you need more you probably should fly twins or chutes the tricky bit is getting some friends to fly with the ferry back on the next high pressure week with sunny VMC, this likely to require two aircrafts to collect one aircraft if the en-route divert choice was away from RyanAir airports…
PS: you will need a strong 2nd crew if it’s electric trim only and both Alt1 & Alt2 failed !
Peter wrote:
IMHO a 4hr backup, after which you lose all gyro instruments, is of limited use.
In many aircraft, unless the failure happens right after takeoff, 4 hr will likely exceed your fuel endurance with reserve. The longest IFR leg I’ve ever done was 5.5 hours, which means most of that flight, a 4 hour battery would have been enough to make the destination.
But an electrical failure in IMC is an emergency requiring a landing at the nearest practical airfield. I’d never contemplate doing anything else, personally. When dealing with a serious in flight failure, the last thing on my mind is the convenience (or not) of getting the plane back to base. Any electrical failure in flight when there’s IMC about, I personally would likely be on the ground within 45 minutes at most. (I’ve done it too, I had to abandon a plane somewhere in west Texas due to a gyro failure. It was a pain getting it back, but not as much as a pain of crashing it by trying to fly it back to Houston in IMC would have been).
I agree it’s about covering emergencies, not finishing your flight (however long) as originally planned. A 4hr backup battery is plenty and the G5 now in our aircraft makes me really pleased that in the event of vacuum failure I am only flying on the TC for the few seconds it takes to switch the G5 from HSI to AI mode. At the very least, I would consider such an event grounds for exiting IMC at the earliest opportunity, if not necessarily diverting. I could in theory fly an ILS like this (with the ILS on NAV2 and the associated CDI) but would probably prefer to achieve a VFR arrival even if it meant diverting.
My point about the vacuum system is really that it’s a known quantity. It can fail, yes, but it only fails in one way. The fundamental issue with any box of electronics is that it can go dark without explanation at any time, for any reason, and usually without you having any idea what the underlying problem is.
I agree that if your electronic systems are separate enough (and operationally, having them very separate might seem undesirable) then the risk is substantially mitigated. However if you already have a vacuum system then adding one piece of electronics to it seems a better (not to mention cheaper) option than pulling out the vacuum system and adding 2x electronics.
On the question of software issues, as an end user I am not going to get into a detailed analysis of whether a G5 and a GI275 are substantially different enough to mitigate the risk of a common software failure. I’m afraid I’m going to assume that if Garmin product A can fail in a certain way which nobody predicted or has seen before, then Garmin product B could do the same thing.
Ibra wrote:
Yes 30min is more than enough in Europe to be on the ground,
That depends on where in Europe you are and what aircraft you’re flying. Assuming you are able to fly an IAP with electrical failure, you must have an IAF no more than 20 minutes away. That’s about 35 NM for a basic IFR aircraft such as a C172 and 50 NM for a typical tourer such as a TB20.
Even in Germany there are places where you’ll likely to be more than 35 NM away.
I have a battery powered radio, BTW.
I hope you tried flying an instrument approach on it in good weather first.
Do you have a battery powered TKS pump?
