There are specific FAA rules for getting rid of the vacuum AI. I recall that a second alternator is mandatory, or a battery backup in the AI (but the fairly common 30 minute battery doesn't meet the requirement so an external battery is required).
Under EASA, I don't know but it cannot be any easier.
It also doesn't make sense to make it easier because that primary AI has to be the one thing you can rely on when you lose all electrics (which is very definitely possible on single alternator aircraft).
Garmin 500 installs usually involve installing a cheap vacuum AI somewhere.
If a second alternator is installed, does it have to serve just the AI? For example if the B&C alternator was installed, would that do - even though it does (if activated) feed the same single electrical system?
The text which I got some someone c. 2009 (probably NCYankee here) is
If you have an electronic display instrument such as the Aspen or G500, there must be a standby attitude indicator that is powered by a source other than the one that is powering the Aspen or G500. The second power source must be independent of the first, such as pneumatic, a dual bus with dual alternator and battery that are isolated from failures of each other, or an independent backup battery system that is charged by the main system and indicates to the pilot that power is being provided by the backup battery. This requirement comes from FAR 23.1311 and applies to all glass systems.
If you install a dual Aspen, one as a EFD1000 PFD and the other as a EFD1000 MFD and the EFD1000 MFD uses an approved external battery backup system (the internal battery in the EFD1000 MFD is not installed because it doesn't meet the power requirements), the EFD1000 MFD has its own ADAHRS and has a PFD reversion mode fully duplicating the EFD1000 PFD. This configuration meets the requirement of FAR 23.1311 and a standby Airspeed and Altimeter is not required. Currently a standby Attitude indicator is still required, but Aspen is planning [12/2009] on obtaining certification for this configuration where the standby attitude is also not required some time in 2010. Around the same time, they are anticipating that the KI-256 attitude signals required by the autopilot will be available and the KI-256 will be able to be removed.
Aspen have since certified their EA100 KI256 emulator box but that doesn't deal with the redundancy...
We have a backup AI that is electric. No vacuum instruments, although we do have a vacuum system.
... but the fairly common 30 minute battery doesn't meet the requirement ... Under EASA, I don't know but it cannot be any easier.
But also not more difficult either. The Metroliner I used to fly (part 23) and the Citations I fly now (part 25, so more restrictive one would think) are "all electric" and have a set of standby instruments that is powered by their own battery for not more than 30 minutes. There is also a vacuum system, powered with bleed air through an ejector pump, but that is only required for the cabin valves and to deflate the deice boots.
Apart from the B&C backup alternator I have a TCW 6A backup battery for the main EFIS and 430
Presumably you don't have a certification requirement though, being a homebuilt?
There is a lot of good stuff available for homebuilts...
We have a backup AI that is electric. No vacuum instruments
Can you write more information on your system and how it meets the cert requirement?
But also not more difficult either. The Metroliner I used to fly (part 23) and the Citations I fly now (part 25, so more restrictive one would think) are "all electric" and have a set of standby instruments that is powered by their own battery for not more than 30 minutes
Surely however on a twin jet you have two alternators and two completely separate buses, one driving LHS systems and one driving RHS systems, with crossover switches, etc?
In that case I don't see any issue. Same in the Cessna 400 which has the same thing (2 alternators, 2 batteries) - the fact that it has only one engine doesn't change anything because you are going down anyway if that goes.
I am wondering, specifically, whether one can replace a vacuum AI with an electric AI, when having a backup alternator, IF that backup alternator is capable of driving the main bus (i.e. it is not dedicated to the AI only). If the backup alternator was dedicated to the AI only, then surely certification would be simple (to the extent that any of this is "simple").
There is a separate issue with replacing a KI256 that is acting as a pitch/roll source for the autopilot, but that's another thing.......... there is a functionally-exact but electrically powered replica of the KI256 out there, TSOd, for somebody who likes generating paperwork.
I also don't think a battery backup is a good solution for a "primary" AI, unless it outlasts the longest possible flight (8hrs).
We have a generator and an alternator. If the generator fails, the alternator kicks in. Dual independent AHARS and ADCs, but also a backup AI, ASI and Altimeter. The AI has a backup battery. The aircraft has one battery which should give you around 40 minutes of electrics should the engine fail - a bit more with load shedding.
Peter, I'm not an electronic whiz but if you're going all electric wouldn't it make sense to have a separate bus for the backup instruments, so that, if you have a generator problem, regulator or short there would be some isolation from the fault? I guess that bus2 could be actuated by the failure of bus1 or even manually switched in.
The TB20 has the supply split into 3 buses, and then these three come through to three avionics buses via the avionics master switch.
I wrote about it here and somewhere I posted a schematic.
Clearly the purpose of this split is to enable isolation of a shorted bus, etc.
The problem is which bus to use for the backup alternator. All three buses are feeding something potentially useful. That is probably because somebody somewhere has thought about it already...
One interesting point of view is that the stuff which is actually most useful to have working in an emergency (autopilot, GPS, EHSI, AHRS heading gyro, NAV1/COM1) draws very little current; under 10A. So all that could go on a separate bus. And to an extent this is already so; BUS3 has most of it.
The problem with the backup alternator driving just one of the three buses is that you now have to have a funny switching arrangement which disconnects BUS1 and 2 when the backup alternator is brought online, which is pointless if the reason for using the backup alternator is simply the failure of the main alternator! And that is far more likely than an electrical short on BUS1 or 2, IMHO.
The B&C backup alternators, when wired as per the STC, do just connect to the same point as the main alternator, so when you bring the backup online you have a choice of which loads to feed and which loads to disconnect.
There are also very small backup alternators which probably can't drive much more than an electric AI, but they are a waste of the spare vac pump drive point, when that could be driving a 20A alternator which can almost power the whole plane.
Surely however on a twin jet you have two alternators and two completely separate buses, one driving LHS systems and one driving RHS systems, with crossover switches, etc?
Not on a Cessna jet... the electric system of a Citation (or any Cessna piston twin I have come across) is really just like a C172 with a second alternator. One battery and two generators feeding a common bus. From which a single avionics bus feeds all the avionics, which is an integrated system anyway with some dual and some single components.
But you can disconnect the battery from the main bus and everything will still work, for example if you have one of the most dreaded emergencies, an overheating battery. One generator alone would enough to power the whole electric system. And if for some reason you have to disconnect both generators and the battery from the main bus, e.g. due to electrical smoke or fire, there is an emergency battery bus through which some essential items can be fed, among them COM1 and NAV1 (but with no display screens and only a very basic frequency selector). The standby instrument (AI, ASI and altimeter all on one tiny little screen) has it's own battery that will last for 30 minutes, but no COM or NAV.
Interesting news from Oshkosh – BendixKing introduces KI-300 Retrofit electronic attitude gyro to replace e.g. KI-256 :
