Notice that this backup battery (AND your aircraft battery) should last at least 30 minutes after failure.
This is a worst case situation. This is why most batteries would last longer, as it is required to do 30 minutes at extreme cold.
- You would want a battery that is in good condition, see maintenance instructions and perform a good (SLOW) capacity check according the battery maintenance instructions. Not performing these test is faking yourself IMHO.
- Alternator maintenance should be performed. Some alternators manufacturers say that their products can go for many hours before the brushes are worn. On most alternators worn brushes will damage the rotor slip-rings. Brushes wear faster under load. I would advise to have brushes checked at maximum 500 hours interval, even in the manufacturer says it will last longer.
Worn rotors on 600-1000 hours are quite common, and expensive, as in most cases it would also require a new rotor.
- A good indication on alternator performance is required. An AMP meter is used on some aircraft, but quite useless. The only good thing is to check the voltage output, that would indicated if the alternator is functional, and will indicate partial alternator failure (1 stator coil, 1 diode for example). Especially on older aircraft this is understimated.
It is not uncommon on older aircraft that an over or under voltage goes undetected untill equipment starts to fail. At this point the 30 minutes reserve has been wasted. The UK CAA has adressed this in CAP 747.
- Reduce load as much as possible, to extend the 30 minutes as far as possible.
I have just been looking at the specification of my new Tecnam, which is hopefully arriving soon. They have added this as the backup display. http://www.flysam.com/
fully charged it can run for 2 hours and is guaranteed for 1 hour.
I was looking at this with the back up battery.
http://www.kellymfg.com
only do so if I can have an attitude system with a battery that will last longer than the fuel in the aircraft
I agree but it isn’t possible with anything other than either another alternator, or an absolutely massive (infeasible) battery IF you want the pitot heat to work.
If you aren’t bothered about the pitot heat, or would be happy to turn it on only at the very end of the flight (would that work – I have no idea?) a smaller (possibly lithium) battery ought to power a COM+NAV (e.g. a KX165A) with a KI204 CDI for long enough. It would be easy to power the radio+CDI from a battery, via a switch.
I agree with the comments above about 30 min not being enough time, it has to be remembered that the 30 min on an airliner is a last ditch measure that will only happen after three generators five or so protected bus bars and a battery have failed. Added to this in the airliner you can get a lot further in 30 min.
I am looking at getting rid of the vac pump but would only do so if I can have an attitude system with a battery that will last longer than the fuel in the aircraft, also I would be looking to split the main bus bar to protect the attitude reverence and one NAV/COM from a main bus bar failure.
Peter, where the MEMs is certified for installation inside a climate controlled environment (I.e cockpit), unless the cockpit heater/furnace has failed or is off, surely the heat generated is the difference between min vs calibrated. It shouldn’t, in theory, generate enough to heat the displays unless the cockpit heat is off? Besides Nine-axis integration via Kalman Filtering can eliminate much noise causing drift.
If this is legal with a part 25 aeroplane, then I bet it will also be allowed for a part 23 one
I don’t doubt it is legal, but that doesn’t mean it is sensible.
In your plane, WN, you have 2 alternators, etc. The chances of you needing this at all are very very low. In mine, the chance of needing it is merely the combination of two events
which is a lot higher. I rarely fly in IMC enroute but it is normal to descend through it, sometimes through a lot of it, on the way down.
That is why I was looking into the backup alternator but that has been shelved because to save about $10 Lyco put in a dummy plug into the extra vac pump drive, and there is no practical way to fix that short of the engine coming right out.
If I was using an electric horizon to replace the vac driven one I would want a 5 hour battery life at least.
while the chips used in AHRS units / OLEDs are different from those in mobile phones
Well yes AHRS units are several generations behind mobile phones
This is because the gyros drift like hell with temperature, and the drift is too bad to compensate for simply by sensing the temperature
Well yes, that’s why you also sense the g(ravity) vector to “erect” the MEMS gyro. Really just like a mechanical gyro. (and yes, it makes sense to have an online estimate of the gyro bias…)
IMHO, a horizon which works for only an hour is no good. It means that if you lose your electrics, in IMC, you really do have to look for a place to land ASAP.
Well, our standby instrument (on a transport category aircraft!) is guaranteed to last 30 minutes. The two main gyros behind the screens are both electrically powered. Thus, in case of a total electrical failure, most likely due to deliberate shutdown because of smoke in the cabin, you have 30 minutes to find a suitable place for putting down your aeroplane. And suitable means among other things twice as much runway as usual because without electricity there won’t be neither flaps nor speed brakes nor thrust reversers. If this is legal with a part 25 aeroplane, then I bet it will also be allowed for a part 23 one. I am told the battery life of the standby instrument is limited mainly by the heater of the standby pitot tube which can not be turned off. The solid state gyros and the small LCD screen consume next to nothing.
