Alternator failure - checklist discussion

[Andy]

The other question I have on this topic is what are the real loads and what is the alternator really supplying?

I was curious too so I added alternator current monitoring to my FX-3. For my configuration in flight with strobes and wig/wag landing lights, and after the battery has recovered from cranking, the alternator current is about 11 A and the battery charge current about 4 A.

Your alternator load will be maybe an amp greater as you have the second COM, remote audio panel, and GNC 355 instead of GPS 175.

[hawgdrvr]

I was curious too so I added alternator current monitoring to my FX-3. For my configuration in flight with strobes and wig/wag landing lights, and after the battery has recovered from cranking, the alternator current is about 11 A and the battery charge current about 4 A.

Your alternator load will be maybe an amp greater as you have the second COM, remote audio panel, and GNC 355 instead of GPS 175.

It's odd that the G3X doesn't show that current for the ALT. Why does it only show what seems to be battery charging? We aren't seeing the real load on the alternator.

[Andy]

It's odd that the G3X doesn't show that current for the ALT. Why does it only show what seems to be battery charging? We aren't seeing the real load on the alternator.

CubCrafters uses one current sensing shunt and it is measuring battery current. You could add a second shunt, or a Hall effect current sensor, if you wanted a direct reading of alternator output. You could also re-wire the shunt to measure alternator current instead of battery current.

The hard part of the mod to add current sensors is working on the GEA 24 connectors.

[kshunz]

Here are voltage traces to go along with the trend plot I posted previously. These are all from the same battery, spread out over about 13 months. They were all at approximately 1700 RPM. I presume the current varies linearly with RPM. I hadn't really thought about that in terms of the voltage drop until now, but I can't imagine my small variations in RPM could account for the spread in voltage drops I'm seeing.

ignition_voltage_drop_traces.jpg

[hawgdrvr]

I'd like to know what the current draw is (amp hours) on 2 x lightspeed ignitions, assuming both remain active in emergency mode.

Then we can deduct from the amp hour size of the battery how long is should last.

My understanding though is when that battery dies it then goes to the starter battery?

[Andy]

Here are voltage traces to go along with the trend plot I posted previously. These are all from the same battery, spread out over about 13 months. They were all at approximately 1700 RPM. I presume the current varies linearly with RPM. I hadn't really thought about that in terms of the voltage drop until now, but I can't imagine my small variations in RPM could account for the spread in voltage drops I'm seeing.

Thanks for preparing those plots. Can you reduce the data density by showing only plots for new battery, mid life, and oldest. I think that would show any trend better.

One thing I noticed about your plots is the quite large variation in voltage before the tests start. My ignition battery usually runs about 0.1 V less than Main bus and is almost constant at 14.0 to 14.1 V. (I would have expected ignition battery to be 0.2 to 0.3 V lower than MAIN due to the diode drop but none of these voltage readings is calibrated and they probably all have at least 0.1 V error.)

I have not attempted to analyse the ignition module current. Like you, I expected current to increase with rpm but it isn't zero at zero rpm unless the module is not enabled by the ignition switch. In a spot check of a recent log I saw -1.3 A at 11.9 V and 1,930 rpm for one test . In another log I found -1.3 A at 12.1 V and 1,810 rpm.

Maybe I'm curious enough to run an rpm sweep while running on the ignition battery.

[Andy]

I'd like to know what the current draw is (amp hours) on 2 x lightspeed ignitions, assuming both remain active in emergency mode.

Then we can deduct from the amp hour size of the battery how long is should last.

My understanding though is when that battery dies it then goes to the starter battery?

Only the RIGHT ignition module can be powered by the emergency ignition battery. It will draw no more than about 1.5 A.

When the emergency ignition battery is exhausted the RIGHT system stops producing sparks.

Nothing automatically reverts to the main aircraft battery but the LEFT system will continue to use it if the switch is on BOTH, the battery is not cut off (EarthX), or discharged.

When attempting to deduce battery useful time from current and AH rating you must consider that the battery AH rating is typically for a load of 0.1 C. (2 AH battery rated for discharge at 0.2 A for 10 hours.) The ignition system draws about 0.6 to 0.7 C so the battery will be exhausted far more quickly than the AH rating would suggest.

I have demonstrated 30 minutes engine run time, in cruise flight, on a good ignition battery. The problem is that they don't stay good for very long.

[cavok84]

Thanks for the comments.

I knew it was too goo to be true that I had anything to contribute

I used the checklist editor as a convenient way to record my thoughts and to have them available for review in the aircraft. I think I know the aircraft systems well enough to shed non essential loads without referencing it.

I'm sure you do; it was more a statement about my thoughts regarding new owners or checklists in general

y IBBS useful time and my emergency ignition useful time are based on actual discharge tests. I have observed an IBBS in good condition power my G3X system for over 3 hours. 2 hours is quite conservative. My low confidence in the ignition backup battery is also based on test data. I plan to replace the ignition battery with a second IBBS.

Interesting, I had no clue. What about the backup battery is lacking? The useful time? When I test my ignition, the backup does show a pretty rapid loss in voltage when under load, so it wouldn't surprise me. I also would be completely fine with trading the IBBS for an additional backup battery.

We seem to disagree about the use of the emergency ignition battery and that's good because it's a subject I'd like all EX and FX owners to think about. I run on the ignition battery for about 10 seconds at every run-up. I periodically plot the data and the loss of capacity with time is very obvious.

yeah i've noticed this with mine too. I wasn't sure if voltage drop would stabilize or if it was indicative of several reduced capacity.

The only thing I really disagree about is the use of the inferior source of ignition power being depleted sooner rather than later (if time is an issue). I'd rather use the less reliable source when I have plenty of altitude and airspeed, rather than rely on it close to the ground, should that be necessary, but as we'll get to, you've corrected my system knowledge regarding a few things.

Master does not control the ignition, ignition is battery direct. Each ignition module is fed by a circuit breaker on the front of the seat pedestal. Unless those breakers are pulled the ignition modules always have power. However, the ignition modules draw no current if the ignition switch is off.

You're completely right... I don't know why I said that...

Selecting emergency ignition at this procedure step tells you nothing except the the LED is working as the engine power does not change when RIGHT is on emergency but LEFT is still powered and On. Remember that the emergency ignition test during run up requires the ignition switch to be on RIGHT only.

But I believe with the ignition in "BOTH", and the emergency switch ON, the right is disconnected from the main battery and does use the backup battery. Your point is that, unless you turn the key to R, with the MASTER OFF, and EMER ON, the left ignition is using the main battery and the right is using the backup battery--masking any deficiency within the backup battery (and using both your batteries at once). Is this correct?

MASTER OFF automatically activates the IBBS as long as it was enabled. That's much simpler than forcing IBBS use when MAIN bus is above the IBBS activation threshold of approximately 11.5 V. However, it has the disadvantage that COM and transponder (ADS-B in and out), lights, strobes, are also lost. I'd prefer to spend a few minutes deciding what I can manage without, rather than rush to kill everything.

Fair enough. I suppose it's very situational, and given YOUR knowledge and time in the plane, it is unlikely to take you much longer than just flipping the switch.

Monitor backup Battery voltage - this won't change as emergency ignition has not been selected on. If there was a procedure step that said Emergency Ignition On then right ignition is fed by the emergency battery and left continues to run on main battery.

This was written when I thought turning off the master would do the trick. I don't know why I thought that because I knew it wouldn't do that at one time, and obviously you have the breakers... I just got back from Japan, so I'll chalk my brain fart up to that. I should've slept on your question before posting.

However, given the steps I listed--backup battery would decrease since I listed running on the EMER backup with Master OFF.. In this case it would decrease and so would the main battery... you'd be using both batteries (one for each ignition system)

I suppose my idea could still work IF:
(MASTER OFF)
IGNITION R
BACKUP BATTER ON

This would kill the left ignition system and run the right from the backup battery

however, I don't know if I'm a fan of this because if you forget to go to BOTH, the engine will die when the backup dies..
It's an extra step that could spell trouble..

I need to think about it. I guess I'll need to edit my post. Personally, I still like the idea of load shedding with the MASTER until you have your airport picked out, but like you said, you lose some equipment that would be nice to have.

I'll get back to you.

[cavok84]

CubCrafters uses one current sensing shunt and it is measuring battery current. You could add a second shunt, or a Hall effect current sensor, if you wanted a direct reading of alternator output. You could also re-wire the shunt to measure alternator current instead of battery current.

The hard part of the mod to add current sensors is working on the GEA 24 connectors.

I wondered about this a few months ago when my amps were periodically going from 2-4 down to 0 and eventually -1, in flight.

Occasionally, the strobe switch would trip. After turning the strobes OFF, the issue stopped. I disconnected the tail strobe as an initial guess, and this corrected the problem (no more tripping and no more reduced BATT charging).

However, for the first time in 6 months or so, I noticed the same issue yesterday during runup. At 1000 rpm, the amps were fine, approaching 1800-2000, the load would drop to 0, and the battery remained around 13.8.

I flew today and did not see the issue a single time. Given the corrective action I saw with disconnecting the tail strobe (and the fact it has addressed the issue thence), I'm puzzled why I would have seen it happen yesterday. The strobe switch never tripped, though.

I'm going to replace my tail strobe at the next condition inspection (November), and my mechanic has suggested replacing the strobes CC uses, as this is a somewhat common issue he's seen on their aircraft. This probably belongs in another thread but any brief thoughts?

[hawgdrvr]

I would pull the three strobe assemblies and just check the crimp connectors as a start. Crimps/connectors seem to be the most common issue in these planes.