Both of these accidents were similar, in the electrical problem caused the CB to trip, killing the engine.
That is a sad correlation. On this thread:
Phase One - Safety there are nine T-51 crashes documented. How many airframes are flying? I wonder how many of those are sudden stoppages with similar failure chains. Has anyone been counting the causes of them, looking for common threads, etc. If there is some hardware or wiring scheme or other issue routinely applied to T-51's that is resulting in sudden engine stoppage, it would behoove all of us to identify it, figure out fixes, notify the community of it, and stop wrecking these airplanes.
That sounds like there's a serious design flaw in the layout of the electrical system. The engine should still function no matter what else dies.
First I am worried over the fleet of T-51's. I support finding the source or sources or these shut downs, fixing them, and letting the community know how to greatly reduce the odds of having these failures. The devil is in the details. Detail needed here is data on the planes out there. Why did the crashes occur to them? Problem Solving has been around a long time, and bunch of recognized tools exist that all need DATA. Without data, we are guessing. Let's gather some data before we go leaping to conclusions:
- Is the rest of the fleet built with the same system details and/or schemes and thus equally vulnerable? This is Search for Causes in Common;
- Is any particular hardware or wiring schemes or other functional issues common to the crashes and not to the uncrashed? This is an Is-Is Not;
- Was there a cascade of issues leading to engine failures? This is Failure Mode Analysis;
- There are more and more will keep being invented...
Yes, single thread power to EFII is vulnerable, and redundant power sources, properly configured for high reliability, and automatically cascaded so it comes up without a lot of troubleshooting is a good thing. Is that the source of the other accidents? We do not know. So, anybody have any real data on other crashes or for that matter on configuration of any T-51's out there along with any unplanned engine shutdowns?
Going from worrying over the fleet of T-51's to worrying over TXFlyGuy's T-51...
As stated here, my system is a dual battery, dual bus system with the Efii Bus Manager.
The system itself is fine.
Given the data we have here, I must contend otherwise:
- The engine cutout on takeoff;
- Something was drawing a lot of power and no one has said yet what that something was or that it has been fixed;
- Something caused a 15 amp CB on the ECU circuit to pop, and the source of that high amperage in that circuit has yet to be identified and fixed;
- The only item reportedly changed is HEGO deletion, but TXFlyGuy says the HEGOs are self limiting on current, so the HEGOs either are not the high draw OR the HEGOs can draw big power. This would require some testing to check before believing you have already got the root cause of the high power draw;
Without the source definitively identified and definitively fixed, this failure is likely still out there waiting to cause the same trouble again. In addition, if one problem was built in, you may have more. Seriously, go looking for more, and then test extensively on the ground before lifting wheels again;
[/QUOTE]Yes, there was an error in the wiring of the O2 sensors with the associated relay. The relay failed to do it's job.[/QUOTE]
IIRC we were told that the Bosch HEGO's shut themselves off if current exceeds some nominal amount. That would preclude the HEGOs from drawing enough power to be our problem, and deleting the HEGOs does not seem to constitute fixing the source of the shutdown. That or the HEGOs in question have been checked and do draw big power under flight conditions, in which case you have closed the loop on HEGOs sucking the batteries dry. But both can not be true simultaneously...
Not to beat a dead horse, but my electrical system was rated at 40 amps max. On the crash flight, the amp meter was pegged at 50+. The ECU CB is 15 amps. It did not require any brains to predict what was going to happen.
I am concerned that you have way more confidence in the electrical system than it has earned... This is your airplane, and your butt will be riding in it. Please be skeptical of the electrical and EFII stuff.
Let's do a "for instance" check on the charging system concern. Let's say we had two 24 Ah batteries, that is most likely a little less than 48 Amp-hours of actual stored electricity. For argument's sake, let's also say 80 amps were being drawn while the alternator was putting out all it could. The alternator is fine with this and so are the batteries, except that they will eventually run out of power. With reasonable batteries, it would still take a half hour (48 Amp-hour/80 Amp is 0.6 hours) of running to draw the battery down and voltage to sag away towards shutdown of electronics. I suspect it took a lot less time than that to get airborne, but maybe the battery capacity is a lot lower than we are postulating. We might still have bus voltage data stored on the EFIS or engine monitor of bus voltage, and may be able to check out this path. I strongly suspect that ECU shutdown did not happen due to a power deficit in the airframe, but data will tell. Maybe the batteries have gone soft and the failure was drawn down batteries not an opened CB.
A circuit breaker opening has been cited as the circuit cutout to the ECU in the accident. Do you think this is what happened or do you know it from solid evidence? ECU CB open at the crash site with no others open would qualify nicely, but was it open? CB's respond to amperes passing through them in the very recent past. Modest overloads take a long time to trip, larger overloads take less time. The CB protecting the wiring to the ECU won't open because battery voltage is sagging or spiking or even melting down and leaving the airplane. If the CB was open at the crash site, replacement is a good idea, but more than that, why did it open? Usually the answer is current through it went high. Until you know why current through the ECU circuit went high and fix it, you are just waiting for a repeat of this event.
An alternate theory is that ship's power was doing strange stuff in response to the overload pulling on the batteries. Hmm. What scenario would make the CB on the ECU circuit pop? CB's and batteries are very resistant to spikes. Batteries will smooth out the spikes and CB's listen to current through them over time, usually by warming up a resister. The black box bus maintainer is another story. Something else likely had to cause the CB to open.
Another theory can exist that the Bus Manager has issues - it is a Black Box, we do not know what is inside it or what faults or undesirable interactions it might have.
Main battery contactors require amps to hold it on, but if the battery was drawn down enough to open the contactor, the ECU was probably out of the game too. Bob Nuckols is telling us that none of us should be powering our ECU solely from the switched side of the battery contactor. Got that? That is what always-hot battery buses and diode bank switching is for. Maybe that function is carried by the bus manager, but you would have to check that path too.
We will look at a switchable battery backup, wired directly to the ECU.
Maybe your troubleshooting brain can manage the airplane and get all the way to engaging backup ECU power aft the engine sags away at 200 feet, but most of us will already be on the ground before we can get that far. I much prefer power to run the engine automatically cascade and throw an alarm I can deal with once we get some safe altitude. Lots of good options exist to do this. AeroElectric Connection is a worthwhile read. There are multiple ways presented to making sure that the ECU is powered despite individual system faults.
Billski
