DA42 double engine failure
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Joined: May 2006
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From: Manchester UK
DA42 double engine failure
Thoughts on this?
http://www.flightglobal.com/articles...ngine-row.html
Evidently, if you have a flat battery in a Twinstar you should only use external power to start one engine, remove the external and wait until the other engine can be started internally or the lack of charge will cause the ECU to shut BOTH of them down, autofeather and cause you to throw an expensive piece of kit on the floor.
Obviously, know your aircraft applies but who would have thought that starting both engines on external power would result in a double engine failure?
I'm sure many of us could have been caught by this and it makes you wonder what other 'gotchas' are out there in the modern systems.
Incidentally, I still think its a fantastic 'plane but after a search I could find no other mention of this on here so I thought I would pass it on for everone's perusal and comment
Regards
Xraf
http://www.flightglobal.com/articles...ngine-row.html
Evidently, if you have a flat battery in a Twinstar you should only use external power to start one engine, remove the external and wait until the other engine can be started internally or the lack of charge will cause the ECU to shut BOTH of them down, autofeather and cause you to throw an expensive piece of kit on the floor.
Obviously, know your aircraft applies but who would have thought that starting both engines on external power would result in a double engine failure?
I'm sure many of us could have been caught by this and it makes you wonder what other 'gotchas' are out there in the modern systems.
Incidentally, I still think its a fantastic 'plane but after a search I could find no other mention of this on here so I thought I would pass it on for everone's perusal and comment
Regards
Xraf
Joined: Jan 2002
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From: Niort
This has been a cause of failure on some of the earlier Thielert conversions. Well the single flat battery issue has....
Interesting mode of failure though - surprising that the ECU goes and resets itself so quickly. However I seem to recall that the Porche conversions in Mooneys had complete dual electrical systems - presumably to make sure this didn't happen?
Interesting mode of failure though - surprising that the ECU goes and resets itself so quickly. However I seem to recall that the Porche conversions in Mooneys had complete dual electrical systems - presumably to make sure this didn't happen?
Joined: May 2001
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From: UK
Diamond have advised all 42 owners and operators of this some while back and the POH has been amended accordingly.
I agree, not necessarily what you would expect, but certainly well documented and no excuse for not knowing.
I agree, not necessarily what you would expect, but certainly well documented and no excuse for not knowing.
Joined: Feb 2007
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From: Amsterdam
Interesting. What it comes down to is that both alternators together are not strong enough to provide the current required to retract the gear. So when you retract the gear, the battery chips in and gets recharged later. Unless the battery is flat, in which case the voltage gets too low for the FADECs to keep on running.
General lesson: if your gear retracts electrically (not hydraulically) or if you have other transient electric loads that are too much for your alternators alone (de-icing comes to mind), don't take off with a near-flat battery. Unless you and your aircraft like flying with a temporarily non-functioning electrical system.
The thing I'm wondering about though is, on a DA-40 (single) there is an ECU backup battery which gets used automatically if the engine master is on, but no current from either the main battery or the alternators gets to the ECU. And this ECU backup battery is protected by a diode and relais in such a way that only ECU B can use it, and only if there is no voltage on the ECU bus (or if you pull just the right combination of circuit breakers), so you can't run this battery down. Doesn't the DA-42 have a similar system?
General lesson: if your gear retracts electrically (not hydraulically) or if you have other transient electric loads that are too much for your alternators alone (de-icing comes to mind), don't take off with a near-flat battery. Unless you and your aircraft like flying with a temporarily non-functioning electrical system.
The thing I'm wondering about though is, on a DA-40 (single) there is an ECU backup battery which gets used automatically if the engine master is on, but no current from either the main battery or the alternators gets to the ECU. And this ECU backup battery is protected by a diode and relais in such a way that only ECU B can use it, and only if there is no voltage on the ECU bus (or if you pull just the right combination of circuit breakers), so you can't run this battery down. Doesn't the DA-42 have a similar system?
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From: EuroGA.org
This seems like a good way to get a double engine failure on takeoff - that's when one retracts the gear.
It's also a good way to get a single engine failure on takeoff - in a marginal power drop situation, one ECU might reset but not the other one.
I am an electronics hardware/software designer and reckon this is very poor design. Handling power transients isn't rocket science. But you should see some of the design defects in Honeywell autopilots. No hardware watchdog at all. For some reason, the people that design electronics in aviation do the most stupid things.
It's also a good way to get a single engine failure on takeoff - in a marginal power drop situation, one ECU might reset but not the other one.
I am an electronics hardware/software designer and reckon this is very poor design. Handling power transients isn't rocket science. But you should see some of the design defects in Honeywell autopilots. No hardware watchdog at all. For some reason, the people that design electronics in aviation do the most stupid things.
Joined: Oct 2006
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From: UK
So why does the FADEC fail to OFF?
Light twin pistons are not my particular area of expertise, but other FADEC systems I have used have, at worst, failed to a fixed power setting. A FADEC that shuts down a serviceable engine in flight sounds to me like an appalling design.
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From: Amsterdam
I'm not 100% sure but AFAIK the Thielert engine simply cannot run without a FADEC. Injection timing comes to mind. So defaulting to "feather" if both FADECs fail sounds sensible to me.
Main question is why the power supply is not sufficiently robust/reliable to ensure power to at least one FADEC in case of a near-flat (main) battery combined with a transient electric load that exceeds the power output capacity of the two alternators.
Main question is why the power supply is not sufficiently robust/reliable to ensure power to at least one FADEC in case of a near-flat (main) battery combined with a transient electric load that exceeds the power output capacity of the two alternators.
Joined: May 2001
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From: England
Anyone remember the DA-40 that had both FADECs A and B shutdown just after takeoff? The FI was able to get the plane on to the runway though. It was about 18 months ago....what was the outcome of that investigation?
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From: Amsterdam
Apart from fuel gauge related problems, one problem our DA-40 had was with the various engine-related switches that are on the top left of the panel. They are very exposed with the hood open, as you enter or leave the aircraft. Apparently one of the switches (I think the flimsy FADEC Auto/B switch) had taken a hit from somebodys foot or so, and caused both FADECs to reset simultaneously, continuously. Pilot was in the circuit as this happened and able to make a safe landing. Since then, the club has put guards on each of these switches so it's virtually impossible to hit the switch themselves by accident, as you climb aboard.
Other than that, and the fuel gauge related problems, I've never heard of anything out of the ordinary with our DA-40. But I've only been flying this one since January.
I, for one, have no problem flying a DA-40 over the Channel.
Other than that, and the fuel gauge related problems, I've never heard of anything out of the ordinary with our DA-40. But I've only been flying this one since January.
I, for one, have no problem flying a DA-40 over the Channel.
Joined: Dec 2003
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From: Tring, UK
I'm very surprised that the ECU's don't have independent power supplies, such as a very small generator that runs off the engine. It seems a bit of a retrograde step from magnetos and the like which are self-powering... 
Joined: Jun 2001
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From: at the edge of the alps
I am not an electrics wizard but wouldn't it be feasible to design a supply layout that shuts everything but the FADEC (plus maybe some vital avionics) off the electrical supply if voltage drops below a pre-determined level?
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From: Amsterdam
Alpine, yes, they probably could, but how do you ever get the gear up then?
Suppose there is a too-high load on the main bus, so a relay is tripped and only the FADEC and a few essentials keep running. At what point do you decide to restore power to the main bus? And if you restore power to the main bus, the gear motor will start whirring away again, overloading the system anyway. And all the while you are flying with the gear partially hanging out. Not locked so you cannot land safely, but also not retracted so you incur a drag penalty with anything you do. Not good. OTOH, having your FADECS (all four of them in case of a DA-42) quit on you is also not a good thing.
I think there's no real solution to this problem other than ensuring that both alternators individually are up to the task of supplying enough current (by themselves) to power the gear motors. But that probably requires alternators that are too big/heavy compared to the average load they need to sustain. Remember that, although items like starter motors, gear motors and such draw a lot of current, they only run for a few seconds. So you can use the battery as a buffer. Unless its flat...
Hence the discussion between Thielert and Diamond on how to solve this. Apparently the "best" solution, given the circumstances, has been to modify the ground-power starting procedure in such a fashion that it is ensured that you never take off with a flat battery: Since you are now required (by the POH) to start the second engine on the aircraft battery (and not on the GPU), it apparently is assumed that if you're able to start that second engine, there's enough life left in the battery to power the gear motors too. Although, obviously, starting the second engine on the aircraft battery might just suck the last few electrons out of the battery anyway.
I personally still favour the solution chosen for the DA-40: an ECU backup battery which is, under normal circumstances, recharged by the alternator via the main bus. But if there is no power on the main bus and the engine master is on, a relay is tripped so that the ECU backup battery now powers ECU B. And there is a diode preventing other electrical consumers from ever using the juice in this ECU backup battery. But then again, I have no idea what the weight of this system is.
Suppose there is a too-high load on the main bus, so a relay is tripped and only the FADEC and a few essentials keep running. At what point do you decide to restore power to the main bus? And if you restore power to the main bus, the gear motor will start whirring away again, overloading the system anyway. And all the while you are flying with the gear partially hanging out. Not locked so you cannot land safely, but also not retracted so you incur a drag penalty with anything you do. Not good. OTOH, having your FADECS (all four of them in case of a DA-42) quit on you is also not a good thing.
I think there's no real solution to this problem other than ensuring that both alternators individually are up to the task of supplying enough current (by themselves) to power the gear motors. But that probably requires alternators that are too big/heavy compared to the average load they need to sustain. Remember that, although items like starter motors, gear motors and such draw a lot of current, they only run for a few seconds. So you can use the battery as a buffer. Unless its flat...
Hence the discussion between Thielert and Diamond on how to solve this. Apparently the "best" solution, given the circumstances, has been to modify the ground-power starting procedure in such a fashion that it is ensured that you never take off with a flat battery: Since you are now required (by the POH) to start the second engine on the aircraft battery (and not on the GPU), it apparently is assumed that if you're able to start that second engine, there's enough life left in the battery to power the gear motors too. Although, obviously, starting the second engine on the aircraft battery might just suck the last few electrons out of the battery anyway.
I personally still favour the solution chosen for the DA-40: an ECU backup battery which is, under normal circumstances, recharged by the alternator via the main bus. But if there is no power on the main bus and the engine master is on, a relay is tripped so that the ECU backup battery now powers ECU B. And there is a diode preventing other electrical consumers from ever using the juice in this ECU backup battery. But then again, I have no idea what the weight of this system is.
Joined: Jun 2001
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From: at the edge of the alps
A backup battery or PMA (dedicated "permanent magnet alternator" sure is the best solution, though probably heavier.
Even if the procedure to start the second engine on ship power only is followed, this would probably not prevent a FADEC shutdown during gear operation in other cases (e.g. single engine go-around with a weak battery or high current draw on the gear motor due to a mechanical gear fault - AFAIK this has resulted in complete loss of electrical power on other light A/C in the past.)
I would have supposed that there is some way to extend the gear without juice to avoid the "neither up nor down" scenario you described.
Even if the procedure to start the second engine on ship power only is followed, this would probably not prevent a FADEC shutdown during gear operation in other cases (e.g. single engine go-around with a weak battery or high current draw on the gear motor due to a mechanical gear fault - AFAIK this has resulted in complete loss of electrical power on other light A/C in the past.)
I would have supposed that there is some way to extend the gear without juice to avoid the "neither up nor down" scenario you described.
Joined: May 2004
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From: N Ireland
Is there no A/L or annunciation to inform the pilot that the battery/ batteries are not sufficiently charged for flight? Would it not be possible to electrically limit the actuation of the gear retract when the batteries are in a low state?
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From: Yorkshire
Not flown one myself but one of our group members went to pick up a Twinstar for the local flying school but found it had a flat battery when he arrived so decided not to take it. He then found out about this incident so good job!
Another thing he mentioned is that when the battery goes flat the gear extends by default so now not only do you no engines but you have massive drag as well and if over water your point of no return is suddenly several miles behind you....
Julian.
Another thing he mentioned is that when the battery goes flat the gear extends by default so now not only do you no engines but you have massive drag as well and if over water your point of no return is suddenly several miles behind you....
Julian.
Joined: Apr 2003
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From: Midlands
If you disable the remote power for start system then the aircraft is safe. If the battery is ok to start the engines you will have power for the gear and the ECU, if the battery is flat, charge it up or replace it.
Rod1
Rod1
