larryb Posted August 7, 2019 Report Posted August 7, 2019 Many of us have had trouble with the flap relays. I had a problem with the relays when I bought my Encore, and the problem has re-surfaced 2 years and 200 hours later. Most of my trips are 1 to 4 hours long, so not many flap operations in 200 hours. Others posting on Mooneyspace has also reported repeated failures. After removing the failed relay from the board I cut open the case and see visual evidence of burned contacts. The relay was stuck in the NC position. I have reviewed the design more critically and find that there are two design flaws likely causing poor life. Fortunately, these are easily corrected. The first flaw is that there is no transient voltage suppression on the motor. When the relay switches off the motor generates a large transient voltage, and this causes arcing of the relay contacts. This is the same concept as the relay coil, which does have transient protection. This can be solved with a BIDI TVS diode across the motor leads. The second flaw is actually the protection diode across the relay coil. This diode is there to protect the various switches in the flap system from voltage spikes. But it has the side effect of slowing down the relay release time which causes more arcing of the relay contacts and contributes to reduced relay life. This can be fixed by replacing the existing diode with the same TVS used for the motor leads. This will allow a faster contact release while still protecting the switches from excessive transients. According to the app note below, relays are spec'd with no coil suppression. I did some bench testing comparing a TVS vs. diode across the coil. There is a clear audible difference in the relay release. Much more forceful release with the TVS. Proposed TVS:https://www.mouser.com/ProductDetail/Littelfuse/15KE43CA?qs=sGAEpiMZZMssaSaMl4au6Cg5qTlymwuc Coil suppression app note:https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3264_AppNote&DocType=CS&DocLang=EN 3 Quote
PT20J Posted August 7, 2019 Report Posted August 7, 2019 Sounds reasonable. Now, is that a minor or major alteration? i have a 1994 J with 1300 hrs. on it that has never had a problem. (Now I’ve probably jinxed it). Previous ‘78 J had 1600 hrs. without a problem. Wonder why some do and some don’t. Quote
kortopates Posted August 7, 2019 Report Posted August 7, 2019 Very interesting and thanks @larryb for sharing this! Like Skip though, I have not had a problem in my '86 252/Encore with 16 years of ownership. But I'll remember this if/when I do. And I highly encourage you to share your findings with Stacey at the factory. I am thinking it wouldn't be that hard for them to create a SB which will also tell us which models/serial #'s (if not all) would the changes be applicable too. 1 Quote
DonMuncy Posted August 7, 2019 Report Posted August 7, 2019 Is this problem and fix applicable to all Mooneys? Any kind of a guess as to what the failure rate might be? Ie. would we all be well advised to go to the trouble? I keep a battery minder on my battery under the theory that, even though I can't prove it will pay off in extended battery life. I keep a engine dryer on my plane under the same theory. Is this on the same level of concern? Quote
larryb Posted August 7, 2019 Author Report Posted August 7, 2019 I never had a problem with my '84 J in the 500 hours that I owned it either. But in that plane there was no take-off position pre-set like there is in my '97 Encore. I have seen several versions of the relay board posted here on Mooneyspace, but I have only analyzed in detail the one I have. I would expect the fix would apply across the board however. Quote
carusoam Posted August 7, 2019 Report Posted August 7, 2019 Something to keep in mind... 1) the relays are rated for a huge number of cycles... 2) one of mine has failed early in its potential life... 3) Something has probably killed it... something not expected by the relay designer... 4) Makes the TVS thing sound more interesting than ever... Nice details Larry! Best regards, -a- Quote
PT20J Posted August 7, 2019 Report Posted August 7, 2019 According to the M20J IPC, the relays changed part number when the preselect feature was added. Not sure if that has any bearing or not. I’ve also seen a thread here somewhere where someone noted that one of the relays can chatter if the flaps are in takeoff position and then pressure applied as might happen with an air load if the microswitches are not set up perfectly. Skip Quote
thinwing Posted August 8, 2019 Report Posted August 8, 2019 Nice work..not sure of the legality of adding the diodes... Quote
N201MKTurbo Posted August 8, 2019 Report Posted August 8, 2019 If the hangar elves were to replace the relays and add a few components, I doubt anybody would notice. In the long run, perhaps you could work with Laser to get it STCed and marketed. They seem to know the process. Quote
larryb Posted August 8, 2019 Author Report Posted August 8, 2019 A couple more points. The relays have a rating of 10A for the NC contact and 20A for the NO contact. We must consider the lower of the two since both are used to carry motor current. Those amperage ratings are for a resistive load. For a motor load industry practice is to de-rate to 20% of capacity, or just 2A in this case. I am not sure what the motor current draw is but my circuit breaker is 5A. The bottom line is the relay specs are marginal in this application. These relays are sealed but do have a vent. The vent must be opened when the relay is installed. Failure to do this can also reduce relay contact life. The reason for the removable vent is to allow automated assembly and circuit board cleaning without getting the cleaning fluid inside the relay. The product is assembled and cleaned and then the relay vent opened. I am not sure if the factory does this or not since my factory relays are long gone. http://literature.cdn.keysight.com/litweb/pdf/5988-6917EN.pdf https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C9136_Venting_Sealed_Relays_AppNote&DocType=CS&DocLang=EN Quote
Yetti Posted August 8, 2019 Report Posted August 8, 2019 I think that these relays and the Comm buss relay are undersized for the load. Seems like the better fix would be to just build a better board with bigger relays. Quote
Steve W Posted August 8, 2019 Report Posted August 8, 2019 23 minutes ago, Yetti said: I think that these relays and the Comm buss relay are undersized for the load. Seems like the better fix would be to just build a better board with bigger relays. Along with the stupid AP Disconnect on the KAP/KFC devices. Although a TVSS might help there too... Guess I should add dragging the scope out to the plane to the list of 32,768 projects on the list. Quote
Yetti Posted August 8, 2019 Report Posted August 8, 2019 1 hour ago, Steve W said: Along with the stupid AP Disconnect on the KAP/KFC devices. Although a TVSS might help there too... Guess I should add dragging the scope out to the plane to the list of 32,768 projects on the list. ya but that old junk can be upgraded with some nice glass panels.... Quote
PT20J Posted August 8, 2019 Report Posted August 8, 2019 If the relays are grossly underrated, I would expect most of them to fail in service. But most seem to be trouble free for many, many years/hours. A failure within 200 hours of 1-4 hour flights is a failure after maybe 100 operations of the relay. Something else is probably going on. Quote
Yetti Posted August 9, 2019 Report Posted August 9, 2019 35 minutes ago, PT20J said: If the relays are grossly underrated, I would expect most of them to fail in service. But most seem to be trouble free for many, many years/hours. A failure within 200 hours of 1-4 hour flights is a failure after maybe 100 operations of the relay. Something else is probably going on. A certified mechanic "fixed" my com relay. Took it out and connected the wires together.... See fixed. Quote
MikeOH Posted August 9, 2019 Report Posted August 9, 2019 3 hours ago, PT20J said: If the relays are grossly underrated, I would expect most of them to fail in service. But most seem to be trouble free for many, many years/hours. A failure within 200 hours of 1-4 hour flights is a failure after maybe 100 operations of the relay. Something else is probably going on. That's my exact take on this, as well. While the TVSs sound great, I don't think that's the fix for just the reason you state. Based on a bad experience replacing a fuel pump relay in a car several times, I determined there is a HUGE difference in the quality of relays (i.e. the replacements are no where near the quality of the OEM part, or even the OEM part is now sub-standard). First thing I'd do is pull apart the factory relay and compare it to the short-lived replacement. Differences can be subtle, too. The type of plating and its thickness used on the contacts themselves, for example. Quote
Releew Posted August 12, 2019 Report Posted August 12, 2019 Any relay will fail and fail quickly if subjected to chatter. Its not like a switch contact. If you have any of the switches in the circuit working when the flaps are static, in the take off position simply pull up on the flap to verify the switch position by inducing aerodynamic loading. in the plane and in the air you can't hear it or feel it, but if your switch is slightly out of position you WILL fry the contacts in the relay and will not be able to retract the flaps. That SUCKS..... because the only way to get back to neutral is to remove the belly and turn that flap coupling by hand about a thousand times!!!!! All that just to get home!!!! Rick Quote
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