gsxrpilot Posted April 12, 2018 Report Share Posted April 12, 2018 So I was going out to do some formation flying today. I started the plane up and taxied off the ramp and towards the taxiway. As I was approaching the runup area, the engine just quit. I "caught" it with the primer switch and it kept going. But if I released the switch, it would die. The 252's have the Klixon switches across the bottom of the left side panel. There is a switch for Low Boost and a switch for High Boost (it has a safety tab that has to be rotated out of the way to activate the switch). These switches are on the left side if the panel. The last switch to the right, is a Primer switch. It's spring loaded and will automatically return to the off position as soon as released. Throttle was just a bit above idle and RPM's at about 1000. I also turned on the Low Boost, but that made no difference. I quit holding the Primer switch and the engine quit. As if I'd pulled the mixture. I noticed as soon as the engine died, that it sounded like the Primer pump was still on. I turned off the Master and it quit. But if I turned the Master on, it came back on. I verified all switches were off. But the Master seemed to activate the boost pump. I got out and we towed the plane back to the SWTA shop. It's nice to be on the field with JD and his shop. JD was away but Dan was there and we started troubleshooting. He crawled under the panel with a multi-meter and determined that the High Boost switch was in the "On" position even though it was physically in the "Off" position. He disconnected the High Boost switch, and then everything worked as expected. It seems that the High Boost switch had failed to the On position. I've only used the High Boost switch once in the two years I've owned the plane and that was more than a year ago. So it basically never gets used. Evidently when it failed "On", it was flooding the engine. At idle, it was just too much fuel and wouldn't run. I immediately thought what would have happened if it had failed 5 minutes later, either on the takeoff roll or while climbing out. The guys tell me that it would have run fine at full power/throttle. But at idle power it flooded and killed the engine. After disconnecting the offending High Boost switch, the engine and plane seemed to run fine. Laura called Mooney and we'll have a replacement switch tomorrow. 4 1 Quote Link to comment Share on other sites More sharing options...
Bob Posted April 12, 2018 Report Share Posted April 12, 2018 Glad the switch failed at a good time! It is my understanding that the low boost switch should provide cruise GPH (about 10 gph) and the High boost switch should provide climb out or full power fuel flow (22-25 gph). 1 Quote Link to comment Share on other sites More sharing options...
Mooney in Oz Posted April 12, 2018 Report Share Posted April 12, 2018 23 minutes ago, gsxrpilot said: As I was approaching the runup area, the engine just quit. 12 minutes ago, Bob said: Glad the switch failed at a good time! Paul, I'm also glad it did not happen when you were airborne. How ironic is it when we are well into a major discussion about engines failing on takeoff? Quote Link to comment Share on other sites More sharing options...
aggiepilot04 Posted April 12, 2018 Report Share Posted April 12, 2018 Wow. It is bizarre to me that a switch can fail while it isn't being (and hasn't been) touched and "flip" itself in the process. I suppose it makes sense for a switch to be designed in such a way that it always fails closed/on though. Curious if that is the design, and also curious if high boost at cruise power would flood the engine. Either way, glad to hear that it happened on the ground. Quote Link to comment Share on other sites More sharing options...
gsxrpilot Posted April 12, 2018 Author Report Share Posted April 12, 2018 The general consensus is that if airborne and at say 50% or more throttle, it would have kept running just fine. Of course it would have died while chopping the power for landing. It was an interesting learning experience. 1 Quote Link to comment Share on other sites More sharing options...
aggiepilot04 Posted April 12, 2018 Report Share Posted April 12, 2018 I don't have a primer switch, but what did that do that made the engine recover? Wouldn't that be more gas? Quote Link to comment Share on other sites More sharing options...
gsxrpilot Posted April 12, 2018 Author Report Share Posted April 12, 2018 We think that with the High Boost switch on, hitting the Primer switch turned it off. They both are switched to the same pump. 1 Quote Link to comment Share on other sites More sharing options...
milotron Posted April 12, 2018 Report Share Posted April 12, 2018 Did the mechanic remove the switch and meter it in all positions? Both the primer and pump switch run the same pump, but the primer also opens a solenoid feeding fuel in other priming nozzles. I suspect there is a diode or two in there to pr cent back feeding either circuit and running the primer corrected this. It sounds like there may be a short in the wiring somewhere rather than the boost switch; it is not expected for any switch to fail to the opposite position like this. iain 1 Quote Link to comment Share on other sites More sharing options...
JohnB Posted April 12, 2018 Report Share Posted April 12, 2018 Very enlightening.. thanks for sharing. Glad you and your plane are ok! Quote Link to comment Share on other sites More sharing options...
Bryan Posted April 12, 2018 Report Share Posted April 12, 2018 Had something similar that happened on my way out to HYI. I had to stop in Longview and pickup something at Maxwells. On approach, it became very bumpy and my hand on the throttle bumped the low boost ON into final approach just bouncing around - my switches are on the right side of the yoke in the Encore inline with the throttle, starting with: High Boost (protected) Low Boost Elevator Trim (spring) Primer Well, when I pulled the throttle back smoothly to idle over the numbers, I felt the engine stumble and stop running. I continued with my flare and landed smoothly but the engine was not running. As I was coming to a rolling stop, I was trying to diagnose the failure. I checked mixture (rich), fuel selector (fullest) and anything I could think of. I had Tower in my ear asking if I needed assistance but then realized the low boost had switched on in the bumps from my throttle hand and flooded the engine. Turned it off, full throttle, lean mixture and a turn of the key fired right back up. That switch along with my Elevator Trim is now apart of my GUMP-S(witches) check twice on final. Quote Link to comment Share on other sites More sharing options...
ArtVandelay Posted April 12, 2018 Report Share Posted April 12, 2018 Your engine monitor didn’t flag the higher pump pressure? Quote Link to comment Share on other sites More sharing options...
carusoam Posted April 12, 2018 Report Share Posted April 12, 2018 Hey Paul, Thanks for the follow-up on the situation you were having. Continental’s tend to use the same pump for three functions... prime. Temporary fuel delivery... low. Low level fuel delivery to keep a cold engine running until it warms up. high. 30gph or so to keep the engine running when the primary pump fails... Sounds like the 252 gets three switches to operate one pump. The O gets two switches, and the pilot gets to decide which one to use for prime. When it is warm, no electric boost pump may be required... Running High boost and the mechanical pump at the same time has a high likeliness of flooding the engine... Check your JPI to see if it recorded how much fuel was being delivered. Keep in mind a majority of the excess fuel may have been shunted back to the fuel tank... like running the boost pump with the mixture pulled to ICO? Also note where the return line is compared to the FF sensor... Hope you can figure out what actually happened and the cause of it... and where the excess fuel went... Wow, Talk about a certified part failure... that doesn’t see a lot of use... Best regards, -a- 1 Quote Link to comment Share on other sites More sharing options...
jlunseth Posted April 15, 2018 Report Share Posted April 15, 2018 Wouldn’t it work to just pull the breaker? Three switches to run one pump, but the primer puts the fuel in a different place than the other two. Quote Link to comment Share on other sites More sharing options...
N231BN Posted April 15, 2018 Report Share Posted April 15, 2018 Wouldn’t it work to just pull the breaker? Three switches to run one pump, but the primer puts the fuel in a different place than the other two. Exactly, you would have to pull the "aux buss" breaker though as they are breaker switches. Quote Link to comment Share on other sites More sharing options...
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