Help please! Lost RPM...away from home

[FlyArmy]

Situation is this, I live in junction city Kansas and just flew to Tucumcari muni in New Mexico to visit my girlfriend who stopped here for the night. A few miles out from the field in NM my GRT EIS RPM indications went crazy then went to 0 and stayed there. If I switch tach input I can get my RPM back with no issue for the other side. It was getting dark when I landed so I couldn't do much troubleshooting, and I have a whopping 15 hours of RV time and limited troubleshooting experience.

I did a mag check and only one side seems to be dropping audibly,but I can't confirm that because of my tach input reading not working on that side.

Please help if you can! Thanks.

Ryan McQ
Rv-8 n484jt o-360a1a

[rvbuilder2002]

If the GRT unit uses the P leads to obtain a pulse signal for the tach value, and you seem to get no audible RPM drop when switching to one of the mags, it sounds like one of your P lead wires probably broke at the magneto end.

[FlyArmy]

Hmm. Is that something that has to be fixed prior to flight or can it be flown 2.5 hrs back home?

[fehdxl]

From what you've wrote, my vote is to take a deep breath, enjoy the evening, fly home, trouble shoot at your convenience.

Before departing, do your normal mag check and use your ear for the mag drop. Even with the screen showing 0, you'll be able to tell if you have a real problem. Now don't start inventing things to hear...you won't ever need a tach to tell you the engine is sick--you'll know. BTW, if your curious, 14 CFR ? 91.205 says that you need a tachometer for each engine...you have that.

-Jim

[FlyArmy]

Thanks for the info. The part that was worrying me was not getting a mag drop on both mags (only one) since it happened. I'll check it out in the morning and hope it does ok on the way back. Thanks again.

[fehdxl]

Originally Posted by FlyArmy

not getting a mag drop on both mags (only one) since it happened

If you believe this to be the case, be very careful moving the prop as one of the mags could be always hot. To check for this, with the engine running at low idle (700-800ish) momentarily (like 0.2 seconds) turn the switch to off, the right back to both. The engine should audibly stop running for those 0.2 seconds. If with the switch in off, and the engine continues to run similar to doing a mag check, then for sure, check that the p-lead is securely attached...it likely is not.

You probably already know this, but the when the switch is in the both position, there is actually a break in both magneto p-lead wires. When you turn the switch to off, it actually makes the connections so that the spark from the magnetos travel to ground (through the p-lead) rather than the spark plugs, hence engine run no more. L/R does the same but only to one magneto at a time.

I say this because it's a design feature that if the p-lead becomes broken or disconnected in flight, the engine continues to run. Of course what's good for getting you safely to an airport is very dangerous once on the ground with the engine stopped.

Best,

-Jim

[tkatc]

Are you running traditional mags or electronic ignition (p-mags)?

[FlyArmy]

They are slicks.

[gtmule]

Originally Posted by FlyArmy

Hmm. Is that something that has to be fixed prior to flight or can it be flown 2.5 hrs back home?

Go East, young man....or wherever home is.....

[pierre smith]

Ryan, FWIW, I was told a story many years ago, of a guy ferrying a P-51 when the tach suddenly went to zero.... His reaction.."Oh ****. lost the motor" so he pulled the throttle back and bellied it in a dry riverbed!...and nevermind the fact that the engine still had been making noise!

Just remember, you can fly day VFR with no gauges of any kind working. Your watch can monitor fuel consumption.

Best,