Jumping Tacho

[float test]

Got a jumping engine tacho on R22.

Sat hovering with a student yesterday. Engine tacho needle just jumped straight off the top of the tacho and then back down several times. The engine then started to increase throttle (overspeed) I then shut the throttle and landed. Gov Off in idle and 104% tacho still jumping round., engine running fine.

Checked the left mag connection good, earth good. Can't get into instrument to check its sealed unit.

Anyone seen this before? Will a jumping tacho effect gov operation?

Thanks

[JCR]

Tacho

It will be the sender on the instument-nothing serious-but get an ENGINEER to do it.

JCR

[enginear]

it the points in the Mag need to be replaced

[What Limits]

OK, this is serious. We have had a number of overspeeds on our R44s where the symptoms were exactly as you describe. In fact we are investigating several of these and we think that it may be down to the quality of the signal from the points in the left magneto going to the Governor Control Box.

I suggest that you get your magnetos changed and please put in an MOR. This could become a major problem if we as customers don't get Robinson to first of all recognise that there is an issue and secondly do something about it.

We are currently in discussion with RHC but we need a lot more ammunition.

Please pm with details and a copy of your MOR.

Thanks for your help.

[float test]

I will file an MOR. The machine has done 870 hrs from new never a problem the Mags just come back from service 9 hours ago?

[staticsource]

Same happened to me last week, it turned out to be the oil seal on the input shaft to the mag. Oil was by-passing and fouling the points. A good indication is oil around the outside of the right mag.

Cheers

SS

[ThomasTheTankEngine]

Had a similar problem my self some years ago, RPM overspeed, confirmed governor was not working, turned governor off and completed flight with no further problems. Engineers replaced magneto which solved the problem.

[enginair]

jumping needle

I have seen this before . I presume tcm sb 663a was carried out tacho points . The trouble mag is right hand on engine and left on airframe . Also had a curruption between tacho and governor where 80% was indicated and aircraft was doing 100% . Just before takeoff t/r drive shaft cut tail boom off

[that chinese fella]

Couple of interesting posts that bought a wry smile to my face.....

It will be the sender on the instument

Thats usually #2 on the engineer 'quick response' to a snag. #1 is 'oh, they all do that........'

where 80% was indicated and aircraft was doing 100%

If ever there was a sign that a pilot skill set has been lost with the takeover of Bell 47's and to a lessor extent 300's, that was it. Whatever happened to using your 'ears' to hear the RPM?

I really cant believe someone could not tell the difference in RPM in that case.

[enginair]

ears

he is a mustering pilot probably had a helmut on . Twin engine pilots still take off with one in ground idle and over torque the whole show probably flew a bell 47 once .

[JimBall]

This has been covered here before and it is NOT an RHC issue. Oil in the magnetos is the culprit - either through a crack in the body or due to over zealous engineer applying oil to magneto. Either - points get fouled, signal corrupted and overspeed happens if you don't have your hand on the throttle.