RPM gauge and trigger query

The size of the gap is a compromise.

Anything bigger than Zero is good.  The smaller the better the efficiency.

The compromise is that if it hits, bad things happen, and the gap is not constant.

As the temperature of the parts change, also does the gap.

So... It has been found from others' misfortunes that 0.3mm is small enough to work reliably but large enough to not disappear when the engine is Hot and running.

The first thing to confirm is the charging circuit.

The battery should measure ~12.6v with all loads disconnected.

Once the engine starts the voltage should move quickly up into the lower 14v range (14.2-14.6).

The battery does not charge unless it gets at least 13.6-13.8v and this is just a Float charge level that will require days to actually charge the battery.

If you do not see >14.2 volts, you will need to address this first.

I suspect you went way overvoltage before the fuses blew and the Tach didn't survive the event.

Be certain the "C" lead is seeing the battery voltage before starting again or you will have a repeat disaster.

The regulator uses the "C" lead to control the voltage.

If the "C" lead is floating, the regulator will try raising the voltage until it maxes out.

Many thanks BiIl

It has taken me three days of talking to all involved with this to get the facts correct, the full story and have a proper examination of the actual wiring of the aircraft (which is a home built not a factory build I might add). It has been like playing Chinese whispers! These are the facts that I have managed to eke out if folks are to be trusted...

The rpm gauge did indeed fail in flight after at least 20 minutes of normal operation and a full power climb out and the needle was observed to drop to zero.

On the ground with the engine off the master switch alone was turned on and the rpm needle was seen to rise up past the vertical before dropping back to zero where it subsequently stayed.

A faulty gauge was suspected so a brand new VDO rpm gauge was installed. This did not work either, the needle never lifted off the stop and this new gauge has not worked on anything else since (Fried)! This gauge had flown at least one hour in another aircraft so it definitely worked before being fitted in place of the suspected faulty gauge.

After the new gauge also failed all wire connections were then tested and the fuses were checked. It was found at this point that just one 20A blade fuse that served the rectifier (not the battery) had melted however, the internal metal connection had remained in tact. Big heat but no blown fuse.

The fuse was replaced and a third working rpm gauge was installed. On flipping the master switch with the engine off this rpm gauge gave the normal little 'blip'. This rpm gauge then worked fine on starting the engine. Great! The rpm was not raised above 2500rpm and was run for just a short while before shutting down.

To protect the now working third rpm gauge its wires were disconnected. The engine was run up to full power. The battery volts were seen to rise from 12V before start up to 14V and no higher after start up and during engine running suggesting the charging circuit including the rectifier is all working fine. All other instruments and avionics were also working fine.

Aircraft observations;

The earth connections all seem to be satisfactory on continuity tests but the primary earth has been wired up different to the manual. It 'should' go battery negative terminal direct to engine block (via chunky black wire) and then engine block earth to airframe. This one goes battery negative to airframe earth connection to engine block.

At the rectifier the B+, R and C outputs all feed into one common wire immediately after exiting the rectifier but all connections seem good.

There is no capacitor or over voltage relay between the rectifier and the fuse box (this is an optional installation on this aircraft type). Next stop master switch (twin pole switch shared with battery feed) then instruments via a 2A fuse. The engine T's and P's instruments all piggy back a common power feed and earth including the rpm gauge.

Questions;

Is it possible to have an intermittent fault in a rectifier that sends out an unregulated burst of AC into the charging circuit?

Is it possible for the connections in the fuse box to 'arc' allowing current, or voltage to flow across the connectors but bypass the fuse itself (hence the plastic body of the fuse melting but the internal metal connection remain in tact)?

On the face of it is working fine now but some 'event' has happened that has caused big heat and a bypassed fuse plus potentially two blown rpm gauges. I need to try and fault find to reduce the risk of it happening again. Rpm gauges are expensive and a cockpit fire even more!

This aircraft has logged 250hrs and has never had any wiring issues thus far. Something seems to have gone 'pop' but I have no idea what.

Any my ideas of where to look next would be greatly appreciated

Having read some other posts and thinking back over the recent history of the running of the aircraft...

Is it possible to cause damage to the rectifier by 'jump starting' with either an external 12V battery or a jump start pack an aircraft battery that has been significantly discharged as a result of the master switch being left on?

I have been told a voltage on the aircraft battery was observed as low as 9.5V prior to jump starting with a jump pack unit. The aircraft was then flown for for at least 45 minutes but the battery failed to turn over the engine satisfactorily on an engine start attempt just a few days later perhaps indicating that it hadn't been sufficiently charged during its flight? It is cold in the UK now so recent cold start ups have been in sub 5degC temperatures.

Basically can a voltage surge as a result of jump starting be sent 'in reverse' from the battery and through the rectifier (via the in line master switch and fuses) and cause damage to any of these component parts en-route?

BATT to FRAME to BLOCK is OK as long as it connects to the frame at a SINGLE Point.

This makes it easier to disconnect the Battery Ground for Maintenance without getting close to the Batt POS.

The melted fuse holder might be because there was a 20 Amp fuse in a 15 Amp holder.

Or it just may have been a bad connection.

The Capacitor is NOT Optional!

It stabilizes the regulator when the Master Switch is OFF (or the Fuse Opens) with the engine still running.

If that fuse had an intermittent connection, the regulator would go crazy trying to maintain 14v with a wildly varying load.

Fuse connection Opens; Voltage goes to 40v.

Fuse connection returns; 10v Volts dumps into load and returns to 14v.

It might do this 700 times a second and beat the poor Tach into submission.

A Capacitor would absorb the surges and saved the day.

Yes, the system ran for years without any apparent problem until the day came that you wish you had taken out (Installed) that Surge Insurance policy.

Jump starting is generally a bad idea.

It is better to charge the battery first.

With a jump start, as soon as the engine starts the generator is looking at dead battery and goes Full-Out trying to get the voltage up to 14 volts NOW!

Is might be what melted the Fuse holder.

A 9-volt battery is a DEAD battery and should be proven reliable before resuming flight.

Consider the outcome if you had needed the battery to restart the engine while in the air.

Charge the battery overnight.

Remove from the Charger.  Voltage >13 ???

Wait 12-24 hours.  Voltage >12.5 ???

Crank the engine for 10 seconds with the Mags OFF.  Actually time it!

Then start the engine.

If it won't pass this test,  Time to replace it.

Many thanks Bill for such a comprehensive answer! Your description is making a lot of sense given the aircrafts recent history and you have increased my understanding of the systems much more also.

Regarding the rectifier is there any way of testing to check if it is working properly or if it may indeed be faulty? What symptoms may be indications of an intermittent fault? I will certainly encourage the owners to install a capacitor but how do we know that the rectifier isn't still hiding problems to cause the same issue again?

One of the owners has checked the fuse box itself and this is tested to a maximum amperage of 30A and is designed for use on kit cars. There is no evidence of any damage to the housing or the contacts that he could see. I see that Rotax advise a 50A fuse for the battery. Why is such a high amperage required?

You mentioned if the fuse opens the voltage goes up to 40V. Is this 40V the voltage that is generated by the internal generator at the coils? If the rectifier fails to regulate the voltage what sort of current may be created to pass through that fuse? Is it current that blows the fuse not voltage (I am struggling a bit with my memory of physics here!)?

Lastly regarding the earth connections, I believe this aircraft relies on TWO points of contact to the airframe if my understanding of what you say is correct? The battery chunky earth wire connects to the airframe on the cockpit side of the firewall via one of the mounting bolts for the rectifier to the aircraft subframe. The earth must then travel via the subframe to another bolt mounted to the subframe on the engine side of the firewall. A smaller diameter wire then connects from the subframe bolt to the primary engine earth bolt on top of the engine crankcase. By ONE point of connection are you meaning that the earth should NOT be required to travel any distance at all through the aircraft subframe before connection to the engine block (ie. The earth connection should be in one side and out the other of one single bolt)?