Generator Regulator Replacement

The original 1960's generator regulator in CF-NZB has been replaced with a new solid state one.  It is amazing that the old one lasted as long as it did and that it failed in such a graceful manner.

I took the aircraft up several weeks ago to refresh my night currency - six takeoffs and landings at night in order to carry passengers at night.  Keeps the options open for some of the longer cross country flights we do in the summer.  However, on subsequent shorter flights, the landing gear motor's circuit breaker would pop as the gear snugged up into the wheel wells.

This is upsetting.  A person wants the landing gear to work perfectly each and every time.  The aircraft underwent several sets of gear swings while up on jacks.  Each time, on the ground, the landing gear would work perfectly.  I would then take it up for a test flight and it would pop the circuit breaker.  Sometimes.

The shunt wound motor on the landing gear is of a type that will attempt to maintain a relatively constant speed.  If voltage is dropping then the motor will pull more current as it slows.  If you stall the motor to a halt it becomes effectively a dead short and draws any amount of current which is why it has a circuit breaker.  The limit switch should stop it before this happens.



The offending original regulator - it's a beautiful artifact

During a very long flight to Saskatoon the circuit breaker popped on the way up.  I pulled the breaker and monitored the ammeter during the four and a half hour flight and noticed for the first time that the battery took a very long time to charge up - over two hours.

Upon return to Langley the battery was removed and I measured the open circuit voltage at 12.5 volts.  On these types of modern absorptive matt batteries that indicates under half charged.  And this is with the ammeter indicating a full charge!  Put the battery on a charging system and I could get it up to 12.9 volts but only just barely, and this is in a battery that is only just two years old.

So what was almost certainly happening is that the charging system in the aircraft is too low a voltage.  So I put the battery back in the aircraft and jammed my son into the back of the fuselage with a volt meter and ran the plane up.  Sure enough, it would only indicate 12.7 volts open circuit at the best of times.  The charging system should be maintaining 14.7 volts.



A view of the cutout relay windings

The maintenance shop had a spare old fashioned relay style regulator suitable for that aircraft and it brought the voltage right up.  Battery charged up to full in 20 minutes rather than two hours.  What was likely happening is that the regulator had worn to the point it could not maintain a high enough voltage.  This caused the landing gear motor to slow down and draw more current than it should.  If the battery got low enough (flying at night draws an extra 8 amps for the navigation lights) then this would slow down the motor even more, and thus draw even more amps, until the circuit breaker would pop before the limit switch shut it off.

What was also interesting is that when the old regulator was removed a couple of the point arc shunt resistors fell off - they had become disconnected.  This was almost certainly accelerating the wear on the points as these items help preventing arcing across the points as the field coil's magnetic flux collapses when the points disconnect. 

The can was also contaminated with dirt and a kind of whitish residue.  With the "newer" regulator installed the radio's were much clearer.  The solid state regulator eliminates this type of spark noise and wear.  Fits in the same spot - even the mounting holes are identical.


Solid state regulator

More of these old items are being found as I have put more hours on this airframe in the last two years than it has felt in the last twenty.  Just in the last few months we have fixed the suction regulator, unplugged the manifold pressure gauge line, and replaced the voltage regulator.  Regular flying smokes out the small gremlins and makes it a much more reliable air craft.