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Never disregard indications of engine trouble

This was detectable 1 or 2 flights earlier, in abnormal EGT indications during the (normally very brief) engine idle test, after the 2000rpm prop + mag check and just before takeoff.

A lot of engines do weird things at low rpm but they are not supposed to.

Shoreham EGKA, United Kingdom


This post looks lonely so I thought it needs support! About 6 months ago I saw a very odd EGT variation on departure
at about 1000’ on one particular cylinder in my aircraft. I reported the cylinder to my Workshop and asked them to check
the plugs on that cylinder.

Result was that one of the plugs in the cylinder was over 4 years old (my current workshop don’t use that type and
hence they knew it was over 4 years old as they have been doing the maintenance during these four years).

I told the workshop not to bother testing it and put a new one in and just test the other one. Result all OK since.

Interesting thing was that the power check pre t/o on the flight in question was fine and the engine seemed normal on
departure and subsequently. Nothing would be known except by looking at the EGT/CHT data.

The workshop also reported that there were still a couple of old plugs in the 8 and I told them to throw those away at the
next annual.

I try to have a look at the EGT/CHT data and other parameters before flying another day.

I fly about 100-110 hrs a year, (t/o to landing).

EGKA, United Kingdom

I had a similar trend indication, with the charging system on my aircraft, just recently. About 10 hours ago the low volts warning light started illuminating with engine rpm lower than 1100rpm instead of the usual 700 rpm. As the Annual was only 6 weeks away I made a mental note to ask the engineers to check the alternator output volts and maybe adjust the voltage regulator.
Then on a recent flight, just after take-off and having retracted the gear, the low volts light came on and the ammeter dropped to zero…an obvious alternator failure.
Inspection showed that the alternator field wire had broken loose from its terminal, easily fixed with a new terminal properly crimped and with a touch of solder applied to the wire tails. The offending wire must have been slowly working it’s way loose until it failed completely, and it was quietly showing me it was going to.

You can have all kinds of subtle fun with alternators. I had this and this and I bet most people would not have noticed either of these. The 2nd one would have been pretty serious if the casing broke in flight.

Shoreham EGKA, United Kingdom

Marchettiman wrote:

About 10 hours ago the low volts warning light started illuminating with engine rpm lower than 1100rpm instead of the usual 700 rpm.

Interesting. What do others see for RPM when the low volt warning light disappears? 1000-1100 rpm is normal for me. Assuming field wire connection is ok, isn’t this determined by the voltage regulator?

LSZK, Switzerland

8-900 RPM (Piper Dakota). Slightly higher if the pilot is the forgetful kind that forgets to turn off pitot heat after landing

EKRK, Denmark

chflyer wrote:

isn’t this determined by the voltage regulator

Yes, but…. When RPM drops, the voltage regulator compensates by increasing the field current to keep the voltage up. I would say that 1,000 – 1,100 is too high an RPM even with the pitot heat on, so for some reason the field current is too low in that RPM range.

It is unlikely to be an alternator limitation – the relevant limit is the current the field coil can take, and it will be designed to take enough current to keep the aircraft powered at lower RPM.

So the question is why – this can be additional resistance in the wiring or the field coil, a short within the field coil, or an issue with the voltage regulator itself.

Or some high load that means the alternator can genuinely not generate enough current at 1,000 RPM.

Biggin Hill

For a given alternator, without faults, the rpm at which its output voltage exceeds the battery voltage (and thus produces a positive charging current) depends on the field current.

So yes a faulty voltage regulator might be the cause. Easy to diagnose if you know about electrics, though with the more modern regulators you should use an oscilloscope to view the field terminal waveform.

But if there is a fault in the alternator, then it could be

  • a break or a short in the stator winding(s)
  • a break or a short in the field (rotor) winding(s)
  • a faulty rectifier

With the full bus voltage across the field winding (i.e. the voltage regulator turned fully on) the required output should be obtained at the specified rpm (say 1100rpm). If not, the alternator or some wiring is duff.

Shoreham EGKA, United Kingdom

Marchettiman wrote:

About 10 hours ago the low volts warning light started illuminating with engine rpm lower than 1100rpm instead of the usual 700 rpm.

This happened to me in June last year landing at Friedrichaven, an unwelcome sight so far from home. However, it was during a very hot spell and the warning dissapeared as things cooled down. Turns out it is the very poor quality of the UK CAA mandated low voltage light, of which I now have a collection. In the Warrior you can see from the ammeter if the battery is charging and the alternator keeping up, unlike the useless centre zero indication in Cessnas. Excepting the unlikely case of a shorted cell in the battery, if the ammeter is indicating current then the voltage must be OK.

EGBW / KPRC, United Kingdom
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