From the archives
(The following is an excerpt from an article I wrote 14 years ago):
Terror at 3100 feet
An exaggeration by Vern Little.
January 24th, 2006—a beautiful, sunny day after a month of steady rain. Freshly armed with the Flight Authority for my freshly minted RV-9A, I arrived at Boundary Bay airport for my first flight.
I had previously decided that a safety pilot would be beneficial, and Mike Langford had volunteered. So we strapped in, fired up the engine, waved to the adoring throng of two persons, and taxied for take-off.
We taxied to position, advanced the throttle slowly and launched ourselves into the calm skies. Cleared to enter Vancouver Terminal airspace by the Nav Canada upper class, we proceeded to level off around 3000 feet and perform some basic flight exercises.
Having spent more than three kilo hours building my airplane, I knew it inside and out. Mike was along in case a ‘situation’ arose that needed my attention.
This was the right thing to do.
Within a few minutes, I noticed a few things on my instruments that concerned me. I had low oil temperature (68 degrees F), no indication of cylinder head temperature on #1, and low CHT on #4 cylinders. Most seriously, was a spurious over-voltage alarm.
Mike took control (he was very happy about this for some reason), while I attempted to make sense of the instruments. After some futzing about, I decided that we were in no danger, but it would make sense to land soon and fix the problems.
After a perfect landing (really, it was), I taxied back in and tore off the cowlings (briefly interrupted by a photo-op).
Here’s what I learned:
• CHT probes work better if you plug them in the socket! Easy fix to #1 cylinder (blush).
• CHT probes will indicate about ½ their normal temperature if one lead is shorted to ground (double blush).
• Sometimes electronic devices fail without warning, as did my Oil Temperature probe. This is exacerbated if you pot them in corrosive silicone sealant. (if it smells like vinegar, don’t get it near electronic devices).
As for the over-voltage alarms, there is a lesson to be learned. Those who have survived my lectures on Aviation Electrics may recall my discussion on alternators, regulators and batteries.
Figure 1 shows how these three major components are related. In normal operation, the alternator is attached to the engine with an accessory belt, which spins the armature.
The armature is wound with a coil of wire that is connected through brushes to a source of power derived from the voltage regulator. The voltage regulator drives a current through this ‘field coil’, thus making a spinning electromagnet.
A moving magnet will induce a current in a nearby loop or coil of wire, such as the stator (non-moving) coils in the alternator. Since the magnet (energized field coil) is moving, it produces a corresponding moving current (alternating current) in the stator coils, which is converted to direct current by semiconductor devices called diodes.
This direct current is used to charge the battery and power the electrical system of the aircraft… if everything works right. The regulator keeps everything working smoothly and sets the charging voltage of the battery.
My over-voltage alarms were caused by a malfunctioning alternator-battery-regulator system. The key data here is that it was ‘too much’ voltage as compared to ‘not enough’ voltage. This meant that the alternator was producing too much juice.
Another piece of data was that it was not a continuous over voltage that would have cooked my battery, but rather it was spurious or intermittent.
Fixing the problems
I found two problems. The first one was that the alternator belt was loose. By itself, this could cause both over and under voltage conditions. A slipping belt would cause the alternator output to drop. This would be sensed by the regulator, and it would send more field current to the alternator. When the belt grabs again, the alternator puts out a large pulse of current before the regulator can catch up and turn it down again.
This cycle can repeat, providing a pulsing voltage, indicated by flashing instrument lights.
After I torqued the alternator spec to 12 ft-lbs at the alternator nut, I decided to keep looking for potential problems.
I turned the master switch to ‘ALT’ (which is the normal position for flight) and proceeded to measure voltages, starting with the main battery, and tracing through the master contactor, main bus, ALT circuit breaker, master switch and regulator input and output.
I found a 1 volt drop between the main battery and the regulator input! This was clearly too much, and my math indicated that it should only be 0.2 to 0.3 volts. So, I had another problem. My voltage regulator was not getting the right voltage input, and so was getting confused.
Further probing found a corroded electrical connection at the master breaker. Simply pulling it off and reseating it fixed this problem.
After flight testing, everything worked fine.
What can be learned from my experiences?
• On the first flight of a new aircraft, it makes a lot of sense to have a safety pilot along to steady the nerves and take over when necessary.
• Things work better if you plug them in.
• Sometimes things fail, and you should know enough to determine (in flight) if it’s serious or just an inconvenience.
• Electrical systems need to be exercised routinely to minimize the effects of corrosion.
• Alternator belts need to be more than ‘finger tight’.
• A basic understanding of how things work allows you to find and correct problems quickly.
• 14.2 volts good. 12.5 volts bad. 15.5 volts very bad.