I've always tested the ammeter/low voltage light in the Cessna airplanes I've flown by turning off the alternator switch (left half of master) during the runup.
I recently flew with a different instructor who discouraged this practice. He suggested abruptly turning on both the landing and taxi lights and watching the ammeter for a slight dip during this voltage draw surge (what's the opposite of surge?).
We were in the runup area ready to go and once we departed we were distracted with looking for traffic and talking about radio freqs, etc...I never asked him why.
Anyone else ever heard of this?
Is there anything wrong with turning off the alternator during the runup?
I can't specifically answer your question, but I can tell you that I've done the ammeter check both ways. I was originally trained to switch off the alternator part of the master. One day during my run up, that technique did not have any effect on the voltage meter. So, my flight instructor offered the technique of switching the landing/taxi lights on and looking for the dip as an alternate check. So, since then, I do the alternator switch check, and if I get no result, then I use the landing/taxi light switches as a second check.
However, I do not know the reasons why one would be more or less kosher than the other. If others have suggestions, then I would also be interested in reading them...
In the 152 was taught to lower and raise the flaps (electrically driven) and watch the ammeter for a dip.
Cycling the lights should do the same thing, but will also shorten the life of the light bulbs. (switching on and off a light is not a recommended thing to do). The next time you need your lights and they don't work, you regret abusing them.
In the Arrow, we just check to see the the ammeter is showing that the alternator is charging the system (positive indication on the gage).
When you're brining the flaps up, take a quick look. Flipping the landing light off and on shortens its life. I let my students know that, because I'm normally the one that grabs the screw driver and swaps out the bulb.
The landing light test is better for the voltage regulation system, and also tells you if the landing light is operational. All you're looking for is an increase in alternator output to verify that the battery is charging and the voltage regulator is, well, regulating.
Switching off the alternator kills battery sense voltage to the field of the alternator, forcing the voltage regulator to go "wide open". When the alternator resumes it's output, it sends the VR into a large transient, and it could fail.
I believe the reason for the practice is to verify that you don't have a hung starter, which turns the starter into a generator supplying unregulated voltage to the system. This check can easily be made during runup by noticing if the ammeter is reading in the "normal" range.
we dont bother with turning lights on/off. just check that the low voltage light is on at start up and then goes out by the end of after start checks and that the ammeter is charging/discharging when it should be during checks.
if anythings iffy then you have a fiddle.
Originally posted by johnpeace I recently flew with a different instructor who discouraged this practice. He suggested abruptly turning on both the landing and taxi lights and watching the ammeter for a slight dip during this voltage draw surge (what's the opposite of surge?).
My understanding is that the gauge shows the charging/discharge current to the battery. By turning off and on the field coil of the alternator, you can check that you are in fact getting current from the alternator and it works. Thus, you are not just running your radio, lights, flaps, etc from the battery and have a chance to test your no radio technique.
If you turn on and off a load, be it flaps, lights, etc, you are just increasing the drain on the battery. I don't see what that proves other than that whatever you turned on does in fact draw power. I don't see how that proves your alternator is producing power.
Doug - the alternator makes up the difference between the load and what the battery puts out. A 20 amp draw should show 20 amps on the positive side of the gauge if the alternator is operating correctly. If not, it'll show 20 amps on the negative side. This is assuming a fully charged battery.
Originally posted by jerryp Doug - the alternator makes up the difference between the load and what the battery puts out. A 20 amp draw should show 20 amps on the positive side of the gauge if the alternator is operating correctly. If not, it'll show 20 amps on the negative side. This is assuming a fully charged battery.
Actually, a 20 amp draw with the alternator operating properly will show 0 draw. The ammeter shows the load on the battery, not the load on the alternator. A loadmeter shows the load on the alternator.
When you start the engine, you show a discharge because you are pulling from the battery. After the engine starts and the alternator comes online, you show a charge to the battery until the battery becomes fully charged. With the battery fully charged, you have the same potential coming form the battery as is coming from the alternator. You show nothing on your ammeter. When you lower the flaps, turn on a light, etc, the electric device has not counter EMF (Electro Magnetic Force) to provide resistance to the current flow. So you initially get a very high current draw. The alternator senses this draw and takes a splt second to match the current. The counter EMF slows the current flow very quickly, so the alternator never needs to put out the full current needed to turn the light on or get the motor started, etc. Since the extra current draw isn't coming from the alternator, it comes from the battery. This is the temporary discharge when you turn an item on. Once the alternator catches up to the current draw, you will see a momentary charge to the battery to replenish it, then back to 0 even though the alternator is now putting out more current.
The reason why turning the alternator switch off is not the best idea is that if you only have radios on and nothing else, you may not be able to detect a siginificant draw to determine the alternator is working. Also, the alternator may only be putting out a limited amount of current, so turning on landing light and moving flaps, etc will place a larger load to insure it is operating normally. If it is going to burn out, better to find out on the ground rather than the first time you go to lower your flaps.
There is no damage or problems caused by turning the switch off however. It is an old wivestail that you will cause the brushes to arc. They arc all the time anyway.