C-172 landing light switch

My '61 Cessna landing light switch has two positions; First notch is the taxi
light, and the last notch turns on the landing light but leaves on the taxi
light. Does anyone know of a legal replacement switch that separates the two
lights? (taxi light goes off on the last notch). The reason I ask is that it
still has the generator and at engine idle the generator circuit breaker often
pops due to the high load and low output. A separating switch would presumably
cut the load way down.

Jim

I don't know very much about light aircraft electrical systems, but they are
very similar to automobile systems, which I do know something about.

And I don't think the light switch is your problem.

In a typical circuit, you will have a load (lights) and a supply (generator
and battery). The circuit breaker will be located between the load and the
supply.

A quick and dirty explanation. Assume a 15A breaker. this breaker will trip
when the current flowing through it exceeds 15A. Let's say that the light
draws a maximum of 20A. If the supply (generator and/or battery) can't
deliver 15A, the breaker will never trip, even through the circuit is
theoretically overloaded. All that will happen is that your light will dim.
So the breaker issue is probably not related to the landing light issue.

Now let's consider the scenario I think you are describing. You are
approaching an airport, turn on the landing light, land, turn on the taxi
light, and begin your taxi, at which time the breaker trips.

Now let's look at what's happening: Your engine has been operating at
"flight RPM", and at some point near the airport you have reduced your RPM's
to flight idle, which is more RPM's than ground idle. So, for all of this
period of time the generator has been spinning fast enough to charge the
battery, which will retain the current. Assuming that your generator is in
good condition and is putting out power, and that your battery is in good
condition and is storing and retaining that power. Assuming everything is
working properly, while in flight, your aircraft is drawing most of it's
current from the generator; current is taken from the battery under
excessive loads.

So, when you land, you should have a fully charged, or nearly fully charged,
battery. If the aircraft needs more current than the generator can supply,
the aircraft will get the additional current from the battery. Consequently,
if your generator is not properly operating, it won't be able to supply
current to the aircraft and it won't be able to charge the battery, no
matter how far you fly.

From what I have read, in many instances the landing and taxi lights work
similarly to the high beam/low beam lights in an automobile. When you turn
on the landing light, one "bulb" comes on; when you turn on the taxi light,
the landing light "bulb" stays on, and a taxi light "bulb" comes on to
supplement the landing light. So, there may be some special requirements for
the switch.

Now to the guesses and suggestions:

You should first have the landing/taxi light circuit checked. Wiring for
these lights typically runs out to the wings or to the lower engine cowl.
This would make them susceptible to short circuits, which would result in
breakers tripping. And blowing breakers is a very bad thing; it can lead to
fires and other nasty stuff.

And while you are having the light circuit checked out, you should probably
also have the generator, battery, and charging circuit checked, if for no
other reason than your own peace of mind.

And if you get any other posts on this subject, go with them; their advice
is probably better than mine!








"JFLEISC" wrote in message
...
My '61 Cessna landing light switch has two positions; First notch is the
taxi
light, and the last notch turns on the landing light but leaves on the
taxi
light. Does anyone know of a legal replacement switch that separates the
two
lights? (taxi light goes off on the last notch). The reason I ask is that
it
still has the generator and at engine idle the generator circuit breaker
often
pops due to the high load and low output. A separating switch would
presumably
cut the load way down.

Jim

I don't know very much about light aircraft electrical systems, but they are
very similar to automobile systems, which I do know something about.

Well Bill, I appreciate the advice, however I also know a bit about cars having
been a NAISE certified auto mechanic for over 20 years. First rule of auto
mechanics is listen to the customer's complaint. I said the generator breaker
was tripping. The landing light breaker is fine and never trips. The give away
here is that (with the fully charged battery, as you surmised) the landing
light does not go out. Neither of them do. In fact more lights come on, namely
the 'no charging' light. Think about that one for a minute. Breaker pops,
landing lights still on...aka landing light circuit working.
One thing I'm not is an electrical engineer. How does it work?...something like
if the draw (wattage) is the same and the voltage drops, the current goes
up?...or I've got it backwards or upside down or inside out etc? :-)

Jim

Do you have a voltage regulator (either internal or external)? That's
probably where I would look based on everything you pointed out.

Very "rule of thumb", volts times amps equals watts...


"JFLEISC" wrote in message
...
I don't know very much about light aircraft electrical systems, but they
are
very similar to automobile systems, which I do know something about.

Well Bill, I appreciate the advice, however I also know a bit about cars
having
been a NAISE certified auto mechanic for over 20 years. First rule of auto
mechanics is listen to the customer's complaint. I said the generator
breaker
was tripping. The landing light breaker is fine and never trips. The give
away
here is that (with the fully charged battery, as you surmised) the landing
light does not go out. Neither of them do. In fact more lights come on,
namely
the 'no charging' light. Think about that one for a minute. Breaker pops,
landing lights still on...aka landing light circuit working.
One thing I'm not is an electrical engineer. How does it work?...something
like
if the draw (wattage) is the same and the voltage drops, the current goes
up?...or I've got it backwards or upside down or inside out etc? :-)

Jim

Do you have a voltage regulator (either internal or external)? That's
probably where I would look based on everything you pointed out.

Oh yes, it definately has a voltage regulator. A BIG heavy one. (I never saw a
'generator' with an internal regulator. Wish I could afford the alternator
STC). Admittedly its been some time since I worked on 'generator' voltage
regulator circuits. Any suggestions on what to look for? Are Delco Remy
aircraft regulators the same as the automotive versions for 1/3 the price?

Jim

I'm afraid I'm at the end of the line on this one; this is where I'd usually
just head to the NAPA store and buy something!

But your Delco comment reminded me of something...

Years ago I needed some 1/4" plastic tubing for something, so I went down to
my local "seal and gasket" place. I told the clerk what I needed, and he
asked me if I wanted "medical grade", "aircraft grade", or "industrial
grade".

I asked him what the difference was between the various grades, and he told
me the "medical grade" was $1.20/ft, the "aircraft grade" was $1.10/ft, and
the "industrial grade" was $1.00/ft. I asked him what the actual difference
was. He told me to lean across the counter and he showed me the spool. It
had three different part numbers on it. The clerk told me that it was all
"medical grade"; they just charged different prices based on what grade you
asked for!

But I would recommend a bit more investigation before you bolt up the Delco!

"JFLEISC" wrote in message
...
Do you have a voltage regulator (either internal or external)? That's
probably where I would look based on everything you pointed out.

Oh yes, it definately has a voltage regulator. A BIG heavy one. (I never
saw a
'generator' with an internal regulator. Wish I could afford the alternator
STC). Admittedly its been some time since I worked on 'generator' voltage
regulator circuits. Any suggestions on what to look for? Are Delco Remy
aircraft regulators the same as the automotive versions for 1/3 the price?

Jim

From what I have read, in many instances the landing and taxi lights work
similarly to the high beam/low beam lights in an automobile. When you turn
on the landing light, one "bulb" comes on; when you turn on the taxi light,
the landing light "bulb" stays on, and a taxi light "bulb" comes on to
supplement the landing light. So, there may be some special requirements for
the switch.

Minor nit: He said the first one ("low beam") is the taxi light, and
then the landing light is "high beam". So, it would be turning the
taxi light on first, then the landing light on (and taxi stays).

Minor nit: He said the first one ("low beam") is the taxi light, and
then the landing light is "high beam". So, it would be turning the
taxi light on first, then the landing light on (and taxi stays).

"Low" beam may be a poor way to state it. It may well be the same bulb as the
"high" beam (landing light) (and I haven't actually checked to see). It is just
pointed much lower to the ground in front of the plane.
And yes, the taxi light stays on with the landing light effectively doubling
the load.

Jim

Jim

Having read the other posts to date.

Have you checked the Circuit breaker? Many times a circuit breaker
will loose its ability to meet rated current after it is popped or
pulled a few times. It's also over 40 years old (if it hasn't been
replaced in that time). Your problem could be as simple as that?

Rots of ruck boy san

Big John
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~`

On 03 May 2004 20:09:50 GMT, (JFLEISC) wrote:

My '61 Cessna landing light switch has two positions; First notch is the taxi
light, and the last notch turns on the landing light but leaves on the taxi
light. Does anyone know of a legal replacement switch that separates the two
lights? (taxi light goes off on the last notch). The reason I ask is that it
still has the generator and at engine idle the generator circuit breaker often
pops due to the high load and low output. A separating switch would presumably
cut the load way down.

Jim

It's also over 40 years old (if it hasn't been
replaced in that time). Your problem could be as simple as that?

As I pointed out in earlier posts, Cessna 172s of that vintage had fuses and
mine was converted to new breakers of the same rating just last year.

Jim