Effect of Leaving Master Switch On Overnight

[eetrojan]

I’m working through a worksheet and am looking for a little help. In case it matters, the questions that I'm working through were written with a Cessna in mind, but I train in a SportStar, so I usually answer them in two different ways if it seems appropriate. All of my “Cessna” answers are based on research as I have never been in one.

The question I’m struggling with a bit is this, “When you leave the Master switch on inadvertently after a flight, what equipment draws current to run down the battery?”

With a Cessna in mind, my reading seems to suggest that if the twin-rocker was left on, ALT and BATTERY, then the two stay connected together and the alternator’s field coil would continue to draw current from the battery. If left overnight, that sounds like a dead battery for sure. And, there’s always the possibility of some light that was also left on.

Am I correct about the alternator? Are there any other obvious current draws? Fuel gauge sensor?

Now, turning to my SportStar. It has a single Master switch as opposed to a split one. In addition, it has a generator rather than an alternator. In the absence of leaving on a light, will leaving the Master on inherently run down the battery in a SportStar?

Thanks for your thoughts!

 

[poadeleted21]

Turn Coordinator.

 

[skidoo]

PFD draws about 3A...

 

[SkyHog]

The avionics fan turns on with the master, doesn't it? Also, the beacon, since you shouldn't be turning it off before the master anyway.

 

[eetrojan]

PFD draws about 3A...

Thanks for that. Do you mean in a Cessna, or in general?

I've been interpreting this question to mean the master switch was left on, but all other switches were turned off so that I'm looking for hidden power thieves. In the SportStar, there's an Avionics switch that controls power to the flight display. Does it still draw power for some reason?

 

[eetrojan]

The avionics fan turns on with the master, doesn't it? Also, the beacon, since you shouldn't be turning it off before the master anyway.

Hmmm. Didn't know there was an avionics fan, but that makes sense. I'll check. Thanks.

Oddly, our checklist has us turning off ALL switches before we hit the Master, even the beacons, but I often find the beacon switch in the ON position when I first get to the plane which suggests that some folks were trained to always leave them on.

 

[eetrojan]

Turn Coordinator.

Thank you

 

[poadeleted21]

Hmmm. Didn't know there was an avionics fan, but that makes sense. I'll check. Thanks.

Oddly, our checklist has us turning off ALL switches before we hit the Master, even the beacons, but I often find the beacon switch in the ON position when I first get to the plane which suggests that some folks were trained to always leave them on.

Not all planes have an avionics fan, some have a tube. I can't speak to the planes you're flying.

 

[Greg Bockelman]

Hmmm. Didn't know there was an avionics fan, but that makes sense. I'll check. Thanks.

Not all do. And they aren't all necessarily on all the time. Depends on how it was wired. A look at the schematic should answer that question.

Oddly, our checklist has us turning off ALL switches before we hit the Master, even the beacons, but I often find the beacon switch in the ON position when I first get to the plane which suggests that some folks were trained to always leave them on.

The idea is that when the master is turned on, the beacon is powered as a warning that something is about to happen with that airplane. I tend to leave the beacon switched on at all times also.

 

[gprellwitz]

The PFD only applies to models of the Cessna (presuming 172) that have the G1000.

It is common practice to always leave the beacon on to serve as a warning if you walk away from the airplane with the master still turned on.

In the Cessnas I've flown, the avionics fan was controlled by the avionics switch, not the master.

As Greg suggests, have a look at the wiring diagram for the planes you're using as reference.

I can tell you that the master needn't be left on overnight for the battery to be drained enough that the engine won't start, at least in a Cessna 182, where a lunch stop of less than 2 hours was sufficient. (No, it wasn't me, and I don't know if the beacon was switched on. )

 

[eetrojan]

As a follow-up, it seems that the SportStar's hidden load is a coil associated with a master relay. Thanks all for your help. Joe

 

[slacktide]

Since you didn't specify WHICH Cessna, I'll also add "Tip Tank recirculation pumps"

 

[Ted]

You need to find ou in your specific aircraft. For example, in the 310 the answer would be:

-Turn coordinator
-Fuel transfer pumps
-Master relay

Since I leave the beacon and nav lights on all the time, they would also be drawing power, but technically those could be turned off - I just choose not to.

 

[steingar]

Damned if I know how, but it drains the battery real good. Don't ask me how I know that...

 

[Dan Thomas]

If there's no separate alternator/generator switch, or if there is and it's left on, too, the field is live and drawing three or four amps. That heats the field coil (not spinning, so no cooling airflow) and the insulating varnish on the wires can burn up so it shorts out soon after the next start. Gets expensive. The battery also sulfates heavily when deeply and slowly discharged, and never fully recovers. Some never recover to a useful state.

Shutdown checklist. It's there for a reason.

Dan

 

[Silvaire]

Barring everything else the master relay that makes the connection between the battery and the electrical buss will very silently drain the battery overnight.

If you leave the beacon light switch on at all times then if someone happens to be around when you are walking away from the plane they can tell you that you forgot to turn off your master. If not then it can do a much better job of draining the battery while you eat lunch.

 

[MAKG1]

As a rule of thumb, the biggest draws in a vehicle electric system are electric motors and electric heaters. Few aircraft have electric heaters, but VERY many have electric motors in the turn coordinator (gyroscope) and possibly electrical cooling system. If the AI is electric, it will also draw quite a bit. But most of those are vacuum driven.

Why does this matter? You aren't seriously going to Jerry rig a TC shutoff, are you?

 

[tmyers]

Damned if I know how, but it drains the battery real good. Don't ask me how I know that...

Is that like cycling your laptop battery ?????

Can't tell you how many time I take just one more look before I close the hangar door to make sure I haven't left mine on. I find it on about one out of 10 looks.

 

[jpower]

Damned if I know how, but it drains the battery real good. Don't ask me how I know that...

Ooooh yeah. I was going up for an early morning flight, and the renter the night before had left the master on. By the time I tried to fly the plane, the battery was so drained that even with a jump start, it wouldn't take a charge.

 

[eetrojan]

If there's no separate alternator/generator switch, or if there is and it's left on, too, the field is live and drawing three or four amps.

My plane has a separate switch for the avionics, and also for the generator (GEN) and auxiliary generator (AUX GEN), so it sounds like they would not be tied to the battery even if the Master were left on. I think, therefore, that my only load with the Master on, and all other switches off, is the coil in the master relay. I'll find out for sure at my next lesson.

Is it correct that a "generator" uses permanent magnets and an "alternator" uses electromagnets? If so, relative to an alternator, does a generator represent little or no load on the battery if left connected to it when the engine isn't running? Or, do the generator coils that are ordinarily excited by the moving magnets steal current from the battery anyway?

 

[NineThreeKilo]

.

Oddly, our checklist has us turning off ALL switches before we hit the Master, even the beacons, but I often find the beacon switch in the ON position when I first get to the plane which suggests that some folks were trained to always leave them on.

Funny, if they trained like everyone else in ALWAYS leaving the beacon on, they probably would not have as many people accidentally leaving their master switches on (which is why I presume they asked that question).

It's alot harder to miss turning the master off when there is a red beacon going round and round as you get out of the plane

 

[Skylane81E]

My plane has a separate switch for the avionics, and also for the generator (GEN) and auxiliary generator (AUX GEN), so it sounds like they would not be tied to the battery even if the Master were left on. I think, therefore, that my only load with the Master on, and all other switches off, is the coil in the master relay. I'll find out for sure at my next lesson.

Is it correct that a "generator" uses permanent magnets and an "alternator" uses electromagnets? If so, relative to an alternator, does a generator represent little or no load on the battery if left connected to it when the engine isn't running? Or, do the generator coils that are ordinarily excited by the moving magnets steal current from the battery anyway?

I very much doubt the plane actually has a generator, regardless of what the switch may say

 

[eetrojan]

I very much doubt the plane actually has a generator, regardless of what the switch may say

Funny. You may well be right. The POH also calls it a "generator" throughout but then again, it calls the battery an "accumulator." That should have been my first clue.

 

[TriGear28]

Hobbs meter? not much draw, but rental co's hate that one. Unless this a/c is different.

 

[JohnAJohnson]

The avionics fan turns on with the master, doesn't it? Also, the beacon, since you shouldn't be turning it off before the master anyway.

On my Cardinal the fan comes on with the avionics switch.

With both sides of the master ON, Alternator B+, Turn and Bank, and whatever exterior or instrument lights are on.

The clock is always on, but would take months to drain the battery.

 

[JohnAJohnson]

Hobbs meter? not much draw, but rental co's hate that one. Unless this a/c is different.

Mine has power all the time (same line as the clock), regardless of master switch position. But it only gets a ground when engine oil pressure goes above 5 psi or so.

 

[TriGear28]

Mine has power all the time (same line as the clock), regardless of master switch position. But it only gets a ground when engine oil pressure goes above 5 psi or so.

Ahh, I see. I know some old ones on rentals used to keep clocking time if the master was on.

Thanks

 

[Dan Thomas]

Is it correct that a "generator" uses permanent magnets and an "alternator" uses electromagnets? If so, relative to an alternator, does a generator represent little or no load on the battery if left connected to it when the engine isn't running? Or, do the generator coils that are ordinarily excited by the moving magnets steal current from the battery anyway?

They both use electromagnets, but some generators had a system that used the residual magnetism in the field pole shoes to generate a bit of electricity that would then turn the regulator on to fire the field. That was on cars and trucks of long ago; I don't think any airplanes used that, so the system has the field running when the switch is on.

The big difference between generators and alternators is that the generator's field is stationary inside the case, and the electricity is generated in the rotor and taken off via massive carbon brushes. The generator needs more RPM to do its work, and those brushes wear out fairly quickly. The alternator rotates its field instead, and uses tiny carbon brushes to carry the small field current. Both of them generate AC; in the generator this is made into DC by the action of the commutator segments and brushes switching the rotor coil polarities as it turns, while in the alternator diodes do it with no moving parts.

Dan

 

[RotorAndWing]

Wouldn't the simplest answer be: "Leave the master on overnight and the battery will discharge"????

 

[eetrojan]

They both use electromagnets, ...

The big difference between generators and alternators is that the generator's field is stationary inside the case, and the electricity is generated in the rotor and taken off via massive carbon brushes. The generator needs more RPM to do its work, and those brushes wear out fairly quickly. The alternator rotates its field instead, and uses tiny carbon brushes to carry the small field current. Both of them generate AC; in the generator this is made into DC by the action of the commutator segments and brushes switching the rotor coil polarities as it turns, while in the alternator diodes do it with no moving parts.

Dan

Thanks Dan. Makes total sense. Appreciate the explanation.

 

[eetrojan]

Wouldn't the simplest answer be: "Leave the master on overnight and the battery will discharge"????

Always true, it seems, but the question was "what equipment draws current"? Thanks R&W.

 

[slacktide]

You need to find ou in your specific aircraft. For example, in the 310 the answer would be:

-Turn coordinator
-Fuel transfer pumps
-Master relay

Ted, ever price out those fuel xfer pumps? Ever compare them to a Facet 24v Gold-Flo pump from NAPA? Ever peel off the Cessna part label to reveal the "NOT FOR AIRCRAFT USE" label underneath?

 

[RotorAndWing]

Always true, it seems, but the question was "what equipment draws current"? Thanks R&W.

My point is, why does it matter?

Procedure calls for turning off master on shut down. What does knowing what is drawing power and how much it's drawing make any difference?

 

[jesse]

My point is, why does it matter?

Procedure calls for turning off master on shut down. What does knowing what is drawing power and how much it's drawing make any difference?

Systems knowledge. Lose your alternator in flight and it might be worth knowing what is running with the master on and how many amps those things draw.

 

[RotorAndWing]

The question I’m struggling with a bit is this, “When you leave the Master switch on inadvertently after a flight, what equipment draws current to run down the battery?”

Systems knowledge. Lose your alternator in flight and it might be worth knowing what is running with the master on and how many amps those things draw.

Above is the original post with the original question.

While systems knowledge is good it, seems nowadays folks are trying to over analyze and over complicate the most basic items.

A better question would be "what is the result of a battery master left on overnight inadvertently?"

 

[AuntPeggy]

Damned if I know how, but it drains the battery real good. Don't ask me how I know that...

+1 .

 

[eetrojan]

My point is, why does it matter?

Procedure calls for turning off master on shut down. What does knowing what is drawing power and how much it's drawing make any difference?

Hey R&W, I appreciate your point, but in my worksheet, I am commanded to explain why. Plus, it's interesting to understand. But, master switch off seems like a good plan. I always double check before walking away.

 

[Capt. Geoffrey Thorpe]

Hey R&W, I appreciate your point, but in my worksheet, I am commanded to explain why. .

Then you need to look at the data sheet for the relay:
http://www.mouser.com/ds/2/459/Pg_048_51-3318.pdf
From that, you find that the coil resistance, is 16 Ohms. Apply Ohms law to determine the current draw. (Add any other loads as necessary.) Finally, divide the amp-hour capacity of the battery by the current through the relay primary coil and you then know how long it takes to discharge the battery.

 

[eetrojan]

Then you need to look at the data sheet for the relay:
http://www.mouser.com/ds/2/459/Pg_048_51-3318.pdf
From that, you find that the coil resistance, is 16 Ohms. Apply Ohms law to determine the current draw. (Add any other loads as necessary.) Finally, divide the amp-hour capacity of the battery by the current through the relay primary coil and you then know how long it takes to discharge the battery.

Only why ("what equipment"), not how long, but I like it!

The only other load I have discovered is the electronic trim control circuit (LEDs and all), but I think I'm splitting hairs already...

 

[SkyHog]

My point is, why does it matter?

Procedure calls for turning off master on shut down. What does knowing what is drawing power and how much it's drawing make any difference?

You must be one of those CFIs that can't or don't explain to students why things are true, leading to the adaptation of "Do as I'm told, regardless of the reason" syndrome.

You know, the ones that breed pilots that believe in flying the step, or that you can't slip a Cessna with full flaps.