Alternator ON or OFF
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Alternator ON or OFF
Many moons ago I was taught to only switch on the BATT side of the dual switches (on the Cessna's) and in the after start procedure then switch on the alternator to check that it is working in the correct sense.
Is their any merit in this system rather than switching on both at the same time prior to starting and then checking the alternator output. Maybe an engineer can shed some light if there is a technical issue at work here.
Spock
Is their any merit in this system rather than switching on both at the same time prior to starting and then checking the alternator output. Maybe an engineer can shed some light if there is a technical issue at work here.
Spock
Last edited by spocky; 15th Jun 2012 at 05:48.
I believe that turning off the alternator switch takes the field off line, saving about five amps, which may be the difference between an engine start and a non start on an almost flat battery. A fairly regular occurence on fixed gear 182 with the regulator in the engine bay and the battery at almost the other end of the aircraft.
This advice is worth what you are paying for it.
This advice is worth what you are paying for it.
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The reason will be so the alternator field winding is not taking electrical load on the battery, and the alternator is not adding extra torque load on the starter.
You can't tell anything about the charging until the engine is running.
It is really just load shedding. Unless someone can think of something else. Having said that mine starts fine with it on, but I don't make a habbit of it.
You can't tell anything about the charging until the engine is running.
It is really just load shedding. Unless someone can think of something else. Having said that mine starts fine with it on, but I don't make a habbit of it.
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Leadsled,
Often your posts are intelligent and full of good material, that last one was as useless as some of mine
The guy wants to understand why? If more people took the time to understand stuff we would not have so many manual following morons around. Sometimes the manuals are wrong, dumb and dangerous. Probably not often, but to have a questioning mind should not be discouraged.
What do you reckon?
Often your posts are intelligent and full of good material, that last one was as useless as some of mine
The guy wants to understand why? If more people took the time to understand stuff we would not have so many manual following morons around. Sometimes the manuals are wrong, dumb and dangerous. Probably not often, but to have a questioning mind should not be discouraged.
What do you reckon?
For a Cessna the manual more than likely says to leave the alterntor off until it starts. I agree with others the reason for this approach is to keep the load down while the starter motor is operating.
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Here is some observations not based on any facts. I'm sure Cessna made a million of these switches and all their other electrical parts as they keep turning up in every model. Think of the courtesy under-wing light switches from 172, a bit white chunky thing, which are still found in modern Caravans.
Thinking that its easier to recycle the generator or alternator than turning off the entire electrical system with two separate switches . Guessing also that these switches were in 100 series when they still had generators.
So I'm saying its a legacy for parts more than design.
Of course I could be completely wrong
Thinking that its easier to recycle the generator or alternator than turning off the entire electrical system with two separate switches . Guessing also that these switches were in 100 series when they still had generators.
So I'm saying its a legacy for parts more than design.
Of course I could be completely wrong
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From The POH
Guys,
From the Cessna 310R POH. Pre Start check list requires Alternators ON. The only time it is different is with Ground Power. For GPU starts Battery and Alternator must be OFF prior to plugging in and they stay that way until after engines are started and GPU plug removed.
Groggy
From the Cessna 310R POH. Pre Start check list requires Alternators ON. The only time it is different is with Ground Power. For GPU starts Battery and Alternator must be OFF prior to plugging in and they stay that way until after engines are started and GPU plug removed.
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Grogmonster, aircraft POH are written by lawyers for the lowest common denominator. 
In this case that means the person who is likely to forget to turn an alternator on after starting.
By recommending it be turned on with the battery you preclude claims for compensation from the dingbat whose radios failed at the worst possible time leading to nervous shock and trauma. 
That said, I have seen recommendations for both actions. One chief engineer reckoned the battery was the cheapest part of the engine start so don't risk damaging the alternators until after the engines are turning. Others just followed whatever the POH said.
I have yet to hear or read a convincing argument for either action.
In this case that means the person who is likely to forget to turn an alternator on after starting.
By recommending it be turned on with the battery you preclude claims for compensation from the dingbat whose radios failed at the worst possible time leading to nervous shock and trauma. That said, I have seen recommendations for both actions. One chief engineer reckoned the battery was the cheapest part of the engine start so don't risk damaging the alternators until after the engines are turning. Others just followed whatever the POH said.
I have yet to hear or read a convincing argument for either action.
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For a Cessna the manual more than likely says to leave the alterntor off until it starts.
From a Cessna 182 POH
MASTER SWITCH
The master switch is a split-rocker type switch labeled MASTER, and is ON in the up position and off in the down position. The right half of the switch, labelled BAT, controls all electrical power to the airplane. The left half labeled ALT controls the alternator. Normally, both sides of the master switch should be used simultaneously; however, the BAT side of the switch could be turned ON separately to check the equipment while on the ground. To check or use avionics equipment or radios while on the ground, the avionics power switch must be turned ON. The ALT side of the switch, when placed in the off position, removes the alternator from the electrical system. With this switch in the off position the entire electrical load is placed on the battery. Continued operationwith the alternator switch in off position will reduce battery power low enough to open the battery contactor, remove power from the alternator field, and prevent alternator restart.
The master switch is a split-rocker type switch labeled MASTER, and is ON in the up position and off in the down position. The right half of the switch, labelled BAT, controls all electrical power to the airplane. The left half labeled ALT controls the alternator. Normally, both sides of the master switch should be used simultaneously; however, the BAT side of the switch could be turned ON separately to check the equipment while on the ground. To check or use avionics equipment or radios while on the ground, the avionics power switch must be turned ON. The ALT side of the switch, when placed in the off position, removes the alternator from the electrical system. With this switch in the off position the entire electrical load is placed on the battery. Continued operationwith the alternator switch in off position will reduce battery power low enough to open the battery contactor, remove power from the alternator field, and prevent alternator restart.
Some people may have educated themselves to a greater level of understanding then mere compliance with the POH, but by the offering of and acceptance of that advice you are replacing the ‘Lowest Common Denominator’ with an unknown denominator for people who have yet to gain their own extended wisdom.
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I was taught to have it on for start.
Looking at the Archer II POH it states that when the master switch is indicated, this refers to both Master and Alt switch and as such they should be depressed simultaneously.
Recently however a long way from home, I attempted a start and was suprised when the starter was not quite able to turn the engine over with the available power.
I tried turning the alternator off and it turned over and started. So I'm convinced the ALT load can make a reasonable difference to your start.
Looking at the Archer II POH it states that when the master switch is indicated, this refers to both Master and Alt switch and as such they should be depressed simultaneously.
Recently however a long way from home, I attempted a start and was suprised when the starter was not quite able to turn the engine over with the available power.
I tried turning the alternator off and it turned over and started. So I'm convinced the ALT load can make a reasonable difference to your start.
I wonder whether we should go crazy and discuss this question on the basis of technical facts.
Please complete the following sentences:
On the aircraft I’m flying, if I start my engine with the BAT switched ON and the ALT switched ON:
- Before I select ‘Start’ with BAT and ALT switched ON, current of [?] will flow through circuits [?, ? and ?] and the consequent risks of that are [?, ? and ?]
- When I select ‘Start’ with BAT and ALT switched ON, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?]
- After the engine starts with BAT and ALT switched ON, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?]
On the aircraft I’m flying, I start my engine with the BAT switched ON and the ALT switched OFF, but switch the ALT switch ON after start:
- Before I select ‘Start’ with BAT switched ON and ALT switched OFF, current of [?] will flow through circuits [?, ? and ?] and the consequent risks of that are [?, ? and ?]
- When I select ‘Start’ with BAT switched ON and ALT switched OFF, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?]
- After the engine starts, but before I switch ALT ON, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?]
- After the engine starts and I switched to ALT ON, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?].
Let’s go really crazy and ask this question:
Does the aircraft I’m flying have an alternator?
Please complete the following sentences:
On the aircraft I’m flying, if I start my engine with the BAT switched ON and the ALT switched ON:
- Before I select ‘Start’ with BAT and ALT switched ON, current of [?] will flow through circuits [?, ? and ?] and the consequent risks of that are [?, ? and ?]
- When I select ‘Start’ with BAT and ALT switched ON, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?]
- After the engine starts with BAT and ALT switched ON, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?]
On the aircraft I’m flying, I start my engine with the BAT switched ON and the ALT switched OFF, but switch the ALT switch ON after start:
- Before I select ‘Start’ with BAT switched ON and ALT switched OFF, current of [?] will flow through circuits [?, ? and ?] and the consequent risks of that are [?, ? and ?]
- When I select ‘Start’ with BAT switched ON and ALT switched OFF, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?]
- After the engine starts, but before I switch ALT ON, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?]
- After the engine starts and I switched to ALT ON, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?].
Let’s go really crazy and ask this question:
Does the aircraft I’m flying have an alternator?
I wonder whether we should go crazy and discuss this question on the basis of technical facts.
Please complete the following sentences:
On the aircraft I’m flying, if I start my engine with the BAT switched ON and the ALT switched ON:
- Before I select ‘Start’ with BAT and ALT switched ON, current of [?] will flow through circuits [?, ? and ?] and the consequent risks of that are [?, ? and ?]
- When I select ‘Start’ with BAT and ALT switched ON, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?]
- After the engine starts with BAT and ALT switched ON, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?]
On the aircraft I’m flying, I start my engine with the BAT switched ON and the ALT switched OFF, but switch the ALT switch ON after start:
- Before I select ‘Start’ with BAT switched ON and ALT switched OFF, current of [?] will flow through circuits [?, ? and ?] and the consequent risks of that are [?, ? and ?]
- When I select ‘Start’ with BAT switched ON and ALT switched OFF, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?]
- After the engine starts, but before I switch ALT ON, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?]
- After the engine starts and I switched to ALT ON, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?].
Let’s go really crazy and ask this question:
Does the aircraft I’m flying have an alternator?
Please complete the following sentences:
On the aircraft I’m flying, if I start my engine with the BAT switched ON and the ALT switched ON:
- Before I select ‘Start’ with BAT and ALT switched ON, current of [?] will flow through circuits [?, ? and ?] and the consequent risks of that are [?, ? and ?]
- When I select ‘Start’ with BAT and ALT switched ON, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?]
- After the engine starts with BAT and ALT switched ON, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?]
On the aircraft I’m flying, I start my engine with the BAT switched ON and the ALT switched OFF, but switch the ALT switch ON after start:
- Before I select ‘Start’ with BAT switched ON and ALT switched OFF, current of [?] will flow through circuits [?, ? and ?] and the consequent risks of that are [?, ? and ?]
- When I select ‘Start’ with BAT switched ON and ALT switched OFF, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?]
- After the engine starts, but before I switch ALT ON, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?]
- After the engine starts and I switched to ALT ON, current of [?] will flow through circuits [?,? and ?] and the consequent risks of that are [?,? and ?].
Let’s go really crazy and ask this question:
Does the aircraft I’m flying have an alternator?
Dr
Last edited by ForkTailedDrKiller; 16th Jun 2012 at 09:42.
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It should not make one iota of difference.
The current draw of an alternator is less than that of a permanently on beacon, and you can add to that load the gyro's on the electrically driven instruments.
The current required for starting is so large, that even the manufacturers recognise this fact by not enabling you to isolate such circuits and permit all allowable amperage to be delivered only for cranking purposes. I.e the draw is so low, that if it won't crank with them on, it isn't going to crank with them off either.
qouting plovett's engineer.
Hmm...my experience is an internal regulator is 1/4 the price of an aviation approved battery.
I'm pretty sure said engineer is rather uneducated on electrics, or just living on OWT's (possibly dating back to generators).
If you consider that most Aviation alternators are an 'approved' version, if not identical to one used in automotive, then things become a little more relevant. For example, a 1967 Cherokee 6 shares the same alternator as a Chrysler of the same era (no not tell you the model). The only difference is that one has an aviation serial number/approval on it.
This leads us to the fact that there is NO fundamental difference between automotive and aviation alternators. They work the same, act the same, generate the same result.
If you turn the alt on before starting, how is this going to damage the alternator? If the risk of damage to the alternator is real, then why is it that NO automotive manufacturer configures their electrical system to switch off the alternator during cranking? Afterall, with their warranties, they would want to spend the 50 cents on a solenoid/relay to do just that, if it was a problem. They don't spend that money, and the power to your cars alternator is permanently live!. don't beleive me...grab a test light and go check for yourself......its the BIG wire on the back of the alternator...compare your car to your aeroplane.
The only benefit of us being able to turn the alternator off, is when we see a problem...i.e overcharge situation (regulator failure), and we can start a process of protecting systems and our lives. In a car, it'll charge until something fails/burns. Usually battery first.
The current draw of an alternator is less than that of a permanently on beacon, and you can add to that load the gyro's on the electrically driven instruments.
The current required for starting is so large, that even the manufacturers recognise this fact by not enabling you to isolate such circuits and permit all allowable amperage to be delivered only for cranking purposes. I.e the draw is so low, that if it won't crank with them on, it isn't going to crank with them off either.
qouting plovett's engineer.
One chief engineer reckoned the battery was the cheapest part of the engine start so don't risk damaging the alternators until after the engines are turning.
I'm pretty sure said engineer is rather uneducated on electrics, or just living on OWT's (possibly dating back to generators).
If you consider that most Aviation alternators are an 'approved' version, if not identical to one used in automotive, then things become a little more relevant. For example, a 1967 Cherokee 6 shares the same alternator as a Chrysler of the same era (no not tell you the model). The only difference is that one has an aviation serial number/approval on it.
This leads us to the fact that there is NO fundamental difference between automotive and aviation alternators. They work the same, act the same, generate the same result.
If you turn the alt on before starting, how is this going to damage the alternator? If the risk of damage to the alternator is real, then why is it that NO automotive manufacturer configures their electrical system to switch off the alternator during cranking? Afterall, with their warranties, they would want to spend the 50 cents on a solenoid/relay to do just that, if it was a problem. They don't spend that money, and the power to your cars alternator is permanently live!. don't beleive me...grab a test light and go check for yourself......its the BIG wire on the back of the alternator...compare your car to your aeroplane.
The only benefit of us being able to turn the alternator off, is when we see a problem...i.e overcharge situation (regulator failure), and we can start a process of protecting systems and our lives. In a car, it'll charge until something fails/burns. Usually battery first.
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Here here jas24, alternators are on every time you start your car & do people regularly pull the T&B breaker every start for load sheddin? I have a feeling this may be another thread out there but I've never had two LAME / old timer pilot s agree on: why do many POHs say DO NOT TURN ALTERNATOR ON IN FLIGHT?
Why why why
Why why why
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While I agree with most of what you are saying in intent, you did also manage to distort some facts.
The Chrysler alternator is fitted to a few of the Cherokee family, most of them and indeed the bulk of that generation of GA aircraft would use Prestolite kit.
Not the same as automotive.
As for an engineer thinking generator, well yes if he was thinking cars. Very few aircraft had dc generators similar to a car type. Some did, but relatively small numbers. I seem to think some of the Airtourers may have been the latest. They have the cut-out relay normally anyway, fitted to prevent the generator motoring.
The LAME was probably talking about alternators fitted to twins where there is quite a lot of added circuitry, mainly to do with load sharing but which could possibly be damaged by cross-generator starts.
The field current draw is only a few amps, the alternator load on the engine (magnetic) would be minimal also at normal cranking speeds.
It depends on the POH but as we know many will just say you can start with both switches on. Some types like the BN2 don't like you switching the field with the engine running.
The load sharing on the Islanders used to be pretty rudimentary and was easily upset.
The British also have an AD on starters, often an isolate switch or warning light is required.
As for errors in the POH. Yes no doubt there still are some but with email it has never been easier to discuss possible amendments with the manufacturer.
I did exactly that just a couple of months ago with one of the Piper models.
Normally the Chief Pilot would be corresponding with the manufacturer over any concerns in that publication.
They are pretty basic handbooks normally though.
A little like changing fuel tanks at regular intervals to balance the lateral weight distribution.
Most of the Cherokee family have you change tanks once a flight. It is there in the 'Cruise' section. How that becomes the 30 minute fiddle I am not at all sure.
So, yes.. many pilots seem to be operating the aircraft contrary to the POH.
The Chrysler alternator is fitted to a few of the Cherokee family, most of them and indeed the bulk of that generation of GA aircraft would use Prestolite kit.
Not the same as automotive.
As for an engineer thinking generator, well yes if he was thinking cars. Very few aircraft had dc generators similar to a car type. Some did, but relatively small numbers. I seem to think some of the Airtourers may have been the latest. They have the cut-out relay normally anyway, fitted to prevent the generator motoring.
The LAME was probably talking about alternators fitted to twins where there is quite a lot of added circuitry, mainly to do with load sharing but which could possibly be damaged by cross-generator starts.
The field current draw is only a few amps, the alternator load on the engine (magnetic) would be minimal also at normal cranking speeds.
It depends on the POH but as we know many will just say you can start with both switches on. Some types like the BN2 don't like you switching the field with the engine running.
The load sharing on the Islanders used to be pretty rudimentary and was easily upset.
The British also have an AD on starters, often an isolate switch or warning light is required.
As for errors in the POH. Yes no doubt there still are some but with email it has never been easier to discuss possible amendments with the manufacturer.
I did exactly that just a couple of months ago with one of the Piper models.
Normally the Chief Pilot would be corresponding with the manufacturer over any concerns in that publication.
They are pretty basic handbooks normally though.
A little like changing fuel tanks at regular intervals to balance the lateral weight distribution.
Most of the Cherokee family have you change tanks once a flight. It is there in the 'Cruise' section. How that becomes the 30 minute fiddle I am not at all sure.
So, yes.. many pilots seem to be operating the aircraft contrary to the POH.
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Breeza,
I disagree about the chrysler alternator point. Why.. I am not going to digress here in public. Happy to pm you about it tho.
----------
As for load shedding, it appears many more owners are replacing their starters with STC'd high torque versions. These starters are even less load critical, so the added load of the alternator field will make little difference.
Whilst I advocate that you can happily start with the alternator switched on, my personal preference is to bring it online during my afterstart checks. It only takes and alternator 30-45 seconds to replace the charge in a battery after start, so i like to bring it online later, so i can observe it doing its job via the amp meter. That way I know its working. I didn't learn this until i did my initial twin rating on Travel-airs.
Procedure taught to me,
Start 1
after start checks
Bring alternator online.
when amps stabilised, start 2.
Funny enough, wasn't taught to take #1 alt offline to check #2 was actually providing some grunt. Might think about that some more.
------------
Lumps
First part of your post makes sense, the second part....you know the bit you put in caps, doesn't.
I disagree about the chrysler alternator point. Why.. I am not going to digress here in public. Happy to pm you about it tho.
----------
As for load shedding, it appears many more owners are replacing their starters with STC'd high torque versions. These starters are even less load critical, so the added load of the alternator field will make little difference.
Whilst I advocate that you can happily start with the alternator switched on, my personal preference is to bring it online during my afterstart checks. It only takes and alternator 30-45 seconds to replace the charge in a battery after start, so i like to bring it online later, so i can observe it doing its job via the amp meter. That way I know its working. I didn't learn this until i did my initial twin rating on Travel-airs.
Procedure taught to me,
Start 1
after start checks
Bring alternator online.
when amps stabilised, start 2.
Funny enough, wasn't taught to take #1 alt offline to check #2 was actually providing some grunt. Might think about that some more.
------------
Lumps
First part of your post makes sense, the second part....you know the bit you put in caps, doesn't.
Jaba,
Sorry, maybe I should have been more expansive.
We have had a number of serious incidents with Alternators turned off, contrary to SOPs, and then not turned on at the next engine start.
Result, on a good day, just depleted batteries and complete radio failure, on a bad day, loss of the aeroplane, and another Coroner's Inquest.
Hence ---- what does the manual say.??
And the POH is the AFM, see CAR 138.
PLovett, maybe there us some truth in who writes AFMs (POH) as a result of court cases, but whether you like it or not, you are stuck with it. In reality, there is the collected experience of the manufacturer in a POH/AFM --- which is a document you are legally required to observe ( unless you are a CASA FOI, of course, then you have a delegated exemption from the laws of physics and aerodynamic, as well as the aviation law) on pain of potentially severe penalties.
Tootle pip!!
Sorry, maybe I should have been more expansive.
We have had a number of serious incidents with Alternators turned off, contrary to SOPs, and then not turned on at the next engine start.
Result, on a good day, just depleted batteries and complete radio failure, on a bad day, loss of the aeroplane, and another Coroner's Inquest.
Hence ---- what does the manual say.??
From the Cessna 310R POH. Pre Start check list requires Alternators ON. The only time it is different is with Ground Power. For GPU starts Battery and Alternator must be OFF prior to plugging in and they stay that way until after engines are started and GPU plug removed.
PLovett, maybe there us some truth in who writes AFMs (POH) as a result of court cases, but whether you like it or not, you are stuck with it. In reality, there is the collected experience of the manufacturer in a POH/AFM --- which is a document you are legally required to observe ( unless you are a CASA FOI, of course, then you have a delegated exemption from the laws of physics and aerodynamic, as well as the aviation law) on pain of potentially severe penalties.
Tootle pip!!
Last edited by LeadSled; 16th Jun 2012 at 13:38.