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:):)

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.

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.

Folks,
What does the manual for the aircraft say??
Tootle pip!!

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?

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.

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 :}

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

Grogmonster, aircraft POH are written by lawyers for the lowest common denominator. :suspect:

In this case that means the person who is likely to forget to turn an alternator on after starting. :ugh: 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. :rolleyes:

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. :suspect:

I have yet to hear or read a convincing argument for either action. :hmm:

Quote:

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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

Quote:

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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.

---

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.

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.

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?

Ohhhhh, my head hurts just reading that!

Dr :8

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.

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

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.

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. :(

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!!

Leadie....

on that post, i think its beyond what the manual says......its how are your checks/scans working?

on leadies edit......

the warrior manual states the same for GPU starts....maybe some call for answers as to why it is so from those in the know.

jass,
I'm with you. In the G.O.Ds, I well remember the guy at Hawker DeHavilland, with his little number punches, putting numbers on the said Chrysler alternators one by one, from a bulk buy at the local dealer, and raising a record, then chanting the magic incantation that turned auto parts into "fully approved aviation quality genuine spare parts".

I can probably dig up an old list I has that cross referenced the commercial sources of just about the whole electrical system on most Piper aircraft.

To this day, this is the mechanism whereby ball an roller bearings from CBC or Bearing Services have the CAR 30 holy water sprinkled on them, turning them into "genuine aviation spare parts".
Actually, this can be a trap for young players, as often auto/industrial grade bearings are too close tolerance ---- the OEM spares are actually out of standard tolerance rejects, when they are rejected because the "fit" is sloppy.

Some of you will remember some unfortunate accidents to a particular model of helicopter, because the manufacturer apparently ran out of "approved" bearings for a rotor, and fitted SAE standard bearings. In part, this is the story of the "dot" after the number on said bearings, denoting the out of tolerance rejects that have enough slop for the aeronautical application.

Tootle pip!!

feeling left out leadie?

I can't understand why you disagree about the Chrysler alternators. I did my Electrical engineering tertiary qualifications in the mid seventies. I have worked on GA aircraft ever since. I own four aircraft. How many Chrysler alternators do you think I have seen recently ?

I work on GA in Aussie, and have done so in many other places about the world.
I have to imagine that there is a stash of aircraft so equipped I have yet to stumble upon.

It has been a long time since I last saw the Chrysler alternator, and I think it would have been on a Cherokee Six. They are just not common like the Prestolite is.

I doubt you are talking from experience but possibly from a secondhand account.
The same as the old generators. They would not have been fitted to many aircraft after the 1960's... we, - the guys that work on the machines just do not see many of them.

I have an Electrical category on my LAME licence so I am not just making this stuff up.

While talk of restamping etc may be a good story it is certainly not prevalent.
The aircraft components are not the same as a car.... normally. Yes I have seen MkIII Zephyr parts fitted by some manufacturers, even Girling brake components... I myself have even fitted toilet seat parts, and that was to a military aircraft. (The seat buffer.)
At the end of the day the machine just has to conform.... checked at each annual review in NZ.

Comparing the average GA aircraft electrical system to that of a car is just not quite accurate. I cannot think of a car, or an aircraft that are the same.

Similar yes, but generally quite a bit different.
That would be why you don't see alternator switches in the average car.

They are Timkin bearings, same part number as those originally fitted and were approved by a CAR35 organisation. No holy water was involved. Each bearing had to be inspected to ensure dimensional conformity. was to do with the internal and external dimensions. Fitted to Bell 47 tail rotor drive shaft.NO!! It was an R22 Transmission that an organisation in NZ(check me on location Beeza) fitted because they were a cheapskate dodgy organisation, had fk all to do with Frank's factory.

Aircraft are just a machine. Made up of various components. Often in the case of GA aircraft there is nothing too special about them.

Sure many of the components are sourced from the same suppliers as other industries. Bearings, light bulbs, electrical components as mentioned earlier, can be identical. Often they are. It is the next higher assembly that will differ, sooner or later.

The boat outboard motor hydraulic pump on a Piper Arrow has to be different somewhere. Indeed we can soon see that not many marine products use Mil Spec 5606 fluid.. maybe that could be an important difference. Same with brake components. I guess what may often look identical may just have some important subtle differences, - the changed part number may indicate different seals have been fitted.

The electrical system on some GA aircraft does use identical components to those fitted to other vehicles. The electrons are the same so yes, - it may be common to see inter-changeability with the component. The overall system may well be different.... as I believe they are.
Cars simply do not require an alternator field switch. They may not have an Overvolt relay added to the circuitry either.
That said, many automotive electronic regulators worked much better on the Cherokees and the Islanders than the OEM products... remember electronics changed a lot during the 70's.
The regulator generally just varies field current in response to alternator output voltage (bus) when all said and done. I am sure many here would have experienced the fluctuating needle movement in some of the original Piper ammeters (loadmeters).

I don't recall the R22 transmission.
I do know much (too much !!) about the R22 drive train failure of the one out of Nelson seven years ago. It was the owner who demanded an incorrect Dash number part be fitted. Saved him a Dollar but cost him his life.
Some guys really do have that mentality...

Blackhand,

I am, in fact, referring to Australian accidents re. helo. not the NZ case, to which you refer.

As to the original manufacturers of bearings supplied by CBC, Bearing Services or other engineering supply houses, I have sat and watched (awaiting delivery) as a felt pen was used to "inscribe" the Part No. on the bearing box, and the release note (on invoice) raised, by the CAR 30 "supplier". The actual bearings never left the box.

That CAR 30 magic is a valuable quantity, a $15 or so bearing was magically turned into a $139 bearing, by said holy water.

Quite frankly, very few organizations would have the equipment to dimensionally test a ball or roller bearing, outside of the OEM.

Tootle pip!!

Possible reasons for the recommendation to switch of the alternator field for ground power starts.
1 . Voltage transients created when switching between GPU and battery potentially damaging diodes in alternator (think jumper leads and computers in cars).
2. GPU not appreciating being 'charged' by aircraft alternator.

I doubt an alternator would be turned fast enough during cranking to generate any power.

Are you referring to the "Do No Turn Alternators OFF In Flight" placard that does NOT appear in POHs at all, but CASA still insists is fitted as a part of a CofA issue?

MakeItHappenCaptain - yes that's what I was referring to. I remember when learning to fly asking what the purpose of this placard is (not in POHs I apologise, it was a while a go and I can't remember where exactly I saw it). Except for the obvious eventual depletion of the battery - my instructor said it was more to do with if you turned the alternator ON in flight - then proceeded to draw on the blackboard the waves of alternating current sprouting something about turning it on with the engine spinning at say 2200rpm would create a large spike in current that would pop your relay/regulator whathaveyou.

We all prefer an explanation of any kind rather than no explanation at all, just wondering if anyone could back this instructor up. He was a good'n

Alternators do not self excite. Will not come back on if battery voltage is too low

So why is Australia still the only country, in this age of comprehensive flight manuals, applying placards that came from the days when flight manuals came off a typewriter and were (agreeably) woefully written.

I deliver new aircraft from a factory overseas and that is the only item that is requred to be added to make the aircraft airworthy in Australia. Bit of a joke, unfortunately.:cool:

Exactly BH.

As we know it would be generalising to say all GA aircraft are the same, they are far from it.
Something like a Cherokee has no control of the alternator apart from the switching, and control, of the field voltage.

ie, no voltage on the field winding means no output... this is what the Alt switch does. The alternator switch does not control the high current output from the alternator through a contactor... most installations don't have such a beast.

The Overvoltage relay, sometimes contained within the regulator, does the same. It just turns off the field voltage which in turn kills the alternator output.

Some salient features of a typical GA electrical system.
The battery contactor is controlled by placing an EARTH at the control, via the Master Switch.
The alternator switch controls the FIELD voltage on the alternator circuit.
There is often no protection on the Alternator output, it does not go to the bus via a fuse or CB.... more likely through an ammeter shunt though.

As mentioned in the previous post, - the battery must have sufficient voltage to operate the Master Contactor and then also be sufficient to provide alternator field.

Turning the Alternator switch off in flight may not necessarily guarantee it will come online again.
Some charging systems can be a little fragile and can do something akin to PIO when the alternator field 'chases' the bus voltage. This could be more likely at higher rpm. Basically a series of high voltage surges as the regulator struggles to come to terms with a very active and lively alternator output voltage. I have experienced the Overvolt relay operating and killing the alternator whenever it the alternator was switched on.
If it persists then the obvious thing to do is bring the engine back to idle before trying again. Switching (shedding) some electrical load may be required also.
EDIT.. Sometimes load may need to be applied, especially if the battery is at a high state of charge already. With very little load on the system, a fully charged battery and a high output voltage from the spun up alternator the bus voltage can very easily rise a few points. An Overvolt relay set at say 14.2 volts could easily trip and open the field. (End Edit).

Although the term alternator implies they are an AC device they are not in a typical aircraft installation. The output is DC as the diode pack is self contained in the rear of the alternator.


Now a completely different subject..

The Ignition switch is independent of the charging circuit.
It does do a number of functions though, some of which are not always understood.
The Ign switch places an earth on the relevant magneto, via the P lead, to turn the mag off.
The switch also takes bus voltage and applies it to the start contactor when the switch is in the 'Start' position. At this position there is also other switching which normally applies an Earth to the right magneto. This is via an external link screwed across the terminals.
The link can be opened if the intention is to have both mags live during the start... if for example the engine has two impulse couplings fitted.

Normally only half the spark plugs are firing during a typical piston engine start. Often the top plug on one bank and the bottom on the other.
So it can be seen that while the switch is held on 'Start' it could be that the engine may only be firing on a couple of cylinders... if the bottom plugs are fouled or flooded.
That is why they will often pick-up when the switch is released.

MIHC

The UK CAA had/has an AD whereby the starter circuit has to be either through a seperate switch or have an indication when the starter is operating.

The Islander has the External power switch to confuse pilots but it is common to see lights added to Cherokees. A Starter Operating lamp.

The placard you speak of is new to me but could be a CASA AD or other certification requirement. Some of those things can take a while to rewrite or sort out.
Often the original incident that prompted the requirement was years ago... but so catastrophic that no-one is prepared to change back from the status quo.

Has anyone put it to CASA ?

Piper does not recommend the alternators be switched off.

It is the master switch and any avionics switches that needs to be switched off during connection and disconnection of the ground power supply to prevent sparking and damage to the aircraft electrical system.

If a GPU battery start is attempted with the master switch in the on position the battery may be connected in parallel and drag the power of the GPU down to its level. That is if you try to start with external power because of a low aircraft battery and leave the master switch on you may experience very low crank rate due the system being dragged down to the lower level of the depleted battery. In cold conditions they state you can use the GPU battery in parallel with a charged aircraft battery to prolong cranking time.

The Piper variants I'm familiar with have checklists that require confirmation of the master switch in the on position before start. Piper refers to the combination split Bat/Alternator swith as the Master switch in most cases unless seperate alternator switches are fitted. Master switch on refers to both battery and alternator on in the split switch situation. After start the checklists call for a simple check of alternator output.

The alternator off switch is provided to switch it off if it fails to prevent drain of battery charge attempting to excite the dead alternator.

As far as load on start the actual cranking of the engine and accessories would far outweigh any load from the alternator itself. Switch on and off the alternator after start to see how much power it drains from the running engine, there will not be much.

Errrmmmm….how do I work out what the alternator itself drains, from switching it on and off? :confused: Even if every system is switched off after start in the aircraft I fly, the battery will charge quite vigorously when the alternator is on. If I leave or switch the alternator off after start, the battery won’t charge.

Is the difference in the on/off current measurements the drain of the alternator?Example number 723 showing why it’s important to have aircraft-specific systems knowledge.

I’ve looked at the circuit diagrams for ostensibly identical aircraft. Some have 2 impulse coupling-fitted magnetos, some only 1, some have retard breakers, some have ‘shower of sparks’ and other start-assistance technology fitted to only one magneto, and some to both. 4 different ignition switch wiring and shorting link arrangements, despite the switches themselves being the same, the airframes being the same and the engine (aside from ignition systems) being the same.

Be careful: all systems on ostensibly identical aircraft may not be the same.

I was not talking about how much current an alternator may draw if left on but the additional load or strain on the starter by having to turn a loaded alternator in addition to the engine and other equipment. In a fixed pitch engine like on the piper warrior any increase in load on the engine will result in less rpm. Items like airconditioning will drain a substantial amount of power and therefor needs to be off during critical phases of flight. In turbine aircraft you can see very slight fluctuations on EGT/ITT when Gens are switched on/off.

I was only stating a test where you could see how little engine power the loaded alternator uses at low engine rpm.

The power required to excite the alternator is very minimal so even if it was drawing some charge during start it would be of absolutely no consequence. The reason it needs to be considered after an alternator failure in flight is that you are minimising to essential power only, cutting any non-essential systems no matter how small.

Very much so.
I did tend to try and state that I was generalising as there is a huge variety of GA aircraft.

Probably all the moreso now as new generation training aircraft are starting to appear.

I know of a Tomahawk with twin impulse couplings, Slickstart on the left mag, and with the ignition switch link opened.
An otherwise identical sister aircraft has the more conventional single impulse.

Going back to charging and electrical systems, - one has the PEP (Ext Pwr) socket and the other does not so quite a difference there also.

I think those two aircraft would be a good graphical example of how the AFM may not accurately reflect the installation on that particular machine.
Many pilots would possibly be unaware of the differences, especially on a training aircraft.

I am not a big fan of turning the alternator off and on as a check after start-up. I would not allow it on my own aircraft, - preferring the landing light to be operated instead. I monitor bus voltage during flight with the Pilot III displaying supply voltage. Some aircraft, like the Tomahawk, also have an Alt Inop light.

Thanks BB. That one has been with me for years

Thread drift,
Recent problem with shower of sparks had this as a symptom.
Had me nonplussed for a while, same vibrator services both engines and was making appropriate noises.
Worked it out in the end - hint push button starter switch

Simple Math, engine at 2300 rpm alternator at max output say 100 amps at 28 volts = 2800 watts

2800 watts / 746 watts/ hp = ~ 3.5 hp needed from the engine to power the alternator plus some mechanical load, friction, windage.

So alternator load is hardly a factor.