2 Checklists, same aircraft....

[halfmilevis]

Hello fellow aviators!

Just a quick question regarding the alternator on start in a Cessna 172, although I suppose the question stands for all piston powered aircraft. I have seen one checklist that calls for the masters (including the ALT) to be on before you engage the starter, and another that says just the battery. As far as my thoughts on the matter, I foresee starting with the alternator on putting lots of load on the system, and a nonlinear current causing strain. I’m fairly certain it makes more sense to flip it on after the engine is started at set to idle at 1000rpm...

Does it matter?

CHEERS!

HMV - Your newly minted PPL pilot!

[cgzro]

Well with the alternator on you are powering the field current which of course reduces the voltage slightly for the starter, also you are magnetizing the field which makes the alternator hard to turn over (because its now generating current) which adds work for the starter to overcome, so two negatives.

So IMHO it makes more sense to keep the alternator, and everything else except perhaps a rotating beacon (to warn people about an impending start) turned off.

[halfmilevis]

I suspected the same!

cheers,

[Big Pistons Forever]

When in doubt go to the POH.

I have C 172, M, N, P, and S model POH's and they all say start with the alternator on. If you have a checklist with something different then I would suggest that the FTU "good idea club" is at work.

[halfmilevis]

Thanks for the reply BPF,

I guess its hard to argue with the POH... I wonder why its written that way though... as per the above reasons...?

[halfmilevis]

Looks like this is a big topic on a few other forums after a Google search.... And it looks like it still remains ambiguous. I get that the POH says throw both on... but maybe that’s so its stupid proof... Even though in your run up you do a “load check”.

To me, it seems that using the alternator during start could potentially damage it... more expensive than a battery. Also, if your low on juice, leaving the ALT off saves a bit of draw and allows the starter more available voltage and amperage.

HMV

[Big Pistons Forever]

Thanks for the reply BPF,

I guess its hard to argue with the POH... I wonder why its written that way though... as per the above reasons...?

I would guess that the extra load is so low it just doesn't matter. I guess theoretically it makes sense but from an operating view point there is no practical reason to not follow the POH.

If you want to damage the alternator follow another common FTU "good idea", test the alternator by turning it on and off at runup RPM. This is specifically contrary to the POH and is a bad idea especially if the battery is a bit discharged because when you cycle the alternator switch back on the regulator will slam a big charge rate into the electrical system.

While we are on the subject of stupid FTU practices I will finish with the start on the left tank. taxi on the right tank FTU checklist silliness. This is not required by the POH, doesn't prove anything, is unnecessary and sets you up for taking off with one tank selected instead of both.

[halfmilevis]

The only applicable reason I can think of for shutting down the fuel tank on Left is because it is the last item on the shutdown, and therefore walking into the aircraft for your flight, you know the shutdown checklist was followed.

Im glad you mention that alternator load ALT OFF, ON thing, because I definitly do that... I load the system, turn the alt off, wait 5 seconds, turn it on and check for charge... Guess its kind of useless...

HMV

[Big Pistons Forever]

Im glad you mention that alternator load ALT OFF, ON thing, because I definitly do that... I load the system, turn the alt off, wait 5 seconds, turn it on and check for charge... Guess its kind of useless...

HMV

It is more than useless as it invites a electrical spike when you turn the alternator back on which is death on avionics.

The correct way to check the charging system is as follows.

At runup RPM with little load the ammeter needle should be either neutral (neither discharge or charging) or slightly on the charge side. You should apply a load to the system by turning on the landing and taxi lights and the pitot heat. As each item is switched on you should see a momentary dip of the needle to the discharge side followed by its return to the middle position. With all the load on, the needle must not be indicating a discharge. This would be indicative of a faulty charging system.

[photofly]

If you want to damage the alternator follow another common FTU "good idea", test the alternator by turning it on and off at runup RPM. This is specifically contrary to the POH and is a bad idea especially if the battery is a bit discharged because when you cycle the alternator switch back on the regulator will slam a big charge rate into the electrical system.

Can you be a bit more precise about "slamming a big charge rate" and why this might damage the alternator? Electricity doesn't have much mass, so I'm not sure "slamming" is a very good metaphor. What actually do you have in mind and why would it cause damage?

On the subject of energizing the field coils with the engine at run up speed causing a voltage spike (which would be bad, if significant) - have you actually checked whether this occurs?

In vehicle electrics, the worst voltage transients are usually while the starter motor is energized; if that's true for aircraft it would make a lot of sense to keep the alternator shut down until the engine is running.

[Posthumane]

While electricity does not have a lot of mass, an alternator is basically a big induction coil. When you try to change the current flow on an inductor, you generate a lot of voltage (and vice versa). Basically if the alternator is putting out a certain amount of current and the load is suddenly shut off, the sudden change in current can cause a voltage spike. Same reason you get a voltage dip when you first turn a load on.

While this probably isn't that great an issue with relatively low output alternators, it is very evident on bigger ones. We have a vehicle at my workplace with a 24V, 400A alternator on it. It had a piece of equipment installed for testing which drew something on the order of 100 amps while operating. Every time the operator tried to use the equipment the electronics on the vehicle went haywire, causing an ABS fault, locking the transmission in a single gear, etc. The problem was that the 100 amp load was suddenly switched on for a few seconds and then switched off (and it was an inductive load - a DC motor). When I put a scope on the vehicle's charging system I found that the voltage was dipping down to around 15V when the load was first applied, climbing back up to 27V, and then jumping to over 38V when the load was removed.

[photofly]

You're conflating two issues. Switching inductive loads like a big motor generates high transient EMFs. Switching resistive loads like those in an aircraft doesn't. (Except of course for the starter motor, so there's an argument for keeping all the electronics - including the voltage regulator - offline while the starter motor is in use).

An alternator contains some coils, but no, it's not an inductor, and it doesn't behave like one.

[Posthumane]

It doesn't matter much if the load is resistive on inductive. In the case I was talking about, the motor was completely cut in and out of the circuit, so back EMF is generated on the motor side of the switch. The issue on the vehicle side was caused by the fact that alternators have a rated slew rate - the rate at which the output is able to switch. In the case of the large 400 amp unit it turned out to be in the order of about 100 amps per second, whereas smaller ones will be faster.

The fact that an alternator has coils DOES matter, and it does have a similar effect as a pure inductor. The field current cannot drop instantly when a load is switched off, which means results in the output power not dropping instantly either. I seem to remember you have a physics degree, so I don't doubt your understanding of induction. The issues caused by current switching are something that have to be dealt with regularly in my field.

EDIT: On the other hand, when you turn off the alternator switch with a lot of load on you are simply switching off the field current, which wouldn't cause a voltage spike on the main bus where your avionics are connected. It would only be an issue when you switch a large load that is connected to the bus.

[photofly]

Sitting between the alternator and the bus is a rectifier and regulator, which I'd expect to handle any load variations.

I'll try to beg, borrow, or steal a storage 'scope, and look at the voltage bus. I suspect that any voltage transients caused by switching the alternator side of the master on and off are either non-existent or too tiny to worry about. In any event, I'd do the test (if I were going to do it) with the avionics isolated.

BPF was concerned about inrush current to the battery after a period of discharge damaging the alternator (or perhaps the regulator) somehow. I don't see an obvious path to damage that way, but I'm open to suggestions.

[Posthumane]

The regulator works by adjusting the field current. If the load change is too fast then the regulator will not be able to switch the field current quickly enough, hence a voltage spike. Most are too small to worry about on small vehicle alternators.

I agree with you that the transients caused by switching the alternator off are not worth worrying about. Regarding damage due to inrush current to the battery, that is also very unlikely. Alternator output is self limiting, and a discharged battery upon start would cause a greater current draw.

[sidestick stirrer]

I thought that was the second function of the battery in the circuit( really, once the alternator is online and it can sense bus voltage, do we really need the battery except as backup?), to absorb and dampen spikes in the system.
There's a reason they're called "accumulators" as they serve the same purpose as their brethren in hydraulic systems.

[Big Pistons Forever]

All the electrical theory is fine but if you have a C 172 with a battery that is weak and "test" the alternator by turning it off and then on at runup RPM then my experience is the ammeter will show an instantaneous initial 40 + amps charge as the regulator senses the low battery voltage and slams the charge to it. I see no scenario where this is going to be good for either the battery or the electrical system in general.

But IMHO the bottom line is simple. If you follow the direction of the POH with respect to how to determine the charging system is operating correctly before flight this situation will never occur

[Cessna driver]

No C.S in here yet??? Shocking!

[trey kule]

I would suggest that the FTU "good idea club" is at work.

Now, that is hilarious if it was not so true.

Anyone think to take a moment and call Cessna tech support?

Sometimes it is not about the starter. It might be that that the wise ones figure if they suggest pilots start with the alt off, they will forget to turn it on, so better to start with it in,...
Some piston planes, like the islander, for example, you do bring the gens on line after the start..separate switched....I cant recall any others, but there probably might be

[PilotDAR]

If you have the discipline to assure that the alternator is functioning correctly before take off, I like the idea of leaving it off for start. If you're lazy or forgetful, and the flight manual allows, start with it on.

I like the start with it off idea for two reasons: Yes, very slightly more efficient use of battery power during the start, and, if the alternator is finicky about coming on line, deliberately turning it on after the engine RPM has stabilized at idle makes the "operation" and "confirmation" more certain. As you turn it on, you will instinctively check that it went on.

Note that a few aircraft have a hidden "self resetting" circuit breaker for the alternator field. When these CB's fail (and they do), they will prevent the alternator from coming on, and that may be difficult to notice. The manual reset makes that failure more evident.

No C.S in here yet??? Shocking!

He's been shocking elsewhere recently, perhaps he's taking a break.....

[Pop n Fresh]

If you have the alternator on during start, the slow motion limits the voltage until the regulator begins to function.

If you bring on the alternator online at speed what limits the initial voltage? You might cook the regulator via over voltage over time. Solid State things are also much more voltage sensitive than the older electronics.

[Posthumane]

While this likely isn't a feature found in older alternators found on most training aircraft, many new voltage regulators that are installed on large trucks and military vehicles have a soft start feature where they won't fully energize the field coil until after the engine has been running for a few seconds. This allows the alternator to spin freely on start without any resistance and prevents a large spike by ramping the voltage up slowly.

[niwre]

What about that mixture check while at runup power? Even had a 'discussion' with a PE about it. POH makes no mention of this yet many FTUs have this in their checklist.

[halfmilevis]

Been away for a bit, excited that this thread got so active!

Prior to takeoff, the checklist calls for MASTERS ON, which would mean that even if you start with just the BAT, and forget the ALT, you should hit it prior to departure (assuming your following the checklist). As for the fuel on both deal, I could get behind the idea that starting with the left tank, runnup on the right and takeoff and flight on both ensures that both fuel tanks are providing fuel. If you start and do all your work on both, you could only be feeding from one and not know it...

I am happy to announce that I did do my run up yesturday and didnt turn off and on the ALT in the load check... instead I loaded the system and ensured no voltage discharge.

HMV

[Big Pistons Forever]

What about that mixture check while at runup power? Even had a 'discussion' with a PE about it. POH makes no mention of this yet many FTUs have this in their checklist.

This is what I teach (carburated engines)

With Runup RPM set apply full carb heat and observe the RPM drop indicating carb heat is working. Wait 10-15 seconds and watch for a rise in RPM indicating carb ice was present and has now been melted. Assuming no rise, which means no ice start leaning the mixture. You should see a rise in RPM as the over rich mixture created by the application of carb heat is corrected followed by a reduction in RPM as the mixture becomes over lean and power drops (you only need to see a drop of 50 or so RPM). This indicates the mixture control is working properly and the mixture control can be returned (Slowly !) to the full rich position. Now reduce the throttle to full idle and ensure the engine does not stall. Your check is now done so you can return to 1000 RPM and carb heat cold.

If the RPM does not rise as the mixture is leaned then the engine is running too lean and maintenance needs to be consulted. I have seen this twice. The first time was due to a dirty carb jet. The second was because the wrong carb had been installed on the engine . Unfortunately in the second case it was too late and continued lean running had overheated and damaged all 4 cylinders.

Unfortunately what I see far too often is that the "mixture check" involves yanking the mixture control out until the engine dies and then ramming it back in. This proves nothing and invites a backfire which will ruin the muffler. Don't so this !

For fuel injected engines lean enough to show a drop in fuel flow and a rise in EGT. That is all you need to see to know the mixture control is working. The test that the engine is not set up too lean is the fuel flow value at full power. At the start of the takeoff run as soon as you reach full power you should check that you are getting the POH listed fuel flow. Any fuel flow value below this should cause an immediate rejected takeoff.