Battery isolator

[Dafydd Llewellyn]

So why not just use the Battery Solenoid switch which then can cut power to the Starter as is possible in a LOT of aircraft ??Jake J

The problem WAS the starter solenoid switch. You don't want to cut power to other than the starter circuit. Yes, I suppose one can use one of those isolators in series with the starter solenoid, and shut it off immediately the engine has started as a standard procedure.

 

 

[Bruce Tuncks]

I take your point Dafydd about using the isolator just after your normal startup. But why not automate this? You could do this by having a second solenoid, this would be in series with the first with respect to the heavy start-current and in parallel for the small actuating current from the press switch.

 

This would be easier to install and lighter if it enabled you to avoid using more heavy cable. But it would add a bit of weight and complexity.

 

Another way would be to connect a buzzer to the starter, you would hear it buzz while cranking and if it kept buzzing then you would know to activate the isolator.

 

I'm surprised at how many people have had an incident.... in 50 years of driving a car just about every day, I've never even heard of a stuck-closed solenoid.

 

... Bruce

 

 

[Dafydd Llewellyn]

Yes, as usual there are about 47 ways it could be done. It's about time people started thinking about it.

 

 

[Guest Maj Millard]

Well like a lot of things it's rare (like structural failurers)...but when it happens you'll wish you'd fitted an inexpensive battery isolator switch................Maj.....

 

 

[Bruce Tuncks]

Thanks guys for the good information. I'm going to see if I can fit one to my SK without using much of the heavy start-current cable. Maybe an extension to the key will be needed. The other thing will be to wire in a panel indicator, probably a red flashing light, which will show if the starter motor is getting 12 volts. This will light up on pressing the start-button and normally go out. Better than a buzzer which might be drowned out with the engine running.

 

... Bruce

 

 

[Dafydd Llewellyn]

Thanks guys for the good information. I'm going to see if I can fit one to my SK without using much of the heavy start-current cable. Maybe an extension to the key will be needed. The other thing will be to wire in a panel indicator, probably a red flashing light, which will show if the starter motor is getting 12 volts. This will light up on pressing the start-button and normally go out. Better than a buzzer which might be drowned out with the engine running.... Bruce

If you simply bung in a second solenoid in the manner you have described, you create what is called a "hidden function" - i.e. one of the solenoids could weld itself in the "ON" position, and you'd not know about this until the second one failed. One can go batty trying to figure the probability of a double failure; but can you think of a way to test them in the course of normal usage - for example, it is normal to use the electric fuel pump to start the engine - if you watch the fuel pressure gauge, it will tell you if that pump is working OK. Then taxi on the engine-driven pump alone, which will tell you that it's OK; then take off using both pumps. That defeats the "hidden function" issue. Like, have two starter buttons and check the starter does NOT pull in if you only press one of them. I'd prefer something that did not rely on the pilot doing something out of the ordinary; a thermal circuit breaker that had a fairly low rating but a long time-constant might be a better answer than the simple limiter; but if you think about it, a slow-blow fuse is essentially just that.

 

 

[eightyknots]

I take your point Dafydd about using the isolator just after your normal startup. But why not automate this? You could do this by having a second solenoid, this would be in series with the first with respect to the heavy start-current and in parallel for the small actuating current from the press switch.This would be easier to install and lighter if it enabled you to avoid using more heavy cable. But it would add a bit of weight and complexity.

Another way would be to connect a buzzer to the starter, you would hear it buzz while cranking and if it kept buzzing then you would know to activate the isolator.

 

I'm surprised at how many people have had an incident.... in 50 years of driving a car just about every day, I've never even heard of a stuck-closed solenoid.

 

... Bruce

I have had this happen to my HR Holden in the middle of Sydney a few decades ago. More recently, I had a stuck solenoid on my Australian built Cox ride on lawnmower in New Zealand. Twice in one lifetime with two entirely different 'vehicles' in two countries. It could be an aircraft next.

 

 

[aro]

I had this on my PA 28; it destroyed the starter and melted a terminal off the battery

PA28s I have flown had a Starter Warning light that is supposed to warn that the starter is still engaged.

 

As others have noted, a master contactor wired to the master switch is common practice. (But not necessarily in RAA aircraft) It should disconnect all power at the battery including the starter circuit so it can be used if the starter switch sticks. I think it is standard in Cessna and Piper aircraft. A starter warning and master contactor has most of the bases covered I think.

 

I second the recommendation of the Aeroelectric connection for those interested in aircraft electrics. http://www.aeroelectric.com

 

No sense in reinventing the wheel.

 

 

[Dafydd Llewellyn]

Mine was an earlyl PA 28-140 and it did not have a starter warning light - I'd assume Piper fitted one to later versions because of the problems with the early ones. Yes, a starter contactor downstream of the master contactor is desirable - but the damage is done long before the pilot will be aware of it, and you can't turn off the master switch before takeoff as a precaution, to prevent a starter run-away. So having a master contactor (which I consider should be there in all aircraft that have an electrical system) does NOT provide protection against a failed starter contactor. It give the pilot something to do after he smells the smoke.

 

 

[aro]

The master contactor performs the function of the battery isolator being discussed, with the advantage that it can be located at the battery with the switch on the panel.

 

I don't understand the failure mode where a high current fuse or second solenoid with multiple switches etc. would help, but a starter warning light (and perhaps overvoltage warning light) and master contactor wouldn't do the job.

 

 

[rankamateur]

I have the exact some one in my Savannah...about 12 bucks at Jaycar

How do you get on when the pax puts the size 10 heel on the key and breaks it clean off? This would happen right after you have just trimmed the aircraft, this usually leads to the passenger moving their feet to stretch their legs, and the nose goes up!

 

 

[Downunder]

How do you get on when the pax puts the size 10 heel on the key and breaks it clean off? This would happen right after you have just trimmed the aircraft, this usually leads to the passenger moving their feet to stretch their legs, and the nose goes up!

If I tried to get my foot to it, my knee would probably be under my chin and mine is under the pilots seat.

 

 

[Old Koreelah]

...the damage is done long before the pilot will be aware of it, and you can't turn off the master switch before takeoff as a precaution, to prevent a starter run-away.

Since a Jab engine only needs the battery for cranking, what if I get into the habit of throwing out my manual isolator for a few moments after startup?

 

 

[aro]

Since a Jab engine only needs the battery for cranking, what if I get into the habit of throwing out my manual isolator for a few moments after startup?

The battery also tends to stabilize the output from the alternator. Disconnecting it might be OK if you have the big capacitor (and it's in good condition), it's possible it might be OK without it, or it might cause a voltage spike that fries all your avionics... It's a gamble.

 

 

[eightyknots]

Since a Jab engine only needs the battery for cranking, what if I get into the habit of throwing out my manual isolator for a few moments after startup?

What about your radio?

 

 

[Dafydd Llewellyn]

The master contactor performs the function of the battery isolator being discussed, with the advantage that it can be located at the battery with the switch on the panel.

No, it doesn't, because it turns off EVERYTHING, whereas a battery isolator switch can be located in the starter circuit alone. That means the switch needs to be downstream of where the starter circuit splits off from the main bus. Used that way, it's ( strictly speaking ) incorrect to call it a battery isolator.

I have not studied recreational aircraft electrical systems, but I suspect some of them fall a long way short in regard to the use of a proper master contactor, and in regard to circuit protection of the starter circuit. Standard automotive practice is not good enough.

 

 

[Dafydd Llewellyn]

The master contactor performs the function of the battery isolator being discussed, with the advantage that it can be located at the battery with the switch on the panel.I don't understand the failure mode where a high current fuse or second solenoid with multiple switches etc. would help, but a starter warning light (and perhaps overvoltage warning light) and master contactor wouldn't do the job.

The failure mode is anything that causes an uncommanded starter solenoid turn-on in flight; that could be a failure within the solenoid itself - e.g. if the spring dies; or it could be an electrical fault that puts power onto the solenoid control circuit. The modern form of starter that uses the solenoid to mechanically pull the starter pinion into mesh with the ring gear before making electrical contact may be less prone to this than types that throw the pinion into mesh via a Bendix drive, where the starter solenoid is not an integral part of the starter motor. The setup that failed on my PA 28 was the latter type; the engine had a direct-drive starter essentially from a 1950 Chev 6-cylinder car engine, and the cheapest-possible FoMoCo starter solenoid mounted on the firewall; and the starter solenoid end crimp let go, releasing the spring pressure so it turned ON under gravity. It was a really Mickey-mouse setup. I replaced it with a geared Prestolite alternator (which involved changing the ring gear) and a proper Cutler-Hammer contactor, and after that the engine started properly and the system gave no further trouble.

In the engine test cell, the starter contactor is powered by a circuit that uses a latching relay that must be triggered via a press-button, before the starter button has any power available to it; and the latching relay coil is earthed via the alternator field winding - so it drops out when the alternator comes on. This makes it "impossible" to inadvertently energise the starter with the engine running by finger trouble - but it does not protect against mechanical failure of the starter contactor itself. However, I'm using the same type of Cutler-Hammer contactor as I used in the Cherokee, so I don't expect it will give trouble.

 

 

[Downunder]

No, it doesn't, because it turns off EVERYTHING, whereas a battery isolator switch can be located in the starter circuit alone. That means the switch needs to be downstream of where the starter circuit splits off from the main bus. Used that way, it's ( strictly speaking ) incorrect to call it a battery isolator.

Except when you have battery backup for your panel......but probably not strobes and lights

 

 

[Old Koreelah]

What about your radio?

I don't power up the radio until after startup, but as Aro points out, the battery is needed to stabilise the alternator. Maybe not my best brainwave. I'll claim the Gareth Evans defence: it was a good idea at the time...

 

 

[Dafydd Llewellyn]

Well, that's not compliant with FAR 23.1361

 

§ 23.1361 Master switch arrangement.

 

 

 

 

 

(a) There must be a master switch arrangement to allow ready disconnection of each electric power source from power distribution systems, except as provided in paragraph (b) of this section. The point of disconnection must be adjacent to the sources controlled by the switch arrangement. If separate switches are incorporated into the master switch arrangement, a means must be provided for the switch arrangement to be operated by one hand with a single movement.

 

 

 

(b) Load circuits may be connected so that they remain energized when the master switch is open, if the circuits are isolated, or physically shielded, to prevent their igniting flammable fluids or vapors that might be liberated by the leakage or rupture of any flammable fluid system; and

 

 

 

(1) The circuits are required for continued operation of the engine; or

 

 

 

(2) The circuits are protected by circuit protective devices with a rating of five amperes or less adjacent to the electric power source.

 

 

 

(3) In addition, two or more circuits installed in accordance with the requirements of paragraph (b)(2) of this section must not be used to supply a load of more than five amperes.

 

 

 

© The master switch or its controls must be so installed that the switch is easily discernible and accessible to a crewmember.

 

 

And does your backup electrical power system also keep your engine EFI system going? And if so, how does THAT comply with a requirement such as FAR 23.1361?

 

 

[jakej]

Whoooaa Dafydd, please back off here, the direction of this post is starting to sound like some LAMEs who don't want us to work on the aircraft that WE built. Some have actually approached CASA about this. We are NOT FAR 23 aircraft & never will be, however that is not to say we shouldn't use best industry practice where possible.

 

IMO (and industry practice, etc,etc) simply put, the Starter should have a Solenoid & the Battery have it's Isolator (switch, toggle or otherwise) too.

 

IF anyone has essential equipment that needs to be on all the time eg EFI systems, then one method could be to have switched/fused power direct from the battery so that when the battery Isolator is off the required systems to keep the engine running are still powered up. In an emergency, or if a Starter is stuck on, then disconnecting the battery for other items like flaps, lights & even radios are not really needed in most cases.

 

Jake J

 

 

[eightyknots]

No, it doesn't, because it turns off EVERYTHING, whereas a battery isolator switch can be located in the starter circuit alone. That means the switch needs to be downstream of where the starter circuit splits off from the main bus. Used that way, it's ( strictly speaking ) incorrect to call it a battery isolator.I have not studied recreational aircraft electrical systems, but I suspect some of them fall a long way short in regard to the use of a proper master contactor, and in regard to circuit protection of the starter circuit. Standard automotive practice is not good enough.

I think the ideal would be two master switches, as follows:

 

1. an isolation switch for the battery, that is, everything will be turned off; and,

 

2. an isolation switch for the starter circuit, that is, the solenoid only will be turned off.

 

In that way pilots have the most control over any potential difficulties that may arise.

 

(with No 2, it is absolutely vital that a light on the dash indicates when the solenoid is energised)

 

 

[Dafydd Llewellyn]

Whoooaa Dafydd, please back off here, the direction of this post is starting to sound like some LAMEs who don't want us to work on the aircraft that WE built. Some have actually approached CASA about this. We are NOT FAR 23 aircraft & never will be, however that is not to say we shouldn't use best industry practice where possible.IMO (and industry practice, etc,etc) simply put, the Starter should have a Solenoid & the Battery have it's Isolator (switch, toggle or otherwise) too.

 

IF anyone has essential equipment that needs to be on all the time eg EFI systems, then one method could be to have switched/fused power direct from the battery so that when the battery Isolator is off the required systems to keep the engine running are still powered up. In an emergency, or if a Starter is stuck on, then disconnecting the battery for other items like flaps, lights & even radios are not really needed in most cases.

 

Jake J

LAMEs who take that view are being extremely stupid in regard to their Hangar Keeper's insurance policy; do they REALLY want the liability of an aircraft that does not comply with a recognised design standard? A few have learned that lesson the hard way. I'm merely trying to point out some of the aspects that are standard practice in aircraft electrical system design. Use FAR 23 as a guide-post, not a hitching-post.

 

 

[Bruce Tuncks]

It took longer to do than it should have, but I've finally got my "solenoid engaged" warning light fitted. It is just a red panel light from Jaycar, with the power feed coming from the starter-motor side of the solenoid.

 

Now if the solenoid sticks "on" then I will know and be able to turn off the mags and hope that the battery goes flat before the starter-motor or something else is buggered. I will be on the ground if (or when) this happens, and able to get out and run away , or maybe try to take the cowl off while avoiding the cranking prop ( just jesting, I think).

 

I reckon this takes care of the safety issue, although not the potential damage issue. What I don't want do do is add weight and complexity for something which has only perhaps a one in a hundred chance of happening. Weight is a real negative safety thing, and you need a good safety reason to add any of it. However, the isolator switch would only need a pull-cable for the once in a lifetime usage, so maybe it can be added with very little weight. ( Yes you are right Dafydd, it is not strictly an isolator switch if it just serves the starter motor, but thats what we have been calling it ).

 

In the meantime, it is satisfying to see the light glow bright red when I press the start button and then go out when it is released. Thanks for all the contributions to this topic.

 

 

[facthunter]

You Need an isolator. I've had to use an electrical crash bar in anger and they are fitted to any "decent " plane. A stuck starter isn't going to put up with it long as it will overspeed and generally the gears aren't lubricated. Also If you have smoke you should be able to cut ALL electrical power. That includes generator/alternators as well as battery. Remember some alternators will not excite without Battery power. If you are crashing in rough timber or rocks you should cut power to everything before impact. Turn fuel off too