Stuck Starter Contactor - Lesson to learn

Heavy use of the starter without engine starting often causes solenoids to stick. as already mentioned, the heat generated from the current will happily weld the contacts together, often to the point where even the “tapometer” doesn’t resolve the situation.

Yes very true about thyristors and DC. These chinese DC SSRs probably use IGBTs, looking at the 2V drop spec.

Myself only a lowly toolmaker, no electrickery guy, I´d think any voltage drop by a SSR looks minimal compared to the drop by high amps from the starter, more than just 2 V I´d say. One can ponder a lot about adverse effects – or lack of any, who knows. I would give it a try and spend € 15.- if I felt the need to find out. My bit of knowledge about thyristors is they don´t switch off with DC once conducting, so only used in AC loads. So in a DC-DC relay I guess any other type of semiconductor is in there. Like in modern welding equipment no more tons of coils, just a collection of power chips for hundreds of amps in a 15 kg machine. Just thinking, voltage drops with these SSRs should be a concern anywhere installed so maybe that effect is overrated and really no factor anywhere? In a 28 V system a starter motor will have the battery voltage come down maybe one third anyway under load, so who would know the SSR contribution ? A test would be revealing, also in regards to voltage spikes. Voltage spikes from semiconductors – or the motor ?? Is there anything at all in such a setup , I don´t know ?? Again, a test would tell and at € 15.- or so I´d throw one or two into the toolbox , cheap and no weight – a criterium for me to accept any electronics anyway, else I´d prefer the more mechanic electric systems for sure – so no electronic ignition in my aircraft !!! Reliable well built magnetos like the Russian types are our choice instead. OK, allright, today the coils in there have grown a bit old so no great idea to go flying with old stock. @ canuck, yes, the buzzer coil in the 18 T failed many years ago, so I found NOS military supply German Prüfrex buzzer coils in Ebay and that was that. And yes, we have the small lever on the electric air valve for the air starter, no problem. Vic

IGBTs have a much bigger voltage drop than MOSFETs.

I think the power relay problems are related to corrosion, possibly combined with lack of use. I have just pulled out an unused Ametek ground power relay (300A N/O contacts and 100A N/C contacts) out of a box and it was stuck. It took maybe a dozen energisations before the armature would fully move. I had it maybe 10 years inside the house and I don’t know where it was before that. A lot of “new” aircraft parts are really old…

I’d have to imagine at these sorts of currents the choice of transistor would be an IGBT rather than a MOSFET. An IGBT would likely be a lot more resilient to spikes, too.

But relays are generally very reliable and last for years, and have extremely low on resistence. It doesn’t really seem worth swapping it for solid state (which would just bring new failure modes, and is not necessarily any more reliable).

@vic relays are still relevant when starting the Yaks. The starting spark mechanism has a an oscillating contact (on-off-on-off…) to provide sparks at top dead centre for starting. I had a problem when this contact stuck in the ‘on’ position which caused the ignition switch/circuit breaker to immediately trip every time you pushed ‘start’ (the lack of power then causes the air solenoid to close…no rotation!). We eventually had to hand prop it to bring it back to base. Funny enough the contact which evidently had temporarily stuck rattled free on the flight home. The mechanics subsequently polished up the contacts and called it good to go.

Unfortunately, I found this out the hard way and was stranded overnight on Alderney (an island). If you fly a Yak, it might be worth finding this device (right hand footwell on an 18T) so that you can rattle it (or tap it with a hammer) or disassemble it and fix it if you are ever stuck.

Also good to know is if your starting air solenoid has a manual override button on the solenoid. Ours did not, but I gather some do. If you have this, I think you can turn the mags ‘on’ and then manually trigger the air to rotate the engine. It might be harder to start (because of the advance in the mags) but at least you don’t have to hand prop it. I haven’t actually tried this, so take the advice with a grain of salt.

In relation to the original post, I was always concerned with the Yak. You spend a lot of time spinning the prop to ensure that you don’t have hydraulic lock. I never had an issue, but a member in the group once felt a blade kick. I don’t know if it actually started or not, but they jumped clear uninjured… no defect was found during the subsequent inspection, although of course the mags could have accidentally been on.

The two links above require a login to Aliexpress (chinese ebay, sort of) but this is the one, I think

and the data suggests a 2V drop which at a few hundred A is quite a lot, especially when you consider that for optimal power transfer the voltage at the load is 50% of the source open-circuit voltage, so in a 12V system the starter motor may be getting only 6V, and you are stealing 1/3 of that.

But for sure an SSR (solid state relay) would work. You would need some serious protection from the inductive spikes, which are massive (easily kilovolts) as these would destroy an SSR.

I would build one with MOSFETs, not thyristors which is probably what these use. You can get MOSFETs below 0.001 ohm which is 0.3V drop at 300A. Probably the best way would be to assemble a lot of small devices on a PCB, in parallel. There are 0.001 ohm devices which cannot carry anywhere near 300A due to other limitations, so this would be a good way.

The downside is that a spike could blow them all up and cause them to fail in a conducting mode. I wonder how all this was solved in the solid state circuit breaker products used in the homebuilt market – example.

I have built power supplies with MOSFETs such as this. This is what a 300A device would look like

Years ago I looked into replacing the TB ground power relay with a MOSFET design. It is a weak point as mentioned above. It is nontrivial but doable. There are some interesting old patents from the automotive sphere, designed to overcome the problem that a MOSFET conducts in the opposite direction even when turned off in the main direction (due to a parasitic diode).

Not applicable to our engine due to air starter, but what do you think about solid state dc-dc relays, 600 A like in the link below ? Legality is another matter for sure, but then it is not essential for flying in itself, only while on the ground. So one could see some modernisation and improvements . . . . Vic

400 A relay

600 A relay

That sounds like the contacts were welded. A tap with a hammer can free them.

They must have become welded just at the moment when the battery finally and totally died, and the continued heavy current would have totally flattened the battery after that.

You can buy Stancor relays from e.g. mouser.com, for far less than the aviation sources.