This was an interesting statement. I’ve never seen an orientation arrow on a contactor so I did a bit of research. We’ll call it a shameless rip-off of Mythbusters.
I checked some aircraft to see if the mounting is always oriented to prevent inadvertent contact or opening of the points on high current contactors. Turns out to be true... some of the time.
I grabbed another airport bum and we looked at his Cessna 177RG with a Lycoming engine (his cowling was off and mine is sealed up,) and a Cessna 152 with a Lycoming.
On the 177 the starter solenoid was facing down and battery contactor was up. That’s in keeping with the suggestion. However, the external power contactor is mounted sideways.
The Cessna 152 had both the battery and the starter contactor mounted up. But note the 152 uses contactors with terminal lugs on the top and not on the side.
So just to be sure what’s going on inside, we grabbed a couple of old contactors and opened them up.
Here’s what we found
If you have the crimped on dome type contactor with terminals on the side the solenoid core moves down when energized. The internal spring holding the contacts open is fairly weak and it could possibly close with a strong thump on landing.
If you have the terminals on the top, the solenoid core moves up when energized. If mounted upside down it could make inadvertent contact with a hard landing. This type needs to be mounted up for the starter. That is in keeping with how they were mounted on the airplane.
So what about the battery contactor? The 177 battery contactor was mounted up so a hard landing would keep the contacts closed. However, the 152 top terminal battery contactor was also mounted up which could possibly open in a hard landing.
Does that bust the theory? Not at all.
What I don’t know is how many G’s it takes to knock open the solenoid contacts when it is energized. The magnetic field on the contactor is very strong. I hooked one up to a power supply and a lamp and slapped it in my hands very hard to see if it would momentarily open. If it opened, the lamp would blink. Not very scientific but certainly more than a few G’s. I was unable to get it to open.
Lastly is the galling issue. The top terminal type has the plastic coated iron core sliding in a smooth phenolic sleeve. It would be near impossible for it to gall when mounted in any position. The side terminal type contactor has a polished metal core with about 1/8 inch clearance sliding in a polished sleeve. It too would be near impossible to cause any galling.
In the end, I would say mounting a contactor inverted for a starter solenoid is probably a good practice and may save you a starter and ring gear. Orientation it is something to keep in mind if you have a bus you don't want powered intermittently should you hit some extreme turbulence or bang a landing.
On the other hand, I think the orientation for a battery contactor is probably irrelevant. I don't think Waldo Pepper could pull enough G's to open the contacts in flight. If you hit hard enough to open the contacts on an energized contactor when landing, the airplane is not likely to be reusable. Mounting sideways should present no problem, either. The solenoid moves once at the beginning of the flight and once at the end of the flight. It's not likely to wear out faster in sideways vs. vertical.
Attached are pictures of the units we looked at for the education.
The first picture is a domed side terminal type. Note the wimpy spring that holds the contacts open. The core moves down to close the contacts.
The second picture is a top terminal type. Note the core must move up to close the contacts.
Third picture is a factory installed external power contactor mounted sideways. It is 40 years old with 6000 hours and has never failed.
One interesting side benefit of opening some contactors was to see the quality. The top terminal type looks far more robust on the outside. However, the contacts on the inside are small in comparison to the large terminal bars and contactor ring of the domed side terminal type. The domed side terminal type is definitely the superior of the two.
Re: Relay Failure
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