Eletrical Problem

[Al Downs]

I am having some electrical problems. Some times when turning on the Master to lower the flaps for pre flight, there is not enough power to operate the flaps. Turning the Master switch off and then on again (sometimes more than once) fixes the problem. My thinking for this problem is maybe the Master switch needs to be replaced.

 

Today a pilot had another problem. Below is the message he sent me regarding what happened.

 

 

I started the airplane, following the checklist in this sequence:

 

Battery on

Choke, Throttle idle

"Clear Prop"

Start engine

Set Generator on

Set Avionics on

 

When I set Avionics on, all of the instruments went dark. So I shut down following the shutdown sequence repeated the checklist. Same thing.

As soon as I turned off Avionics, the instruments (EFIS) would come back on, if I turned the Avionics back on, they would go dark again.

 

This problem seems different but may be connected.

 

I went to the hangar to see what was happening and of course everything worked just fine. Took the plane out and put on .5 hours with no problems.

 

Anyone have any ideas?

[Mike]

I would say you have a bad earth connection probably the one under the instrument pod through the bulkhead

 

Mike

[FastEddieB]

First issue might be more likely to be a bad master relay than a bad master switch.

[FlyingMonkey]

I usually hit the battery and generator breakers at the same time, before engine start...I see you hit the gen after start. is there any problem with doing this either way? I do make sure Avionics are off during engine start or shutdown.

[Tom Baker]

Fast Eddie, no relay, just a CB switch to carry the load. There is a starter relat though.

[FlyingMonkey]

Hi Andy,

 

No problem. I do it your way too and so do manyof the guys I know, but have seen it done both ways.

 

The only reason I like doing it that way is because I'm forgetful and don't want to take off with the gen off.

[josjonkers]

I usually hit the battery and generator breakers at the same time, before engine start...I see you hit the gen after start. is there any problem with doing this either way? I do make sure Avionics are off during engine start or shutdown.

That is the way I have been doing it for 8 years

[Al Downs]

Grounds were all torqued about a month ago so I am pretty confident that they are good. I have never seen it do what the pilot said it did last night.It is intermittent so I will keep an eye on it and try to pinpoint the problem.

[FastEddieB]

Fast Eddie, no relay, just a CB switch to carry the load. There is a starter relat though.

 

That sounds unusual.

 

No plane I've owned has had all the current flowing through the master - they have ALL had master relays.

 

Can someone post a schematic!

[Tom Baker]

That sounds unusual.

 

No plane I've owned has had all the current flowing through the master - they have ALL had master relays.

 

Can someone post a schematic!

 

Eddie, it is not just a switch, it is a circuit breaker designed to carry the load. I would bet that all of the airplanes you were talking about had all of the current flowing through a circuit breaker. You have to remember that we don't have high current draw, so we don't need a high current relay.

[Rich]

I agree with Tom on the current draw and the need for not having a master relay.

 

I've had intermittent electrical problems before with the same symptoms described.

The EFIS can run on the back up battery , if installed, so when the normal power is interrupted everything goes blank but the EFIS.

 

I found that the avionics switch/circuit breaker was the culprit. I removed the switch/CB, opened it up and found mis-aligned contacts that were causing pitting and carbon build-up. This caused the intermittent loss of avionics power.

 

This worked well for awhile, then I started having a similar problem. Well, I figured I'd send the plane back to the dealer for inspection and to fix a few other items that needed to be corrected on the aircraft, Sting, S4. They found a cracked Master switch/CB. It was replaced and I've had no more problems.

 

If the switch/CB on your plane is not tripping, I'd put a jumper around the Switch/CB and see if you have the same problem. If not, replace the SW/CB.

 

The Sting has a redundant CB system. The master and avionics both have a separate CB and they both have a separate SW/CB.

 

Rich

[sandpiper]

I usually hit the battery and generator breakers at the same time, before engine start...I see you hit the gen after start. is there any problem with doing this either way? I do make sure Avionics are off during engine start or shutdown.

 

So that's what's wrong with my plane!! :wacko:

 

Just joking. I've done it that way for 6 years and everything is just fine.

[Doug G.]

Wonder why the CTLS checklist says to wait until after engine start? A little less resistance on startup?

[FastEddieB]

Eddie, it is not just a switch, it is a circuit breaker designed to carry the load. I would bet that all of the airplanes you were talking about had all of the current flowing through a circuit breaker. You have to remember that we don't have high current draw, so we don't need a high current relay.

 

Tom,

 

Never too late to learn!

 

I really thought all planes had master relays, based on the fact that I think every plane I've ever flown has had one. My Sky Arrow with similar current draw as the CT (or slightly less) has one.

 

Of course the idea is to not need wires substantial enough to carry full system current run all the way to the master in the cockpit and then back to the main bus. And the master itself need not be heavy duty enough to carry the full load either.

 

But I suppose the FD engineers weighed all that against the advantage of reducing the parts count and a failure point by eliminating the master relay entirely. Good on them for thinking outside the box.

 

I also assume they sourced a breaker designed to also serve as a switch. I mention this because, in general, the type of breakers in most planes that can be "popped" manually are not really designed to routinely act as switches. Certainly they can be used as such on occasion, but I don't think they are tested for nearly as many cycles as regular switches are, since they are not primarily designed to be switches.

 

Just curious - does anyone here know of any other planes that did away with the master relay?

[Tom Baker]

Eddie, the + wire from the battery goes to the starter relay, and the wire to the master connects there. It is not a very big wire compared to older GA aircraft. The master switch does not have to carry the load of the starter.

[FlyingMonkey]

There may be a certification reason you see a relay on most GA aircraft. SLSA are a like a hybrid of experimental and certified airplanes, so since they are free of part 23 standards and the ASTM standards are different and generally less stringent, that might be why do you could see some different setups on LSAs. And of course "certified" does not mean better, it just means at some point a bureaucrat said "this is how it should always be done" and made it stick.

[FastEddieB]

Eddie, the + wire from the battery goes to the starter relay, and the wire to the master connects there. It is not a very big wire compared to older GA aircraft. The master switch does not have to carry the load of the starter.

 

I assumed there was always a starter relay with large cables attached, in addition to whatever "main" set-up there was - breaker or relay.

[paul m]

But I suppose the FD engineers weighed all that against the advantage of reducing the parts count and a failure point by eliminating the master relay entirely. Good on them for thinking outside the box.

 

I also assume they sourced a breaker designed to also serve as a switch. I mention this because, in general, the type of breakers in most planes that can be "popped" manually are not really designed to routinely act as switches....

 

Eddie,

This seems to have been answered already, but as I was reading the CTLSi Supplement to the POH last night I remembered this discussion so thought I would point out the language in the POH.

“The master breaker function of the electrical system is achieved by dual breaker-switches. The breakers are designed and qualified to be used as switches. This allows to reduce the amount of components in the system, and therefore reduces the possibility for system failures.

[FastEddieB]

As I suspected. Thanks!

[Rich]

Eddie,

 

The Sting S-4 does not have a master relay. Like you said, one less failure point and the draw is not significant to warrant one.

 

The S-4 has redundant CB's for all circuits. A main CB for each item and a CB/Switch combination for those items.

 

On my previous home built, a Challenger II, CWS/LSS I did not wire in a master relay.

 

Rich

 

Rich