Aztec, Battery master relay 5 amp fuse

[Way2Broke]

Has anyone ever heard of a "battery master relay fuse" blowing on a aztec and causing the entire system to die? What would cause this and why in the heck would piper design a electric system that could go dead when a 5 amp fuse gos out, why not give the pilot access to the fuse or better yet why in the heck would they not make it a circuit breaker. Why why why?

Little help?

 

[avbug]

The battery master relay is a little like a faucet. The electricity flowing to and from the battery is the water, and you're turning it on and off. Except instead of having a little wheel to turn, you use a very small current of electricity to do it. A small current flows through your battery master switch and powers the relay which closes, and allows a lot more electricity to flow to the aircraft.

Fuse and circuit breakers don't protect components, they protect wiring. (A lot of pilots don't appreciate that fact, and use circuit breakers for switches). Wiring runs are required to be protected. Protection can take the place of circuit breakers or fusible links, or inline fuses. Generally things attached to the battery, including "hot battery bus" items, or items which attach directly to the battery, use a fuse, whereas circuitry items dependent upons switches from other busses, use circuit breakers. High amperage items generally use a "slow blow" fuse called a current limiter.

In this case, when you turn on your battery, the power has to come from somewhere, and that somewhere is the battery. So, ,your battery master relay is really a hot battery item...one that's attached to the battery all the time. If anything goes wrong, shorts, the only thing there to disconnect it, especially if you haven't selected it, is the battery master relay fuse. Not all installations use this same thing, but something has to protect the circuit to the battery master relay, to protect the wiring in the event of a short to ground. If anything gets hot and burns, it will be that wiring, and that's the purpose of the fuse.

 

[Way2Broke]

The battery master relay is a little like a faucet. The electricity flowing to and from the battery is the water, and you're turning it on and off. Except instead of having a little wheel to turn, you use a very small current of electricity to do it. A small current flows through your battery master switch and powers the relay which closes, and allows a lot more electricity to flow to the aircraft.

Fuse and circuit breakers don't protect components, they protect wiring. (A lot of pilots don't appreciate that fact, and use circuit breakers for switches). Wiring runs are required to be protected. Protection can take the place of circuit breakers or fusible links, or inline fuses. Generally things attached to the battery, including "hot battery bus" items, or items which attach directly to the battery, use a fuse, whereas circuitry items dependent upons switches from other busses, use circuit breakers. High amperage items generally use a "slow blow" fuse called a current limiter.

In this case, when you turn on your battery, the power has to come from somewhere, and that somewhere is the battery. So, ,your battery master relay is really a hot battery item...one that's attached to the battery all the time. If anything goes wrong, shorts, the only thing there to disconnect it, especially if you haven't selected it, is the battery master relay fuse. Not all installations use this same thing, but something has to protect the circuit to the battery master relay, to protect the wiring in the event of a short to ground. If anything gets hot and burns, it will be that wiring, and that's the purpose of the fuse.

Thanks as always AvBug. My point was why in the heck would Piper design a electric system that if a 5 amp fuse blew (that the pilot does not have access too, granted I would rather is blow than catch fire) there is no redundant back up. Thus causing a complete loss of all power. Very poor engineering.

 

[avbug]

My point was why in the heck would Piper design a electric system that if a 5 amp fuse blew (that the pilot does not have access too, granted I would rather is blow than catch fire) there is no redundant back up

But you see, that is the point. It's good engineering. Piper didn't want you to be able to reset anything, or reroute the power. The fuse has done it's job...if you were able to reset anything to restore the circuit opened by the fuse, then you would be negating the value of the fuse. Hence my prior comment that pilots often improperly use circuit breakers as switches.

What do you suppose caused that fuse to open? Don't you think it might be important to find out, before you restore power to the device that controls the flow of more amperage than any other item or appliance in the aircraft...and consequently is also your biggest ignition source ready ignition source on board?

The fuse is there to protect the wiring. The wiring will burn if it's not protected. When I say protect the wiring, I don't mean the device is there to preserve the economic value and beautiul intactness of the wiring harness. Bottom line is it's here to save your life by keeping a fire from occuring in the wiring, to where ever the wiring shorts. Ever stretch a piece of steel wool between the terminals of a couple of d cell batteries? It's a boy scout thing...it's a way to start a fire, and the wiring in your aircraft is no different. Fuses and circuit breakers are there to prevent that from happing by opening the circuit and de-energizing the circuit before something burns up, welds shut, or fails in a way you can't control. It's all about protecting you.

This brings up several points, which I'll address in turn. You've just discovered a very important principle. Sometimes pilots think of the battery as a backup power source...lose your vacum, or lose your generators, and you still have battery power. It's not a backup source, and you just found out why...you might have generator power available, but no battery, or you might just lose everything...always plan your flights accordingly, as you just found out exactly what can happen...you probably never realized it before. Just imagine how many other things can go wrong that you're not aware about yet...and plan accordingly for those by always giving yourself multiple backups and plans.

Imagine the potential in a single engine airlane in isntrument conditions. Elsewhere on this board a thread has been hashed out at length by all the road warriors who profess allegiance to the concept of single engine IMC...but here you encountered a situation in which you lost everything. Imagine that in IMC. Knowing the airplane, and knowing the limitations is very important. Having alternate ideas in mind, just as you plan for an engine failure or other abnormal situation, is not only a good idea, but critical to your health.

You're also discovering the reason for fuses. Pilots often treat fuses as switches. Oh shucks...breaker's open. Let's reset it and see what happens. Probelm is, you might be starting a fire, or in the case of wiring which has shorted to other wiring...powering an entirely different circuit...what has opened may not even be the problem circuit. Many cases of this very thing have occured over the years. If something breaks or opens, then follow your checklist in addressing it. In most cases, if a breaker is to be reset, you'll be allowed one shot at it, but be careful what you're doing, and know what that breaker does before you touch it. In his case, Piper didn't want you to be able to reset anything...because what has failed dictates that you get a mechanic to look at the system and address the possibilities.

Some systems to offer some level of redundancy in having multiple batter master relays from separate batteries. However, if you have but one battery, this becomes a mute point...one battery, one battery master relay. Many larger aircraft also have external relays when using external power for an engine start or when powering the airpalne on the ground, but still require battery power from inside the aircraft. You can have all the power outside, but unless the aircraft battery has at least a mnimum value of usually about 16 volts, the relay won't open, and you can't get to the aircraft battery with external power...this isnt' a mistake. The manufacturer doesn't want you charging a dead battery while it's inside the airplane; they want you to remove it and have ensured that you can't charge a dead battery on board using external power, for several reasons (thermal runaway, battery overheat and potential gassing, corrosion, explosion, sparking in convined spaces, potential of battery boiling, hydrogen gas buildup, etc). Important stuff.

Resetting a breaker isn't always a good idea. If a component has opened, be it a breaker, current limiter, or fuse, something has caused too much current to pass through that circuit protection device, and you need to find out why. Was it a momentary spike? Has there been a short somewhere? Perhaps a faillure in the battery master switch, or a short in the wiring from the switch...the circuit has ensured that you can't turn it on in this case, because in the event of a fire, you'd have no way to shut off the battery if this hadn't occured. Again, it's designed that way for your protection.

A number of circuits can be had in various aircraft which aren't resettable in flight. Pilots sometimes grumble to find that circuit breakers or devices are made inaccessible, placed in external compartments or other places that a pilot can't reach in flight. This is never by mistake. You aren't supposed to be able to reset them in flight. The manufacturer knew you'd try, and doesn't want that. Your Janitrol heater snaps off in flight, perhaps...you may have to land to push the littler red button on the side to reset it...this is no mistake...it shut off for a reason (they're not called "bombs" for nothing).

Finally, this underscores the need to know both your airplane, and the proceedures for your airplane. This is a good example of needing to know what to do with the situation, and where to look to find the checklist items.

Last year I flew an airplane which had a new electro-hydraulic dual system on board; my first flight with the new system. Previously, the aircraft hydraulics had been engine driven. Prior to my taking the airplane, someone made a mistake in attaching ground power, which apparently fried a relay closed before the attached wiring had a chance to protect it. I did not know about it. During the subsequent flight, the hydraulic motor caught fire due to the relay being weldled shut, and I landed the airplane with a cockpit full of smoke from a burning hydraulic pump. As I cleared the runway, the hydraulic accumulator breathed it's last, having given it up in braking on the runway, and I was left in a large tailwheel airplane with the only steering being brakes...and no hydraulic pressure to power them...and on fire. In such a case, having a circuit protective device that would have failed and been unresettable would have been far preferable to a component that burned up instead.

Look for the cause of the open fuse before looking to replace the fuse or reroute power...there's always a good reason it's blown, and restoring power may not be in your best interest. Rest assured, the design is not by mistake.

 

[FrozenPilot]

So to sum up avbug's post, theres a lot more going on with that 5 amp max circuit than it appears. Since we're talkin a solenoid, its possible that if there was a short in the coil that it could keep the master on permanently. Other issues can arise of course, short circuits are never good.

avbug, ever consider a law career?

 

[Way2Broke]

Thanks for the posts AvBug. My father is a engineer and this is where we always bump heads. I understand the need to take the operator out of the picture for safety's sake sometimes. However, in this case I don't see where a 10 minute cool off period and one reset would hurt anything. I don't like it when some engineer somewhere decides that I am too stubborn to know when something is not operating to spec.

 

[icefr8dawg]

Very insightful avbug.

I'd note that with thousands of hours in piper twins I've never once experienced that part of the system fail. It's all the parralleling (sp) and impedience sensitive parts of the charging system that have failed...

 

[avbug]

However, in this case I don't see where a 10 minute cool off period and one reset would hurt anything. I don't like it when some engineer somewhere decides that I am too stubborn to know when something is not operating to spec.

That's precisely why you shouldn't have that control. Think about it. Until now, you didn't even know the fuse was there. What else don't you know, and why should you be playing with things you know nothing about? Do you know why the fuse opened, yet?

 

[Way2Broke]

Avbug,

No we do not know why the fuse opened yet, nor do I think we ever will. The mechanic troubleshot the system for the cause, but concluded, "Maybe it was just its time to go." Pretty darn good mechanic too, he went straight to that fuse when I told him what had happened. What I do know is that it has not happened again, and if it does happen again that plane will not see the sky again until they do find the cause. Oh, BTW. I got my CD today. 80 bucks well spent. Thanks for all the insight. Can I vote you for poster of the year?

 

[FL420]

avbug.....you have a real talent for putting complicated(to the layman anyway) concepts in simple terms with understandable explanations. I sometimes wondered why some fuse and circuit breakers were placed in some pretty strange places. Now I know and have a new-found respect for the dangers of resetting CBs. Thx!