Friday, October 09, 2009

Another Reason Not To Outsource

While looking for the date of the last pitot-static certification, I found the following snag and rectification in an aircraft journey logbook.

Avionics master comes on with battery switch. Found stall warning power connected to avionics bus and stall warning system connected to pilot instrument light power.

I picture some inadequately supervised apprentice somewhere in Temiscaming or Chatanooga playing with the panel wiring like an old fashioned telephone switchboard. That would be why our PRM has decided not to have remote shops supervise the maintenance work.

The engineer comes by to tell me which breaker he will put the new tachs on. It's so completely unrelated that I've forgotten what it was, maybe with the landing light on the CB for the "rear cabin door open" warning light (I just looked it up).

"There are a couple of unused circuit breakers behind my elbow in the cockpit," I mention, assuming that it is neater to have every piece of equipment on its own. He already knows about the spares, but explains that it would take another hour of aircraft disassembly to wire the tachs in there. He has done an electrical balance and there is plenty of room for them to share. A light bulb goes on for me, as I now realize why two airplanes of the same type don't have the same equipment on the same circuit breakers. Anything that isn't original factory equipment is just wired into whichever breaker was convenient to the person who installed it.

8 comments:

zb said...

Yay Electrical!

A Light bulb also goes on for me. It is rated beyond the trip current of the circuit breaker in my head lableled "Is this really a good idea?" Aren't avionics and their respective wiring complicated enough already? The wiring may be completely fine by all applicable standards, but individual CBs for groups of related equipment would really be nice... I'd hate to read words like these: The NTSB also found the non-standard assignment of panel instruments with their respective circuit breakers to be a contributing factor because of the distraction caused for the pilots when they were troubleshooting the reason for the non-operative tachs during a critical phase of flight.

Aviatrix said...

The thing is, there is no standard location for the CB for a digital tach. They weren't invented when the airplane was designed. The CBs are all labelled. And you don't troubleshoot an inop tach during a critical phase of flight. You make sure the propellers are actually going around, then you fly the plane.

I strongly disagree on the niceness of having related equipment on the same CB. I don't want a group of related equipment on the same CB, because I don't want to lose a lot of related equipment at once. I really don't mind so much if my tachs take out my landing light. If my fuel flow gauge takes out my fuel quantity gauge, that's more of an issue. I don't care so much if my flaps take out my starter, because by the time the flaps don't work, the engine is running.

When related CBs are near each other, that is nice, and most of them are. The tach wouldn't have fitted in with the behind elbow set, for example. Those are to do with auxiliary, non flight systems, line power for the computers in the back.

Sarah said...

For those with the luxury (?) to be always flying the same aircraft, we can make neat Dymo(tm) labels or paper charts of what is on which circuit.

On one club airplane the c/b's are way over on the right hand panel, knee level. I doubt very much I could read any labels in flight from the left seat, certainly not in turbulence at night when trying to troubleshoot an electrical problem. Maybe I should take a photo and put a label sheet in my checklist book.

Jimmy said...

"Anything that isn't original factory equipment is just wired into whichever breaker was convenient to the person who installed it."

That is so true. When we did the LSTC for our equipment install, I had to do the electrical load analysis. This entails figuring out which CB had what on it. Total nightmare to say the least.

What amazed me most was the draw of some items, especially the anti-ice. My heated windshield requires 40 amps at 28 volts, more than a complete LIDAR system in back for example! All I know is I never want to lose an alternator at night in icing conditions. That was a situation that never really bothered me before the analysis.

I use a Brother P-touch label maker to keep the CB panel readable...

Lawrence said...

Hey I work as an apprentice in Temiscaming! It must have been Chatanooga.

A Squared said...

And you don't troubleshoot an inop tach during a critical phase of flight.

That's kinda what I was thinking... Actually I was thinking that anyone that crashes an airplane because of an inop. tach is better removed from the gene pool

zb said...

Ah, I get it. I agree it makes sense not to put all the dashboard lights and instruments on one CB, just because all are "instrument panel stuff" and therefore related. There is a logic behind this, deeper than I could fathom...

Aviatrix said...

I amended the post to note that the tachs are on the CB for the "cabin door open" warning light. I'm a little surprised they are both on the same breaker, but perhaps that's the way the wiring works.