Thursday, April 22, 2010

Bonus Electrical Post

Commenter Mark identified a number of potential problems with the heater installation in my airplane, pictured in Monday night's post. The heater looks perfectly normal to me, and I've flown a number of these airplanes maintained by various people. This particular airplane has been in and out of various shops and no one has complained about the heater installation, but standards vary. I'm neither an AME nor an electrical engineer, so I'm not really qualified to pronounce one way or the other. I printed off Mark's comments and gave them to an AME. He chuckled a bit, but went back to arguing with a recalcitrant grease nipple on my nosegear without offering any specific rebuttal.

But as a bonus to Mark for his interesting analysis, I present a photograph I took some time ago, of wiring in an actual working airplane. I took this in a maintenance hangar some time ago. I no longer remember why.

Mark?  Mark!  Are you okay?

15 comments:

Anonymous said...

Hehehehe... I responded to your aviation software post a while back and this is precisely what I had in mind when I was describing modern commercial software. That's exactly what it looks like on the inside assuming half of those wires go to something that doesn't exist anymore. :-)

Hooray technology!

Mark said...

Aviatrix.. I will be OK, once I un-faint.

dpierce said...

Where does the white wire go?

Critical Alpha said...

Mark: This is what I meant when I said you'd be calmer if you saw some of the messes I've seen. I haven't seen this actual aircraft but I've seen plenty which were wired in the same style.
Neat isn't it -;)

Aviatrix said...

Anonymous 00:31: It's very likely that half the wires go toward equipment than doesn't exist anymore. After all, if you removed an old DME receiver from the dash, would you go into that mess to find all the associated wires?

Mark: Glad you're still with us.

Warn said...

I missed the previous post at the time but I do have this advice if someone hasn't already mentioned it. If the heater switch has a "blower" option, that is, it's a 3 position switch, leave it in blower for a little bit before you turn it off all the way and it wont pop the overheat breaker as often.

If it doesn't have a blower only setting figure out which combination of air valves will allow airflow to keep flowing over it without it being on. Once the air starts cooling off you're okay to turn it off.

Sarah said...

I want to be an instructor some day, just so I can play this trick: Before the lesson, burn some plastic and capture the smoke in a jar. During the lesson, open the jar surreptitiously and ask the student... "what's that smell?"

And, it has to be mentioned: Current xkcd

Aviatrix said...

Warn: It has a fan position and also a squat-switch linked automatic fan, for operation on the ground. I usually turn it off on final, to avoid having to follow the instructions and run it on fan for one minute before shutdown, but where it has been tripping is on the ground at the start of the flight. You're supposed to be able to run it on the ground, but I'm not going to anymore.

Sarah: That's a great one. I never thought of that, and I'm the one who has put Chinese takeout red sauce on the gear to see if the student catches the "hydraulic leak".

Anonymous said...

Looks like behind the panel in my airplane!

zb said...

@ Anonymous of 00:31

Just this week, my co-worker who does firmware programming mentioned that the code she is currently re-viewing and making ready for certification was a total mess. Being a hardware developer, I just said that the chip holding the code looked very nice and clean, no matter what.

@ Sarah

The smoke in your jar would be Magic Smoke, as is also mentioned in the current xkcd.

Jimmy said...

That wiring may look horrible but at least it is all tie wrapped into place.

When we took my dash apart for a new transponder we discovered unsecured bundles everywhere. A couple were wrapped around the elevator torque tube(!) and the back of the ADF was resting on it as well. The connector had a neat semi-circular shape to it as a result... It took one AME three days to clean the nightmare up.

As Trix said most of that stuff is legacy wiring. I had a dozen connectors going from nowhere to nowhere.

Curt Sampson said...

zb: Sure, the chips are nice and clean. They always have been. I was impressed myself by all of the nice little PCBs, each with a few 7400 parts, all in a nice card cage, that made up the CPU of a PDP-8. Then I saw the wire wrap on the back panel....

When I'm a syadmin, I always remove my unused cables. I'm anal about cabling. But then again, I know when I bring it back up there's still my alternate system working as it was while this one was down. It's rather like being able to fly in two airplanes at the same time, and even though you have to fly 24x7, if the newly upgraded one crashes, the old working one is still going.

zb said...

Curt, the thing that bothers my co-worker most about the code she is reviewing is that there seem to be tons of unused variables left over from earlier versions. The ICs are clean, but the types of mess in code and in hardware seem to be quite equal. But you can't use duct tape, hot glue and wire fasteners for loose pieces of code, heh.

Also, I am very glad I have never had to troubleshoot a wire-wrapped board. They are a mess, and they have really bad electrical properties, too -- cf. p. 99 in this most awesome application note (pdf, 5.1 MB), that just says "No" to wire wrapping. (pp. 28-31 are a great flip book!)

Curt Sampson said...

You may not be able to use duct tape, hot glue and wire fasteners to fix the code itself, but all of those come in very handy when, err, "fixing" the programmers who made it. :-)

As for p.99, that is a particular bad example of what not to do a those speeds (long wires are antennas!), but when properly done wire wrap is good. Wikipedia says it well: "Wire wrap construction can produce assemblies which are more reliable than printed circuits--connections are less prone to fail due to vibration or physical stresses on the base board, and the lack of solder precludes corrosion, cold joints, dry joints, etc. The connections themselves are firmer and have lower electrical resistance due to cold welding of the wire to the terminal post at the corners."

But boy, is that a wonderful application note. Of course anything you build yourself in the high-speed world is going to be fun, since it's all radio in the end.

Aviatrix: I'd say sorry about being off-topic, but then again, it was you who posted the picture of the heater circuit breaker on the Internet. Now you see where this can lead....

(Heh heh. "Lead.")

zb said...

Curt, awesome, thanks for your suggestion. I almost fell off my chair laughing. I'll let my co-worker know about the uses of these devices in the (not so very?) tidy world of firmware programming.

It'll be fun to watch my entire team getting strapped into their chairs within few days once they know, because all we do since the beginning of this year is reviewing each other's designs or code to make it ready for a safety approval. You can guess that things get emotional every once in a while doing something like this for such a long time.