In my experience CFM and Pratt & Witney make the best electrical harnesses. I don't think I've ever had to repair one. They are solidly built and are almost completely enclosed in heatshrink with bonded boots.
Rolls Royce on the other hand their wiring is pretty poor in comparison. Numerous chafing wires, badly routed harnesses and little protection in the way of heatshrink and boots.
Having said that, if you were to get a wiring fault on a CFM or P&W, a repair in-situ is almost impossible, you would have to replace that section of harness. On a Roller you can easily do repairs. EGT harnesses are tricky to repair due to the fact that it has to be a continuous loop. In line splice repairs are not allowed.
A heat gun can be used on wing provided it is not a naked flame type. I have used one in the past that used a nitrogen supply and a halogen heat source that was approved for in-situ use.
Could you be more specific about, what A/C you are working on. Spotheaters are good to make a soldersleeve, but warming up a boot? The A/C Manufacturs are not following their own rules. The WDM chapter 20 rules. Boeing + Bombardier is what i am thinking about.
I have worked on A-320 with V-2500 + CFM. Not much splicing to do, for en Avionic. Of course firewarning is the same shit as on other aircrafts. Especially the hot section strips. They are ending up in kind of a box below the pylon. For engine disconnect. That box is of course often filled with water. To troubleshoot that, you have to use a capacity tester. Not a megger. Hhmm.. I read in a book some years ago, that resistance and capacity, in princible is the same. Yes: "Viva la resistance"!. The fwd FW are gasloops. Easy to troubleshoot. Often oil in the connector at the pressure tx. Other faults? Vib. system. Use a low voltage megger. My question to Airbus: You want me to use an AC- or a DC- megger? They never explain why! Takes a week, to read it up. Other faults? Thrust reverser. But that mostlly ends up to be "mechanically". Time for the Avionic, to find his couch, and see the last two episodes of "Jackass"!
Well there you go, you learn something new every day!
But you're quoting the A/C SWMP and not the engine repair manual. Sometimes they do contradict each other. I was lead to believe that RR do not permit ILS in thermocouple harnesses, but that's only my recollection. If in doubt go with the engine manual.
But what about the thermocouple terminals? On aged A/C like Douglas/Dehavilland, you are allowed to solder them. It`s a process, where you use 5 different chemicals. But takes ten minutes. First time our company bought the chemicals, they bought so much, that we can supply the rest of the free word (including China) for the next 50 years. But there must be a way, to crimp these bastards. We have these terminals in stock. But what about the crimp tool? Ohh, the engine-manufacturer says, they have to be soldered. But you are allowed to use splice in the thermocouple system. What`s the difference?
Any contradiction in repair authorisation can be verified by tech query with the manufacturer then remedied by tech publication amendment. Ignoring these things doesn't help the next guy and perpetuates any confusing instructions?
I know when the ERJ 145 came into service - a load of us practically rewrote the maintenance manual for Embraer, it was such a confused little collection of books!
Location: Across the road from the remains of deHavilland
Age: 61
Posts: 2,167
Quote:
But you're quoting the A/C SWMP and not the engine repair manual. Sometimes they do contradict each other.
They serve different purposes. In general, the SWMP applies when the engine is "On-Wing", the engine manual applies on the cradle. In 'On-Wing' cases that aren't covered by the SWMP, the engine manual may be applied.
You must also bear in mind that in many cases the wiring harness on a QCE is an 'aircraft part', designed and produced by the aircraft manufacturer and not the engine manufacturer.
The same applies to other major components such as APU or landing gears.
I remember when we had a fault on our RR Trent Airbus 330. The EGT was causing erroneous indications. We traced the fault to a chafed wire on the egt harness onto the CNA. It was the green one and I can't remember it that's alumel or chromel.
An easy repair would be to cut at the chafe put in in line splice and heatshrink sleeve, a permanent repair as per Airbus chapt 20. However our RR wiring practices manual expressly prohibited in line spice repair on EGT harnesses. We put in a query with our technical service boffins and we were advised to replace the entire harness. A big job on a Trent!
So what would have taken 20mins to fix as per Airbus, took an entire day because of Rolls Royce.
Location: Across the road from the remains of deHavilland
Age: 61
Posts: 2,167
Sounds like you asked the wrong people. A better (and legal) way to handle it would have been an "On-Wing" repair per Airbus process with an outstanding defect or equivalent raised for a wire bundle replacement at next engine removal. Perhaps your "tech services boffins" aren't approved for producing repair schemes? If so, they certainly ain't boffins.
Blacksheep, that's our problem. Even as licenced engineers we have to submit our repair schemes through tech services which then get signed off by Quality. Tech servs get their nod from Airbus, Boeing, RR etc through either direct from Tolouse, Seattle or Derby.
I feel that this somewhat undervalues our qualifications as licenced guys. After all, we're the ones who sign it off in the log. Having said that, if you sign it off in accordance with a tech services repair scheme, the onus is then on them and Quality. However I wouldn't know the recourse on how that would stand up in court as ultimately it's my signature on the CRS.
the most common repair on our fleet, on the RR side of the house, is the 'overtie' repair - see ATA 70. Very useful. As for splice repairs - if you have to make the wire bigger by splicing and protecting with a sleeve, you can often a) spend more time and effort than an overtie and b) potentially cause clearance issues with the 'thicker' repair area.
As for the CFM vs RR harness issue - it's $30k plus for the RR harness and only about $8k for the CFM......
Because we don't own our aircraft we lease them. If you've ever done a lease hand back you'll know what an absolute nightmare it is.
The leasing surveyor will go over every repair (structural or electrical) and may insist on "as new condition" What may be considered as a permanent repair for an airworthy issue may not necessarily satisfy the owners. That's why tech services and quality get involved. Sometimes it's cost effective to try and do an "as new" repair over an acceptable one.
From then on it gets political and way over my pay grade. I just do as I'm told!
Also with this EGT issue we had a conflict of manuals as stated before. My inclination would be to go with the engine SWPM over the A/C one on this issue. After all, Rolls Royce made the engine, not Airbus. Rolls obviously have a reason why they prohibit ILS on an EGT harness. I don't know why, maybe it's because of what Vortechs Jenerator said!
My guess is that it upsets the Wheatstone bridge and that you shouldn't be adjusting the bobbin resistors to trim the ballast. (Used to do that on VC10 Conways) When you change the entire harness, you get the ballast resistor value needed with it.
I'm now going to get controversial and state another reason why electrical repair schemes may need prior approval and that's because these repairs are now supposed to be done by an A&C engineer!
I started life as an aircraft electrician but carried on along the B2 route. When I converted my licence to EASA B2, they also gave me a restricted B1 to allow me to carry on being an electrician! Learning about the minefield that is cables and connectors and picking up the required skill to use precision termination tooling (PTT) is not something you can do by subscribing to club66dotcom and sitting a multi-choice exam!
That's how I got the rest of my B1 modules. I am now qualified to do a structural repair. But would I do one? No chance, I would leave that to a proper tin basher.
It looks like the FAA is working on an international standard for repair and installation of wiring ,"the holy bible". Called F39. They are talking about a yearly update. Go to : http:www.astm.org/COMMIT/F39.htm
the overtie repair for the a330 (trent 700) and other RR family engines was put into the Amm by Airbus of course, but this was at RR's recommendation. So it did have blessing from the original OEM mothership.
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