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Mike Busch |
This article originally appeared in the July 1998 issue of CESSNA PILOTS ASSOCIATION MAGAZINE.
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In
my roles as a frequent contributor to AVweb's maintenance section, and as a
member of the technical staff of the Cessna Pilots
Association, I spend a good deal of time helping other aircraft owners to
troubleshoot and fix problems with their airplanes. From time to time, however, I have to
deal with glitches on my own airplane.
A good case in point was the charging system problem that I encountered recently on my
1979 Cessna T310R. There was nothing particularly unusual about this problem or how it was
ultimately resolved, but I thought some of you might find it interesting to read about the
thought processes that one maintenance-involved aircraft owner (me) went through in
troubleshooting the problem and dealing with it.
The problem first arose during a 4,000-NM cross-country trip in early April that took
me from my home base at Santa Maria, Calif. [SMX], to Champaign, Ill. [CMI], for three
days of simulator-based recurrent. The flight from SMX to CMI was mechanically
uneventful-luckily, since the flying weather was simply awful-and the training at
Recurrent Training Center went well.
My itinerary for the return flight westbound included stops at Wichita, Kan. [ICT] (to
visit with some folks at Cessna), Independence, Kan. [IDP] (to see how things were going
at Cessna's single-engine plant), and Tulsa, Okla. [TUL] (for a meeting with Lowrance
Electronics) before finally heading back to California.
First sign of trouble
As I started engines at CMI for the leg
to ICT, I noted that the amber "Right Alternator Fail" light did not extinguish.
After muttering the obligatory expletive, I confirmed the indication by shutting off the
left alternator and using the cockpit volt/ammeter to verify that (a) the right alternator
output current was zero, (b) the battery was discharging, (c) the bus voltage was 24 volts
(rather than 28), and (d) the red "Low Voltage" light came on. Clearly, there
was a problem with the right alternator. [From engine start to confirmed diagnosis of
charging system failure took perhaps ten seconds…probably less time that it took you
to read this paragraph.]
At this point, I had to decide what to do. The most conservative decision would have
been to shut down and find a mechanic. That also would have been the FAA-approved thing to
do, since both alternators are required equipment and a twin Cessna with one
non-functioning alternator is technically unairworthy.
On the other hand, this would probably mean that I'd have to cancel or reschedule my
appointments at ICT, IDP and TUL. Furthermore, Wichita seemed like a much better place to
deal with a mechanical than Champaign, since ICT has one of the best twin Cessna shops in
the country (Yingling Aircraft), not to mention the world's best supply of twin Cessna
parts. The weather between CMI and ICT was severe VFR, so I didn't consider a flight with
one alternator inop to be even slightly risky, even though it would have been technically
illegal.
Transient anomaly?
As I was wrestling with all these go/no-go considerations-it was now perhaps 30 seconds
since engine start-something quite unexpected happened that simplified my decision making
considerably. The "Right Alternator Fail" light went out! I quickly repeated my
volt/ammeter confirmation procedure and there was no question about it: the right
alternator was working just fine now. This made my decision a no-brainer…I'd fly to
Wichita and sort things out there.
As fate would have it, the right alternator performed flawlessly for the balance of the
three-hour flight to Wichita. Two days later, I fired up the airplane once again for the
short 30-minute hop to Independence, and again the right alternator seemed just fine. It
was also fine on the leg to Tulsa. And on the following day, during the eight-hour flight
back to Santa Maria, Calif. (with a fuel stop in Albuquerque), it never once hiccuped.
I've developed a term for problems like this that appear briefly and then go away
without explanation. I call them "transient anomalies." I've encountered lots of
them in my 30 years of aircraft ownership, and I've learned not to let them keep me awake
at night.
My theory about the best way to deal with most "transient anomalies" is quite
simple: sit tight, wait awhile, and see what happens. It's next to impossible to make good
maintenance decisions about a transient anomaly in its "transient" phase,
because a mechanic can hardly be expected to troubleshoot a problem that he cannot
reproduce.
If you ask your mechanic to fix such a problem, about the best he can do is to use a
"shotgun" approach: make a guess at where the problem might lie, replace the
suspected component, and hope the problem goes away. If it doesn't, he'll take another
guess and replace some other component. (Traditionally, the first component to be replaced
is the most expensive one.)
On the other hand, if you just sit tight and wait awhile, one of two things is likely
to happen: either (1) the problem will get worse, or (2) the problem will go away and may
not recur for years (or perhaps ever). If the problem gets worse, it's a lot more likely
to be reproducible and therefore amenable to troubleshooting. If the problem goes away,
it's not worth worrying about (although it's undeniably aggravating that the cause remains
unknown).
Now although this "sit tight and wait awhile" philosophy has served me well
over the years, I hasten to point out that it's appropriate only when dealing with
problems that are clearly non-life-threatening. For example, if you observe major
fluctuations in engine oil pressure for 30 seconds, after which everything returns to
normal, I would definitely NOT recommend taking the "sit tight and wait
awhile" approach. Prudence would demand landing at the first opportunity and
inspecting the engine oil filter and propeller governor gasket screen for the presence of
metal. Transient oil pressure fluctuations are most often caused by a chunk of something
getting caught in the oil pressure relief valve and interfering with its ability to
regulate oil pressure, and must be considered a Very Bad Thing.
But I digress. We were talking about my transient alternator anomaly, weren't we?
Sure enough, it got worse!
After returning to California, I didn't have occasion to fly the airplane again for
nearly three weeks. By now, it was the second week of May and I needed to fly to Tucson,
Arizona, for the Cessna Pilots Association convention.
When I fired up the airplane, the right alternator did not come on-line…seemingly
a replay of the scenario I'd faced at Champaign. I waited for a minute to see if the
alternator would "cure itself" as it had before. Then I waited another minute.
No dice.
Once again, I weighed my alternatives. I knew that there's an excellent shop at Tucson
where the problem could be worked on while I gave my seminars. I also knew that there was
no chance anyone had a replacement Prestolite 100-amp alternator on the field at SMX.
Besides, I had no way of knowing whether the alternator was at fault. The symptoms
could have just as easily been caused by a bad regulator, a bad field switch, a field
fuseholder, or even a fractured wire, loose connection, or cold solder joint. Clearly,
some serious troubleshooting was the first order of business.
The weather for the flight to TUS was VFR all the way. Making the flight on one
alternator would be perfectly safe, I told myself, albeit technically illegal. I started
taxiing out to the runup area, still undecided about what to do. About halfway there, the
right alternator decided to come on-line!
That settled it. I took off and flew to Tucson. The right alternator worked fine.
"Just as soon as I get back," I vowed to myself, "I simply must
troubleshoot this problem and get it fixed." I knew that I had a very long trip
scheduled for next month (June) that would take me all the way to Key West, Florida, and
then over Cuba to the Cayman Islands (where I'd been invited to give a maintenance seminar
at Cayman Islands International Aviation Week). No way
was I about to undertake a trip like this with a flakey charging system!
As if to hold me to my promise, the right alternator failed again during the return
flight from TUS to SMX, and never came back on-line. I was actually glad about that. It
would make my troubleshooting easier.
Troubleshooting time
The next day, I headed down to the airport with my trusty Fluke 77 volt-ohmmeter, a
couple of alligator clip leads, and my parts and service manuals. I was determined to get
to the bottom of this. I had no idea whether the alternator, regulator, or wiring was at
fault, but since the regulator was the easiest component to get at-mounted beneath the
copilot's seat-I decided that was as good a place to start as any.
I powered up the aircraft and measured the voltage
between the regulator field terminal and ground. It was 24 volts, just as it should be. I
then disconnected the field lead from the regulator terminal and measured the field
current going to the alternator. It was about 1.75 amps, which once again looked normal.
I concluded that the regulator was probably working okay, and that the fault was more
likely in the alternator itself. (This was not the result I'd been hoping for,
since the alternator is something of a pain to remove and reinstall, and is quite
expensive to replace.)
With the meter still hooked up to measure field current, I got out of the cockpit and
walked around to the front of the airplane. I grabbed a propeller blade on the right
engine and started to rotate the crankshaft while watching the alternator field current on
the volt-ohmmeter. Aha! As I turned the prop, the field current suddenly started jumping
all over the place, from zero to 1.75 amps in highly erratic fashion. Clearly, there was a
mechanical problem in the alternator field.
Then, as I continued rotating the prop, I heard a funny noise and something caught my
eye. I quickly realized that what I was hearing and seeing was electrical arcing visible
through the end cap of the alternator. Every time the field current jumped from 1.75 amps
to zero, I could see and hear a big spark coming from inside the alternator, in the
vicinity of the brushes and slip rings.
Clearly, then, the alternator was at fault, and would have to come off the engine and
be repaired or replaced. The troubleshooting phase was complete.
Nothing I can't handle, right?
Removing and reinstalling the alternator seemed like something I should be able to
handle myself. On the other hand, since I'm not an A&P and since removing and
reinstalling an alternator is not on the list of preventive maintenance items that an
owner/operator is permitted to do on his own recognnaissance (FAR Part 43 Appendix A), I
was going to have to find an A&P willing to supervise and sign off my work.
I went upstairs
to the Cessna Pilots Association office and found Tom Carr, a member of the CPA technical
staff who is an experienced twin Cessna mechanic and who served as the IA on my last
annual inspection. I explained to Tom what I'd found, and asked him if he'd supervise and
sign off the removal and reinstallation of my right alternator. He said okay. I headed
back downstairs to the airplane, and about 30 minutes later (with Tom's helpful
encouragement) I had the alternator off the airplane and in my hands. Getting it off was
easy enough, but in the process I could tell that reinstalling it was going to be a lot
trickier.
An ohmmeter check across the alternator field terminals confirmed that the resistance
jumped all over the place as I rotated the alternator shaft by hand. Definitely a field
problem, probably brushes or slip rings.
Okay, now what?
Who ya gonna call?
I checked my maintenance logs to see how long the alternator had been in service. The
answer turned out to be only about 400 hours. That's pretty youthful for an alternator
that theoretically should last for a full engine TBO.
At this point, most maintenance shops would simply order a rebuilt alternator on an
exchange basis. But because my alternators are massive 100-amp Prestolite ALV-9510s (used
on most known-ice-equipped twin Cessnas), I knew this would be an expensive way to go. To
confirm this, I called Electrosystems in Ft. Deposit, Alabama, and Aero Electric in
Wichita, and both quoted me an exchange price of about $1,000 (plus a hefty core deposit)
for a rebuilt ALV-9510.
This seemed to me like an unnecessary waste of money, since my alternator had such low
time on it and probably just needed new brushes or something else equally simple. I
briefly considered the possibility of opening up the alternator myself to find out exactly
what was wrong with it, but after discussing that idea with Tom Carr, we concluded that we
didn't have the necessary tools to take the thing apart, nor the necessary parts to put it
back together and bench-test it properly.
I decided my best strategy was to find an aircraft accessory shop that was qualified
and willing to do an "IRAN" on my alternator. ("IRAN" is an acronym
for "Inspect and Repair As Necessary," and is an aviation term-of-art meaning
"don't do a full overhaul, just fix what's broken.") I asked my colleagues on
the CPA Technical Staff (John, Steve and Tom) if they knew of any shops that would do this
for me, but no names came immediately to mind.
CPA has a saying: "if we don't know the answer to a Cessna-related question, we
usually know who does!" So I prevailed upon Steve to phone a friend at Tom's Aircraft
Maintenance Inc., a well-known Southern California twin Cessna maintenance facility,
explained the situation, and "if this was your alternator, where would you send
it?" Tom's suggested a small shop that none of us had ever heard of: Baker Aero
Electric (310-539-9789) in Torrance, Calif.
Dave to the rescue
When I phoned Baker Aero Electric, Dave Baker answered. I explained my predicament and
asked if he'd be willing to open up my alternator, figure out what was wrong with it, and
repair only what needed to be repaired. "Sure," Dave replied. I boxed up the
alternator and took it over to the local UPS office. Dave received it the following day
and had it repaired, tested, and on its way back to me within 24 hours.
I called Dave to
ask what he'd found wrong with my alternator. He said he found the brush holder was bent
(something that probably occurred when the alternator was last overhauled), and that the
brushes weren't properly aligned with the slip rings. As a result, the brushes gradually
developed a "lip" where they hung off the edge of the slip rings. In time, the
lip got big enough to touch the rotor and prevent the brushes from making proper contact.
The solution was simply to bend the brush holder back to its proper shape and install new
brushes. Dave said he also cleaned the slip rings, inspected the rotor, stator and diodes,
reassembled the alternator and bench-tested it. He even threw in a new mounting gasket.
The tab for all this (including return shipping and insurance) was a mere $103.00!
Gee…don't you just love it when a plan comes together!
Finishing the job
Shortly after UPS delivered my care package from Baker Aero Electric, I showed Tom Carr
the repaired alternator and the accompanying maintenance release paperwork, and told him I
was ready to reinstall the alternator on the right engine.
The alternator is secured to the engine crankcase with four mounting studs, two of
which are very difficult to reach. It took me the better part of an hour to get the nuts
started on those two lower studs, but I finally succeeded (and was very proud of myself).
I looked up the proper torque for the four mounting nuts, but found that I simply could
not get a wrench on either of the two lower nuts, even after exhausting every ½-inch
wrench in my toolbox (including offset, crow's foot, dog bone, etc.).
I went to Tom for help, and he gave me that "been there, done that" smile.
"I've got some special 'trick' wrenches in my toolbox that should make the job much
easier," Tom revealed. I prevailed on him to let me borrow his special wrenches
(which were obviously non-standard tools that had been ground down and bent just for
situations like the one I was facing). Tom's trick wrenches made torquing down those
otherwise-unreachable nuts a piece of cake. Then all that was left was to reattach the
wiring harness to the alternator's five screw terminals.
I told Tom that I was done with the installation and ready to cowl up and give the
alternator a "smoke test." Tom inspected the installation very carefully, poking
here and tugging there while I watched nervously, and finally gave it a thumbs-up. I
re-cowled the engine, pulled the aircraft out of the hangar, fired up the engines, and
gave the charging system an extra thorough functional check. Everything appeared normal.
I brought my maintenance logs to Tom and told him that I'd finished ground-running the
engines, and the charging system seemed to check out 100%. We stapled the Baker Aero
Electric paperwork in my right engine logbook, then I made and signed a logbook entry for
the alternator removal and reinstallation, and finally Tom made and signed an entry
approving my installation work.
Just to make sure, I took the airplane up on a 30-minute test flight. Everything worked
exactly as advertised. I was a happy camper.
So what's the point?
My purpose in writing this article is to underscore a few of the ways in which an
aircraft owner can benefit by getting more involved in the maintenance of his aircraft.
Let's review.
First, in the face of an annoying but intermittent problem (what I call a
"transient anomaly"), a maintenance-savvy aircraft owner will consider sitting
tight and waiting to see whether the problem gets worse or goes away by itself. If it gets
worse, it'll be much easier to troubleshoot. If it goes away, it's not worth worrying
about unless and until it reappears.
Second, a maintenance-savvy owner will always make a real effort to investigate and
troubleshoot a problem so he can tell his mechanic exactly what's wrong ("the right
alternator has an intermittent field, with electrical arcing visible through the end cap
as the propeller is rotated by hand-probably bad brushes") rather than offer only a
general description of the problem ("the right alternator fail light is coming on
intermittently"). This is especially important for problems that cannot easily be
reproduced on the ground.
Third, a maintenance-savvy owner can frequently save a lot of money by researching
lower-cost alternatives to expensive parts replacement. In this case, by making a few
phone calls, I was able to resolve the problem with a $100 repair instead of a $1,000
overhaul/exchange. This is an area in which one phone call to the Cessna Pilots
Association technical support line can easily repay your membership dues a dozen times
over. CPA can usually suggest the most cost-effective way to resolve almost any Cessna
maintenance problem…and if we don't know the answer, we know who knows!
Finally, a maintenance-savvy owner who is so inclined can perform virtually any sort of
maintenance or alteration to his own aircraft, provided his mechanic is willing to
supervise and sign off the owner's work. If you enjoy swinging wrenches on your airplane,
it pays to develop a relationship with an A&P who is comfortable in dealing with
owner-performed maintenance, and then work to earn his trust by showing him that (1) you
won't start swinging wrenches without his prior approval, (2) you have reasonable
mechanical knowledge and aptitude, and (3) you know when it's time to ask for help.
Keep in mind that you probably won't save a lot of money by doing your own work,
particularly if you place a high dollar value on your time. I'm sure, for example, that
any professional twin-Cessna mechanic could have removed and reinstalled my alternator in
half the time that I took to do it. On the other hand, if you're interested in learning
more about the mechanical aspects of your aircraft, there's simply no better way than to
jump right in and get your hands dirty.
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