RecipTec ech hnology
1 0 0 2 s m e t s y S r e w o P e c a p s o r e A y l l e K f o y s e t r u o C s o t o h P
By Winston Greer & Mike McC luskey of Kelly Kelly Aerospace Powe r Systems. Systems.
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f you've ever had an “out-of-box failure” of a replacement alternator then you need no description of the frustration you felt when that happened. If you haven’t had that experience then no description will do it justice just ice.. Techn echnical ical Rep Repss and Warran arranty ty Admi Administ nistrato rators rs devel develop op this same sense of frustration when a returned “out-of-box failure” unit tests to specifications. Aircraft Airc raft chargin charging g systems systems occasio occasionall nally y present present a real trouble troubleshoo shooting ting challenge. As Tech Service Reps, we would like to minimize your frustration and ours by offering the following insights for solving commonly encountered problems. Given the complexity of the charging system, the information presented in this article will be necessarily brief. We will focus on the “high spots” of alternator troubleshooting. Other related components and some of their effects will also be addressed. Throughout the years, we’ve received numerous calls requesting technical assistance with charging system problems. problems. About the time we think we have hear heard d it all something something new comes along. along. The old adage that “You can never know it all” certainly applies to alternators! The only consistent factor is that two basic scenarios exist. The first scenario is “It was working OK and now it isn’t,” and the second scenario is “I just installed it and it doesn’t work.” This second scenario can be much more insidious and difficult to troubleshoot, especially if you are uncertain as to the failure mode that caused you to change the component in the first place. At a risk risk of poin pointing ting out the obv obvious ious,, your your first step shou should ld alway alwayss be to verify that the alternator was correctly installed. Is everything properly routed and secure? With belt-driven units, is the belt tension proper? These questions and others equally apply to units that “fail” soon after installati installation. on.
“If all all else fails, fails, read the directions” Manufacturers frequently include installation instructions and some even 1
April 2001 • Ai Aircraft rcraft Maintenance Technology •www.AMTonli line.com ne.com
RecipTechnology include a test report packaged with the unit. Occasionally, parts dispatchers remove this information so as to include them in the customer’s file. Sometimes these papers actually end up being discarded with the thought of “helping ” the mechanic by removing extraneous tags, etc. If you deal with a dispatcher in your organization, ask if you can look over any information that came with the unit. If a manufacturer ’s bench test report shows that the unit came off their production line operating properly, then it’s not likely any fault lies with the replacement unit. If you are the one who actually opened the box and you are sure there are no directions or test reports included, look for other contact information such as the manufacturer’s telephone or fax number on the box or data plate, even a web site or email address. In many cases, the manufacturer has a toll-free number to call for technical assistance. Spend a few moments reviewing installation instructions and performance criteria (see Figure 1). This exercise may prevent you from removing a good replacement unit and installing yet another potentially misdiagnosed “out-of-box failure.” Confirming how the installation is to be performed correctly,either through written instructions or with coaching by a Technical Service Representative, can likely save you that “out-of-box failure” experience and get your customer back in the air sooner.
Systemsimilarities & differences There are three basic types of alternators used on general aviation aircraft: the Ford style, the Chrysler style, and the Prestolite style. The basic principle remains the same for all three models: the alternator ’s job is to produce AC and convert it to direct electrical current before leaving the device. In so doing, the alternator provides the direct current required by the aircraft instrumentation and equipment. Distinctions between these models are minor. The greatest difference lies in the wiring configuration of the voltage regulator and the alternator. In some installations, the currentcontrolling element of the voltage regulator is in series between the A/C bus (direct current) and the alternator field
Figure 1: Technical documentation – a valuable source of instruction, product per formance data, and manufacturer’s contact information.
(F1). Only one field terminal will appear on the back of the alternator, with another being internally grounded. Alternators with single-lead brush racks will always be wired in this manner. In other installations, the current-controlling element is located between the alternator field (F2) and ground. In these installations, the alternator will have two field terminal connections available. Alternators with two-lead brush racks can be wired either way.
Grounding of field terminals One common technical assistance request is diagnosing a newly installed alternator that doesn’t work. Most Prestolitestyle alternators have two field terminals: one must be grounded directly or through the regulator. Some airframe manufacturers install a very small metal tab going from the F2 terminal to the brush holder screw. Better look close, though; given a well-used alternator, if you don’t know it’s there, you’ll never see it (see Figure 2).
Identifyingthe failure mode/isolating the cause Now back to the first scenario: “The alternator was working OK and now it isn’t.” Presuming that an alternator bearing failure is not involved, that connections all check OK, and correct belt tension has been confirmed for those belt-driven
“Radio Noise” It’s w ell know n that charging system components can affect radio communication in the form of noise. Failing diodes or a bad stator w ill normally generate a whine i n the headset that w ill vary in pitch w ith the R.P.M . of the engine. Radio noise falsely attri buted to leaking diodes or a partial short in the stator can be one of t he hardest problems to track down and solve. Given a new manifestation of radio noise, one of the easier checks is for alternator wires situated too close to the antenna cables. Alternators inherently make some electrical noise and if w iring has recently been re-routed too close to antenna cables, the radio will pick it up. If you have shielded w iring going to or from t he alternator, make sure both ends of the shield are properly grounded and that you have good continuity to the airframe. It’s also advisable to inspect the connections at the batt ery. These connections should be perfectly clean, bright and tight. Often overlooked is the connection of the ground strap from the engine to the engine mount. These connections tend to corrode over time and you cannot see t he corrosion w ith2
February2001 • Aircraft Maintenance Technology • www.AMTonline.com
out taking the ground strap off. If both ends of the connection are clean and tight and the problem persists, then unfortunately everything else has to be checked. Be certain to look closely before deciding all is OK; the problem of corrosion can be very subtle. It only takes about 0.2 ohms resistance on the ground plane to cause radio noise (and a variety of ot her problems). A friend had an older airplane wit h the battery mounted in the rear. He had radio noise, slow cranking R.P.M ., apparent alternat or problems, and several other electrical problems. It was recommended that he run a dedicated ground w ire from the battery to the firew all. W hen he did, all his electrical problems w ent aw ay. Over the years, the aluminum skin of the aircraft had oxidized w here it w as overlapped and riveted, putting too much resistance in his ground plane. Regrettably, all solutions are not this simple. Once all potential sources of radio noise have been investigated and eliminated, you may have a genuine avionics problem. Your local avionics shop wil l have to be consulted for further investigation and diagnosis.
RecipTechnology Figure 2: Example of a ground strap on the field for a Prestolite-style alternator. These can be easy to overlook, perhaps the reason a mechanic had marked this one.
units, then use your multimeter to check and see if there is resistance on the alternator field. If the field is open, then the culprit is a bad rotor or brushes. If the field checks OK (generally 3 to 25 ohms), the next step would be to make sure voltage is getting to the field. If not, then it’s the regulator or the wiring. Determine whether or not voltage is getting to the regulator; if so, then the regulator most probably is the culprit. If everything is checking as it should, by default, things continue pointing to the alternator as the source of the trouble. There is one more test to make before you remove it from the engine — use an analog ohmmeter to check the resistance between the output terminal and ground (a digital ohmmeter won’t work). This is a reverse polarity test so you have to ground the positive probe and contact the negative probe on the terminal. The reading should be between 30 and 50 ohms; a lower reading than this indicates the stator or diodes are gone, and the alternator must be repaired or replaced.
resistance of the coil, thus lowering the magnetic flux of the rotor and the output of the alternator. Brushes are yet another culprit that can certainly contribute to low current output. Be certain to inspect the brushes. Are they worn? Are they making positive contact with the slip ring on the rotor? Most alternator manuals give minimum lengths for brushes. If you happen to disassemble an alternator for any reason and it ’s been in service for a while, always measure the brushes. For gear-driven alternators, the last and certainly ugliest suspect for low output is the one that your customer really doesn't want to hear about: “The coupling gear is slipping.” As you may already know, the reason your customer doesn’t want to hear about this is that in many cases, the replacement cost of a coupling gear is two or three times the replacement cost of an alternator. Coupling gears have rubber inserts that act as a torsional buffer; the inserts are designed to shear to prevent damage to the engine in case the alternator stops suddenly (for example, if something gets inside the alternator and locks it up). The rubber material of the insert tends to harden with age and heat and will eventually allow the two halves of the gear to slip. A good indication that the coupling gear is responsible for low alternator output is when the system works fine with light electrical loads but the amperage and/or the voltage starts dropping as you add additional load. Engine manufacturers’ service bulletins explain how to test coupling gears for minimum slip torque. Also, if you are experiencing high voltage with low output, this can be caused by leaky switches and circuit breakers. If these components have never been replaced, then now is the time to consider doing so.
Lowcurrentoutput The next most common variant of the “It was working OK ” scenario is low current output (this can cause a technician to consider changing careers!). As with most other problems, there can be several reasons for the manifestation of low current output — the most frequent one being a shorted or burned stator. A failed diode is the next most-common suspect immediately following a suspected stator. With a failed diode, you will likely experience radio noise. Modern diodes are much more reliable and durable than those used even just ten years ago, having a much higher mean-time-to-failure life expectancy. Core units returned to our shop have had the stator shorted and burned with fully functional diodes; however, this is the exception instead of the rule. The point is that diodes today can handle the stress of stator failure better than ever. Still, it’s not advisable to reuse diodes that have survived a stator failure due to induced stress, coupled with a high probability of damage and subsequent loss of durability. Another possible cause of low current output is a partially shorted rotor. In this case, the wires in the rotor coil short to each other but not to ground. This lowers the
Pulsingammeter Another “first scenario” common complaint is an oscillating or pulsing ammeter needle. Like all other troubleshooting problems, knowing where to start looking for solutions will conserve your time and temper. Usually this problem is caused either by the field circuit breaker, the alternator switch, or both. However, the regulator over voltage sensor might also be bad. To diagnose the problem, turn the electrical system on without starting the engine and measure the voltage coming off the output terminal of the alternator. Next, measure the voltage on the input (power) wire of the regulator and compare the two readings. If there is more than one-half (0.5) volt difference between these readings, then, as you recall from our earlier comments, either (1) the circuit breaker is Figure 3: Just as overworn brake pads grind brake rotors, brushes that have gone too long and been worn too far cause irreparable harm to mating slip rings.
www.AMTonline.com• Aircraft Maintenance Technology • February2001
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RecipTechnology defective; (2) the alternator switch is defective; or (3) the over voltage sensor is bad. Note:You could have any combination of the three. At this point, check the input voltage of the circuit breaker and compare it with the output voltage. Again, if there is more than one-half volt difference, the circuit breaker must be replaced. Then perform the same input-voltage versus output-voltage test on the alternator switch. There cannot be more than one-half volt difference or the switch must be replaced. Voltage differences inside the switch or circuit breaker originate at the contacts. When they lose the dielectric
Figure 5: Insulated wire used as the brush retainer prior to re-installation of the brush block. Bare wire can scratch the slip ring when removed to release the brushes
grease they are packed with, they arc and pit and oxidize on the contact surface. These compromised surfaces eventually lose the ability to properly conduct current. The outcome is very much like magneto breaker points that have been run a long time. Although it is rare, one last possible cause of an oscillating or pulsing ammeter is the regulator itself. There are some early regulator designs that operate on such a low frequency that they will cause the ammeter needle to pulse at low RPM with a moderate load. If you have one of these regulators you have two choices: (1) live with it; or (2) upgrade to a newer, high frequency regulator.
apparent reason. A simple cycling of the alternator switch temporarily corrects the problem. For obvious reasons, this is frequently referred to as “nuisance tripping.” The first thing to check for is a poor connection in the remote sense or field wires. Some regulators are very sensitive to ambient electrical noise and any intermittent condition will cause the system to trip off line. If connections are confirmed as good and the problem persists, then look at the alternator itself. Even the slightest scratch on the slip ring of the rotor can cause nuisance tripping. Inspect the brushes for excessive wear; especially note if the copper wire is showing, and if it is then the brushes must be replaced (see Figure 3). When the brushes are too short, the brush spring may no longer maintain proper pressure for brushes-to-slip ring contact. Before reassembly, the alternator slip ring must be resurfaced. Don’t ruin your good work on the slip ring (see Figure 4); use a piece of plastic or plastic-insulated wire (you can even use a toothpick) as a brush retainer in the brush block (see Figure 5). Bare metal wire can scratch the slip ring when it ’s pulled out of the brush block to release the brushes once reassembly is complete. It doesn’t take very much arcing
Figure 4: How your slip ring should look when you are ready to reinstall the rotor.
between the brushes and the slip ring to trip some systems off line and even a tiny scratch can initiate or perpetuate the “dropping off line” problem. If you’ve looked at the alternator and are confident that it’s not the source of the problem, it is very probable that the regulator overvoltage circuit or separate overvoltage sensor is failing. Some systems have no overvoltage protection at all, some have regulators with overvoltage protection built in, and others have a separate overvoltage sensor. Make sure you know which you have before replacing them. This has been a very basic survey of single engine electrical system troubleshooting. Just remember one thing: if you can’t figure out what’s wrong don’t hesitate to call a manufacturer’s Technical Service Representative. They probably can save your time and frustration, your customer ’s money and both you and your customer a few gray hairs! A M T
Winston Greer is Vice President of Quality and Mike McCluskey is a Technical Service Representative for Kelly Aerospace Power Systems in Fort Deposit, AL.
“Nuisance tripping” The final problem we’ll discuss could fall in either of our two scenarios: an alternator that drops off line for no 4
Figure 6: One manufacturer ’s technical service contact information.
February2001 • Aircraft Maintenance Technology • www.AMTonline.com
Reprinted with permission from Aircraft Maintenance Technology , April 2001 KA40301
RecipTechnology
Multi-engine alternator charging system troubleshooting By Winston Greer and Mike McCluskey
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ulti-engine electrical systems are nothing more than single-engine systems coupled together to make them work as a unit. Continuing the treatment of alternator troubleshooting from our initial article on this subject in the April 2001 issue of AM T , for this installment we will be discussing twin-engine applications. If you are an optimist, then you might consider twins an “opportunity” for: 1. Performance improvement 2. Increased safety through engine redundancy 3. Revenue-enhancement of your operation If you are a pessimist, then you might think of twins as just a way of doubling your “problems.” Somewhere between these two extremes lies reality. With more knowledge about troubleshooting twinengine charging systems, you are more likely to adopt a more optimistic perspective.
Basic theory Let’s review the basics. An alternator is an electromechanical device that converts mechanical energy to
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electrical energy. A regulator is an electronic device that controls both the voltage and the field current of an alternator. Fundamentally, all of this sounds quite simple — and it is, but interesting outcomes can occur if everything is not “ just right.” Many of the topics covered in this article are expansions on the fundamentals discussed in the April 2001 installment, Taking Charge of Alternator Problems. The fundamentals of troubleshooting singleengine charging systems are the same as those for troubleshooting multi-engine applications. However, significant differences do exist between single and multiengine applications. This expanded
September 2001 • Aircraft Maintenance Technology • www.AMTonline.com
RecipTechnology treatment of the subject should help to resolve some of the more perplexing problems associated with multi-engine alternator systems.
HighFrequencyRegulators Recently, the major reason for changing regulator equipment is to upgrade to newer designs that operate at a higher frequency than earlier designs. Higher frequency (faster) regulators tend to eliminate the “ wagging needle” on ammeters that earlier-designed regulators were prone to cause. These older regulators generally operate at a frequency close to the alternator frequency. As a result, the regulator tries to “follow ” the alternator and vice versa, causing the pulsing or wagging of the ammeter indicator. Another major reason for changing to newerdesign regulator equipment is the advantage of built-in ground-fault protection. From a cost-to-benefit ratio standpoint, this is one of the great newer developments in regulator design. With earlier-designed regulators, a short in the alternator field would “blow ” the regulator. With ground-fault protected regulators, the regulator is not damaged, and an LED comes on to alert of the ground-fault condition.
ALU-8521 LALT.
This is also a great diagnostic tool. If an alternator keeps dropping offline and the LED comes on, then you know the failure mode is the field circuit. This feature saves a lot of time (and money) when a ground-fault problem occurs. Some technicians keep this type of regulator in their toolbox and utilize it as a diagnostic instrument when the airplane they are working on doesn’t have the newerdesign type of regulator.
Paralleling Paralleling remains the most challenging aspects of installing alternators or regulators in twinengine applications, and it is here where those “interesting outcomes” might first become apparent. An airframe manual will, in most cases, give you instructions on the proper procedures to follow. Electrosystem’s Paralleling Procedure Service Instruction SI-0101 may also prove helpful, especially if the aircraft electrical system has been upgraded. One circumstance that occurs with increasing regularity is the installation of regulators that are not original equipment for that particular aircraft. In this case, the manufacturer would have included regulator-specific paralleling
instructions and may differ from the original equipment regulator paralleling instructions outlined in the aircraft manual. These instructions are specific to the initial installation, and may differ from those supplied by a different regulator manufacturer. In most cases, when upgrading to the new, high frequency regulators, they must be replaced in pairs to allow for the units to “talk ” to each other. If you replace the old style Linear regulator with the new style high frequency switching regulator the units won’t talk to each other and paralleling the system will be virtually impossible.
Systemoutput “imbalance” Although single-engine troubleshooting tips apply easily to multi-engine applications, multiengine systems can present a host of unique challenges. Always remember to start with the obvious solutions: make certain that the alternators on both engines are properly installed, the wiring is properly connected and routed, the belt-tension is appropriate for belt-driven units, the battery is adequately charged, etc. However, many pilots and mechanics think that after paralleling is complete, both alternators should produce
+
– F2
ALU-8521 RALT.
24-VOLT FIELDPARALLELING SYSTEM
F1
5A
AIRCRAFTBUS
. 0 2 N I N M A
OS75-28
GND
F2
OS75-28
(over voltage sensor)
S U B
S U B
VR286 (28V)
F1
5A . 0 2 N I N M A
(over voltage sensor)
FIELD
–
+
PARALLELING PAR
FIELD
VR286(28V) GND
www.AMTonline.com • Aircraft Maintenance Technology • September 2001
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G e n e r a t o r t o A lt e r n a t o r C o n v e rs io n s If you have an aircraft that uses generators, then you might want to consider converting to alternators with a Supplemental Type Certificate (STC). Since most general aviation generators have not been manufactured in over 30 years, approved generator parts are getting harder and harder find. In most cases, alternators weigh less than generators, and are more reliable. Furthermore, in some cases, it costs less to convert to alternat ors than to have the generators overhauled. When considering the conversion from a generator system to an alternator system, the first t hing to determine is w hat STC’s are available for your aircraft. One good place to start looking is the FAA w ebsite http://av-info.faa.gov/stc/ w here you may search using many different parameters. STC’s come in every conceivable form, from “ paperwork-only” where you obtain the parts yourself and make the w iring harness, to companies that provide the STC and perform the installation. STC installations at the STC-holder ’s facility are generally very good, and even more attractive if
exactly the same amount of current on each side. This is not usually true. As much as 10 amperes difference may be normal. Many variables such as the wiring, the regulators, the ground system and the alternators themselves may contribute to the imbalance between the two sides. If the difference is less than 15 percent (e.g., about 10 amps for 70-amp alternators), then searching for the cause may require more time than you can spare, in order to achieve very little, if any, improvement.
Air gap differentials If imbalance in the two alternators exceeds about 15 percent, then improvement should be possible. Certainly, no two alternators perform exactly alike — this is especially true in overhauled units. Most of the components in 14
you are close to one of their facilities and convenience is the greater consideration. But, getting an STC kit seems to be the most popular option. There are many compelling reasons for this popularity. A good STC kit has everything needed for the conversion including instructions, parts, and required FAA paperwork; all you have to provide for your customer is the installation labor. STC-holder installations may be too expensive or located too far aw ay to be convenient. With a paperwork-only STC, you have to find the parts: alternators, circuit breakers, switches, wire, brackets, pulleys, etc. Then you must make the harness, and perform the installation. It is not a good idea to merely add the extra w ires required for the conversion; the old wires probably have insulation that is hard and cracking and could cause new problems in a relatively short period of time. In fact, old w iring alone in some cases might be j ustification enough to perform a conversion. No matter which option you choose, you can know that your electrical system is up-to-date, with parts and service more readily available.
an overhauled unit have been used at least once. These components will, in most cases, have wear differentials that contribute to output imbalance. A good example is the air gap between the rotor and the stator, which may cause a considerable difference in the current output. All else being equal, if one alternator has a 0.010-inch air gap, and a second has a 0.005-inch air gap, there will be a noticeable difference in the output, especially at low RPMs. While the physics are not necessarily intuitive, the effects of the difference in the magnetic flux producing the current vary exponentially with distance. Simply stated, doubling a difference in the air gap of two alternators will result in a rate of change greater than double that value. Just remember that small differences
September 2001• Aircraft Maintenance Technology • www.AMTonline.com
in air gaps can have a large effect on output differences. When the installation, or more likely the customer requires a closely balanced system the best option may be to install new units instead of overhauled ones. Even then, they will not likely produce exactly the same output, though the difference should go unnoticed.
Drive gears and belts In the case of gear-driven alternators, serious imbalance may be caused by the coupling gears. If one coupling gear is slipping, then the alternator may not turn fast enough to produce the same amount of current. There are service bulletins addressing this problem with detailed instructions on how to properly test the coupling gears. It is a good idea to test the coupling gear any time the alternator is changed. These gears have rubber inserts that tend to become hard and brittle with age and heat. Unfortunately, in most cases the coupling gear costs much more than the alternator, so be absolutely sure that it is defective before you purchase a new one. On belt-driven alternators, proper belt tension is very important. Engine and alternator manufacturers publish service instructions on the proper tensioning of the drive belts. To ensure performance and reliability, these service instructions should be followed closely. If the alternator belt is too loose, it may slip, and the alternator will not turn at the speed necessary to produce the required output under load. If the belt is too tight, it may cause bearing failure.
Wiring, Terminals, and Connectors As previously mentioned, another unique challenge is properly maintaining the electrical system wiring. It doesn’t require higher math to recognize that there
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tors is temperature. Alternator current is inversely proportional to temperature; that is, output decreases as alternator temperature increases. You might have two alternators perfectly paralleled when cold, but after they warm up they become unacceptably out of balance. Have 1 0 0 the pilot verify the balance 2 s m on the first flight after an e t s y alternator is replaced or S r e reinstalled; you may have to w o P adjust the system when the e c a plane returns. p s o Some regulator models r e A are also more heat-sensitive y l l e K than others. The installation location of the regulaParalleling remains the most challenging aspects of installing alternators or regulators tors might also increase (or in twin-engine applications. decrease) this effect. Some regulators even require a may be at least twice as much electrical wiring in a heat sink to dissipate the heat that they generate, so twin-engine system as there is for a single-engine sys(once again pointing out the obvious) be sure to read tem. This means, of course, that there are more areas the manufacturers' instructions. If you don’t have suf where insulation can become chafed, and more potenficient information, don’t hesitate to call the manufactial for connectors to loosen and/or corrode. Make turer for technical assistance. sure that the terminal ends are clean and tight and that When aircraft charging systems go wrong, it can there are no broken wires or wiring with chafed insulabe tempting to look at the situation from the bad side — tion. Good luck this can often be like finding the and get frustrated. But bad perspectives can result in proverbial “needle in the haystack.” bad outcomes. Hopefully these articles on charging system troubleshooting have provided information Groundsystemproblems that can help keep your perspective optimistic. Here's As we noted in the April 2001 AM T “Radio Noise” to your successful outcomes. A M T sidebar, the ground system can present some of the Winston Greer is Vice President of Quality and Mike most serious troubleshooting challenges. Many techniMcCluskey is a Technical Service Representative for cians change alternators, regulators, wiring and other Kelly Aerospace Power Systems in Fort Deposit, AL. charging system components only to find that the aircraft has a bad ground system. If you have more than 0.2 ohms resistance in the ground system from either of the alternators to battery ground, it is time to start cleaning connections. Recall that other “usual suspects” of a bad ground can be the field circuit breakers or alternator switches. To troubleshoot this, measure the voltage at the bus bar with the system turned on and the engines not running, then check the voltage going into the regulator. If any breaker or switch has more than a 1/2-volt difference from the input side to The Source the output side, then it is probably defective, and Additional resources.... should be replaced. To determine which one, you will have to go directly to the switches or circuit breakers. Kelly Aerospace Power Systems Product Support (888)-461-6077
Anomalies associatedwithtemperature variations Another consideration in setting up the alternawww.AMTonline.com • Aircraft Maintenance Technology • September 2001
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