April 10, 2002 An Avionics Annual

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by	??? (Original Author Unknown)
	This article originally appeared in the March, 2002 edition of LIGHT PLANE MAINTENANCE and is reprinted here by permission.

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The FAA recommends an annual inspection for your avionics and we recommend it highly as well. We’ll give you the gist of it and then some in the words that follow. Next month, we’ll also give you some low-buck alternatives for test equipment to perform more detailed avionics system checks. By the way, don’t count on your IA doing these avionics checks during your annual. His plate is quite full just doing all the FAA required mechanical and paperwork inspections without doing all the recommended avionics things as well. The reasons are clear: time and money. People want things done quickly and as cheaply as possible.

Adding in all the extras to give your avionics and instrument systems a health check would drive your annual expenses too high, without your understanding of the importance and advantages of these checks. Nevertheless, an avionics inspection is really time and money well spent, particularly if you fly IFR, but even if not, why wait for an in-flight failure to discover problems?

These inspections can run the gamut in level of detail and difficulty, from simple visual inspections to using test equipment to check how well your avionics are performing. The majority of this inspection process doesn’t require an A&P rating, since most things are simple checks where no structural disassembly is required—only the expenditure of time and attention to detail.

The payoff is finding problems before they are in-flight failures—which we all know seem to happen at the worst possible times. Additionally, you can help to make sure your equipment performs up to specification. A secondary payoff is increased troubleshooting ability when things do go wrong en route. A third payoff is being better able to describe the nature of a malfunction to avionics shop personnel. Sounds like a win, win, win, at the expense primarily of a little time.

What follows is a list of suggestions as outlined in FAR 43 Appendix D, plus our take and recommendations. One caveat is that if you find a situation that requires disassembly outside the guidelines of preventative maintenance outlined in Part 43, Appendix D, you need the supervision of an A&P to stay 100-percent legal. 

Inspection Guidelines

Access under the panel varies from bad to worse in many airplanes, but if your body allows it, inspect all wiring bundles for proper security, and that they are not rubbing on any controls when you move them through their complete length of travel. There should be no signs of overheating or wire discoloration.

If you find lots of wayward unmarked wires not properly secured, it may be time for an avionics shop to have a look, as this is a sign that unqualified people were doing repairs with who-knows-what types of unapproved wires from Radio Shack or surplus city.

Older airplanes should be particularly checked for deteriorating wires such as cracks in the sheath that invite moisture and ultimate failure or even a fire. Some shops recommend five-year change outs of wiring harnesses, which is not going to happen. Most people simply will not spring for wholesale replacement of a functioning system at this interval—and it’s certainly not required for Part 91 airplanes. In fact, time limited parts are rare in our class of airplanes; most parts are replaced on condition based on an IA or shop inspection.

That said, even the best wires (with the possible exception of Teflon) will deteriorate over time, and cracked wires are downright dangerous, so you need to decide if this is a danger you want to risk. It’s not only the failure that is troubling, but also the fire potential. Don’t count on the IA finding this cracked wire, unless you specifically ask him to check.

Don’t confuse surface cracks with imminent failure. Factory original wire particularly has two coats of covering, a slick plastic-like outer film, and an inner insulator. The outer film generally goes first, while the inner covering is still intact. If you find this, consider it a flag to plan for replacement relatively soon—at least those wires exhibiting such deterioration if there are only a few. 

Coax Cables

This wire also fails, but generally it’s a performance failure from age or corrosion. If coax is improperly installed with tight bends rather than right angle connectors, the inner conductor will migrate and change impedance from ideal to arbitrary. The result is poor avionics performance—particularly in comm radios. Junk connectors or bad installation of good connectors has a similar result.

Coax sitting in the fuselage belly is particularly prone to problems since this is where most moisture will be found if any. Coax connectors like standard BNC types are not weatherproof without the help of an external covering, so be on guard for problems here. Coax should not be able to flop around in flight, as this guarantees its early demise.

So if you find any loose coax in the fuselage, it needs to be properly secured and run away from any power lines. In spite of an external shield, it will succumb to interference if run too close to power carrying wires.

Some coax is nearly physically bulletproof, such as RG-142 BU, which has a Teflon outer jacket, heavy shield braid and an inner insulator of Teflon rather than polyethylene. It’s also extremely expensive and very stiff to work with. You may find it in a DME or transponder installation, since it has significantly better performance at those frequencies. The connectors are still not immune from weather problems, however.   

Mechanical Connections

Avionics gear is generally secured both at the front and back of the box depending on weight and length. If the rear ends of the radios are allowed to wag in flight, as some amateur installations are, the radio may well snap the front mounting screws or other mounting hardware. Make sure mounting racks are well secured, and that there is no lose hardware. Even the newest avionics should be installed with some clearance between boxes for air circulation.

Also make sure avionics can breathe. If there are cooling ducts, make sure they are free and unobstructed. Something could come loose in flight and block a cooling duct, or a wire could come loose to a cooling fan. Check this. 

Antennas

Antennas can take quite a beating. Some can be observed in flight vibrating to beat the band. Obviously the mounting and antenna itself is subject to deterioration both from the weather and the flight environment.

Check the security of your antenna mounts, and be sure there is no deterioration of any seals. If water gets in there, it can deteriorate the quality of the ground point and reduce or lose antenna effectiveness. If the antenna is physically loose, you have a problem that needs immediate attention.

Antennas encapsulated in fiberglass or plastic housings also can suffer from a crack in the housing, which allows corrosion and deterioration of the active antenna element. Check to be sure the outer antenna sheath is in good shape. Also, be sure that antennas are not painted inadvertently, as paint will affect an antenna’s performance. 

General Once-Over

Go over all the switches and placards in the aircraft to make sure everything is properly labeled and everything works as advertised. Also make sure fuses and or circuit breakers work properly. These things do occasionally quit simply from old age or use.

Circuit breakers that have popped several times may need to be replaced, as they may not be able to meet their amp rating under load. Do not install higher rated breakers, however, to cure a popping breaker. The wiring may not be able to support the additional load.

In this era of GPS wonderboxes, it’s easy to ignore the other navcom gear until there is a problem with the GPS. Check out all the avionics, frequencies, etc., within the limits of the airport policies on communications, VOT check, etc. Include a check of speakers (which you may not normally use because of a headset) and spare microphones used for backups.

Make sure transmissions both transmitted and received are clear, sidetone is working properly, and squelch systems do their job without excess signal reduction. Radios sometimes work OK on some frequencies, and not at all well on others higher or lower on the band, so a comprehensive check will help avoid a surprise while away from home and an unusual frequency is called for. Stay within prescribed AIM rules for transmitter checks.

Even if you don’t fly IFR, performing the VOR operational check outlined in the AIM, Section 1-1-4 or 91.171 of the FARs for OBS operation, flag check, needle accuracy, etc., is a good idea for all pilots. This can be performed on the ground at airports VOT equipped, or in flight otherwise. While these checks have to conform to a schedule and recorded for IFR flight, annual checks should be considered at minimum for all VFR only fliers. 

ELTs

You can also check your ELT, even though it should have been part of the annual inspection, we are aware of more than one instance where this has been ignored and assumed to be OK, or only the battery expiration date checked. According to FAR 91-207(d), ELTs must be examined once a year for proper installation, operational problems, battery corrosion, and sufficient radiated signal. Normally this is done as part of the annual inspection.

Make sure there is no corrosion not only on the battery but also the connector pins, and that the mount itself is secure and the ELT is secure in the mount. Make sure the orientation is proper for crash activation, and that any cables have proper slack. You cannot legally move an ELT anywhere you like for easier access without the proper paperwork approval from the FAA.

Operational checks for crash activation can be done by rapping with the palm (TSO-C-91 types, or by a rapid forward and reverse throwing-type motion on TSO-C91a type units). These checks are limited by FARs to the first five minutes of the hour and for only three pulses (beeps) of the system on 121.5.

Later TSOs, such as those for 406 MHz units spell out check procedures. We don’t recommend these operational checks be done by owners, but only to assure that these checks are being done as required, rather than blown off.

One check that can be easily owner performed is using the ON or ELT activation switch and listening on 121.5 for a strong signal. The test time and signal length guidelines apply here also.

Installation of other avionics systems antennas can seriously impede the ELT antenna’s ability to send a strong signal, and those planes with new antennas are particularly important candidates for a thorough ELT operational and signal strength check.

By the way, most IAs we have checked with as well as the FAA says there is no prohibition from an owner changing an ELT battery, provided no critical structure (for safety of flight) must be removed for access to the ELT. It’s not specifically called out on the preventative maintenance section of the FARs, but battery changing in general is allowed, and it does not limit that to the starting battery. It’s one of those gray areas. 

Summing Up

As you can see, there are a great many areas that can easily slip by an annual—either as not specifically required or sometimes incompletely checked as in the ELT. Knowing that your avionics and instrument systems are up to snuff sure can take some of the anxiety out of IFR (or even VFR) flying.

I know first-hand that these systems or components seem to break at the worst possible time when you need them the most (Murphy’s Law). If you have personal awareness of the layout and condition, you are that much closer to either avoiding a problem in the first place or more quickly finding the problem when it does occur.

If you don’t want to do these checks yourself (which we think you should) for any reason, then by all means arrange with whoever is doing your annual inspection to be sure to add these checks to his list—and expect to pay for his added time to do that which is above the minimum legal list (which is what most annuals are based on price and time-wise).

A good but potentially expensive alternative is to have a trusted avionics shop give your plane the once-over annually if you fly IFR. It’s money well spent for peace of mind. Just don’t hand them your credit card and say fix it without a thorough briefing to you of proposed fixes and costs.

Don’t forget about making sure you have other recurring checks for IFR flown aircraft such as altimeter and pitot/static checks and biannual transponder checks too.