Biennial Certs — Who Needs 'Em?

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Every two years, the A&P or radio shop tells us that it's time for those pesky biennial altimeter and transponder certification tests. What exactly do they test, and what's the point?

AvionicsThere are two FARs that dictate the biennial checks for most general aviation aircraft: FAR 91.413 for the transponder and encoder, and FAR 91.411 for the static system and altimeter.

First, notice that I didn't mention the pitot system. While any sane pilot would like to know that his airspeed indicator is reasonably accurate, the FARs do not require any test of the pitot system. Go figure.

An avionics shop worth its salt will check the pitot system and alert the owner if there is excessive error. My experience indicates that it is not at all unusual for airspeed indicators to have errors of 10 knots or more, particularly in the normal cruise speed range. We check the airspeed calibration at a number of points, especially at the stall and flap speeds.

Transponder/Encoder Certs

FAR 91.413 calls for the transponder to be tested for proper output power, frequency, bit encoding, ident time, and a host of other items. It also calls for the encoder to be correlated to the altimeter; in other words, whatever the altimeter reads when it is set at 29.92, the encoder must read the same within fairly close tolerances. This test is quite elaborate and takes some time to perform.

FAR 91.413 must be complied with regardless if the aircraft is flown IFR or not. We call it the "VFR FAR" because even VFR-only aircraft must have it done. Even mechanics sometimes get confused about this. Recently, some maintenance people were fined by the FAA for returning an aircraft to service without this FAR being complied with.

Static & Altimeter Certs

FAR 91.411 applies only if the aircraft is to be flown in IMC or on an IFR flight plan. It requires that the static system be tested to make certain it doesn't have leaks greater than a certain threshold. The permissible leakage depends upon whether the aircraft is pressurized or not. In addition, the altimeter must be tested for friction, scale error, hysteresis, and accuracy at a whole series of altitudes from sea level up to the maximum altitude that the instrument is certified for (usually 20,000' for normally-aspirated aircraft or 35,000' for turbos.) Aircraft with air data computers require more elaborate testing.

If you are interested in seeing a "spec sheet" that shows just exactly what tests have to be run to comply with these FARs, give me a call at (805) 922-2580 and I'll send you a copy. The tests can get quite complex and time-consuming, particularly in pressurized aircraft.

Unpleasant Surprises

Frequently, an owner will taxi up to the shop for biennial certs and tell us that everything has been working great, only to discover later that we found problems during the tests. Common problems are weak transponder output, a Mode C report that differs from the altimeter, or a leaky static system.

This scenario is far more likely if the certification tests haven't been done for many years. In cases where the biennial certifications have been kept current, usually problems are few and any repairs are inexpensive.

I recommend that these certifications be done religiously every two years. If the aircraft is not flown IFR, then you can save some money by complying only with FAR 91.413.

Transponder Tips

If ATC reports that you have a problem with your transponder or Mode C altitude, be sure to verify this with a couple of other ATC facilities before you panic. It could just as easily be a problem with the controller's equipment as with yours.

If your transponder is weak or intermittent, check your antenna. We often see these symptoms being caused by nothing more than an accumulation of oil or dirt on the transponder antenna, causing the signal to be attenuated. An intermittent DME can be caused by the same thing.

These antennas are of the "stubby rod" or "shark fin" variety, and are usually mounted on the belly where they are prone to getting coated with oil, exhaust, and dirt. I recommend wiping down all belly-mounted antennas at every preflight. Your avionics shop will do the same thing, but they'll charge you fifty bucks.

Another frequent cause of intermittent transponder operation is poor cooling. The Cessna/ARC transponder must be cooled with forced air or it will fail. A good avionics cooling fan is a must. I've actually seen them catch fire and burn up the main printed circuit board! This destroys the transponder, of course, and maybe some other stuff as well.