This month I want to talk about some “out of phase” maintenance items. Out of phase inspection items are defined by Transport Canada, with the primary ones being a compass swing and ELT inspection every 12 months and a recertification of your altitude reporting and altimetry equipment every 24 months for aircraft flying in “Mode C” airspace.

Over the past few months, I’ve been trained in performing these tasks and authorized, under our avionics shop, to recertify the components. I wanted to share with you what is involved in testing the various components of the dreaded and often neglected two-year recertification.

This process starts with removing the altimeter, transponder and encoder from the aircraft. The transponder is set up on our bench tester so that the antenna feeds directly into the test unit. The tester acts like an ATC station forcing the transponder to reply. From there, we can perform the various tests stipulated in CARs Std 571, Appendix F, for those interested in the gory details. Essentially, we are looking for the frequency, power output, and signal quality of the transponder. We also check to see if the correct codes are returned for, the squawk code, altimeter and “ident” functions.

The most common problems, especially in older units, is the deterioration of the “cavity” (essentially an old-fashioned tube component) causing a decrease in power or a frequency shift. Newer units don’t have a cavity and are more reliable.

The next step is to determine the state of the altimetry devices, namely the encoder and altimeter. Here, we are looking for three things: the accuracy of the altimeter, the correlation of the encoder to the altimeter, and whether the altimeter case has any leaks. To do this, the altimeter and encoder are hooked up to a variable vacuum/pressure source. A calibrated pressure altimeter is also attached to compare with. As dictated in CAR Std 571, Appendix B, the test begins with an accuracy test of the altimeter by adjusting the vacuum and pressure in the test system to simulate pressure altitudes from -1000’ up to the certified maximum altitude (usually 20,000’ or 35,000’ for light aircraft). Accuracy is checked at each altitude stated in the CARs. The tolerances vary, getting larger as the altitude increases. If it isn’t within limits, some adjustment can be made to fix it, but occasionally, the unit can’t be made to fall within all tolerances, and we have to fail it.

Once we reach 18,000’, a case leak test of the altimeter is performed. The static line to the altimeter is sealed and left for one minute to ensure the altitude drops less than 100’.

Additional tests are performed on the altimeter to make sure that the altimeter setting window is correct, the needle doesn’t stick and to determine how long the altimeter takes to relax to its original setting after being at its maximum altitude.

While these tests are going on, we check the correlation of the encoder with the altimeter. A test unit is hooked up to the encoder that reports the altitude. An encoder only reports altitude in hundreds of feet, and ideally switches from one altitude to the next at the 50’ marks. For example, as you climb through 5000’, the encoder should switch from 4900’ to 5000’ at 4950’, then back down to 4900’ as it descends through 4950’. Interestingly enough, the tolerance here is 125’, so you could be switching as early as 4875’ or as late as 5125’. We check to make sure that switching is within tolerances at a number of altitudes as prescribed in the CARs and adjust the encoder accordingly. This is why brand-new encoders still need to be tested before installation.

Once the components are returned to the plane, tests are performed to make sure that the transponder is performing within limits using a portable test unit and that there are no leaks in the system. At this point, you’re ready for another two years!

The last item I wanted to talk about this month is a bit of news on the ELT front. There are a few new options now available and some that will soon be available.

One of the manufacturers, ACK Technologies, has just announced full certification on their model E-04 406 ELT! This unit should be shipping later this month and, unlike the current low-cost Artex ME-406 and Kanad AF-Compact (which run about $1200) it will have a GPS interface that will talk with your GPS to send out your location with the signal. The neat thing is that the list price is just under $600!

Another option is the Ameri-King AK-451. The street price on the non-GPS model is about $750 and the GPS interface model is about $1100. However, I couldn’t find out if they were actually shipping yet.

Finally, if you were an early adopter of the Artex ME406 (like me), the ME-183 GPS interface should soon be available allowing you to connect to your GPS for location information at a price of about $260.

See you on Thursday!