Static System and Transponder 24 Month Cert Question

[Bruce Fenstermacher]

It's not that it's a bad idea just perhaps overkill at least every year or if you suspect nothing is wrong with the static system for the VFR flying most of us do in our 170s. A transponder check only in my area of eastern Pa. cost between $80 and $100.

[CBogle]

All:

I took the plane in last week for the transponder check and the tech insisted on certifying the altimeter also. His reasoning was, if he didn't run the altimeter up on the bench, he wouldn't have any reliable way to check the accuracy of the mode-C altitude being sent out by the transponder. After he checked the altimeter on the bench to 18,000 ft., we re-installed it in the plane, then, with a vaccuum device attached to the static port, we ran the altimeter up and compared it's readout with the readout being sent by the mode-C. (He had already checked the transponder on the bench)

I guess this all makes sense. For the past two tests, I had the IFR certification done and was charged $225. The same shop charged me $150 for this last certification as it was only "VFR." Seems fair enough.

It's strange, but, I've talked with a couple of aircraft owners at my airport this week on this topic and they said they weren't even aware that a 2 year transponder certification was required and had never had one done. One was strictly a VFR guy and thought it wasn't required, but the other is an IFR guy with a glass panel in his plane and he was calibrating his altitude readout from the static system with that indicated on his WASS GPS, and, he would adjust the static system to match that shown by the GPS. His logic was that the GPS was the more accurate of the two. I have no idea if this is correct logic or not but both said that they were going to get their systems certified now that I made them aware of the requirements. Makes me wonder how many others are out there flying without the 2 year certifications. Also, makes me wonder if an A&I signing off on an annual inspection would be expected to review the aircraft logs and catch the fact that these certifications had not been done.

Regards,

Curt

[GAHorn]

Curt, if all you are doing is flying around VFR you only need to have your txdr/encoder recertified. This is usually done quite inexpensively on the ramp with a portable test unit.
If you are flying IFR you need that PLUS your altimeter recertified. (Usually requires altimeter AND encoder removal and bench check, as your guy noted. But it sounds as if your guy might be a little confused...because is you don't fly your airplane IFR your altimeter does not need regular recertification.)

As for your friend who thinks it's a good idea to rely upon a WAAS GPS altimetry as more accurate than an altimeter....he's wrong and you should tell him so. It is not acceptable to rely upon GPS altimetry for IFR purposes such as you described your friend to believe. A certified altimeter and static system is the only correct method to determine altitude for IFR flight. GPS is only reference information. GPS altimetry can be several hundred feet off. (WAAS capability does not take into account predictive RAIM at present position and other errors.)