May 2012
Q: Hi Steve,
I am pretty new to airplane ownership and I have a question about a test my mechanic told me my airplane needed. He said that the pitot static system needed to be checked. He also told me that I would have to fly 35 miles away to the only avionics shop around here to get the check.
I know that the pitot tube is connected to the airspeed gauge and the static system is connected to the vertical speed indicator, the airspeed gauge and the altimeter.
No one has touched those instruments since last annual, so why do I need to pay $250 for this test?
—Testy
A: Dear Testy,
Welcome to the world of aircraft ownership. I hope the gift of flight in a small airplane is as amazing to you as it is to me. Here’s the story on what are commonly referred to as pitot and static checks.
The rules requiring these tests are listed in FAR Part 91, General Operating and Flight Rules. 91.411 is titled Altimeter system and altitude reporting equipment tests and inspections. 91.413 is titled ATC transponder tests and inspections.
Some pilots—and even some mechanics—believe after reading these regulations that airplanes are not airworthy unless both tests have been done within the preceding 24 calendar months. That’s not totally true.
Careful reading of 91.215 (titled ATC transponder and altitude reporting equipment and use) does reveal that there are some instances where an operative transponder is not required, but these exemptions are narrow and are designed to accommodate antique-type aircraft that left the factory without electrical systems. Each ATC facility may grant authorization for these non-transponder aircraft to operate in the airspace it’s responsible for when a request is made at least one hour before the proposed operation. There are also paragraphs stating that ATC can authorize flight in any airspace for aircraft that are not equipped with automatic altitude reporting equipment and aircraft with inoperative equipment, if requested.
For the average light plane owner, these exemptions are confusing and restrictive. Most of us are happy to pony up the money for the freedom to be able to fly freely in U.S. airspace.
The following outlines the point of confusion.
91.411 states that an airplane or helicopter may not be operated in controlled airspace under IFR unless the static pressure system, each altimeter instrument, and each automatic pressure altitude reporting system is found to comply with Appendix E of FAR Part 43.
If you don’t file IFR flight plans, you don’t need the 91.411 test. But this is often misunderstood because 91.411 refers to the static pressure system, the altimeter and the automatic pressure reporting system (the encoder) and 91.413 does not.
91.413 states that no person may use an ATC transponder as required in 91.215 unless the transponder has been tested and found to comply with Appendix F of FAR Part 43. There’s no mention of the altimeter or static system instruments. However, the altitude reporting part of the transponder system—the encoder—is connected to the static system, so it follows that the static system is checked during the transponder/encoder test even if it’s not specifically mentioned.
There are two tests mandated in 91.411. The first is for system leakage of the static system. The second is for scale error, hysteresis, after effect, friction, case leaks, and barometric scale error of the altimeter. The most common method of doing these tests is to hire an avionics shop with a certified ground test box. The shop will connect the test box to the airplane static port and the pitot tube and conduct the static system and the altimeter checks and certifications at the same time.
It’s also okay for an airframe-rated mechanic to remove the altimeter and send it to a repair station with an instrument rating for recertification, then reinstall it and perform the static system leak and inspection check.
91.413 mandates tests of the performance of the transponder and altitude encoder. This testing measures the power of the replies to ATC inquiries, that the replies fall within a narrow frequency range (1090 +/- 3 MHz), that the suppression mode works, and that the receiver sensitivity is within published parameters.
Since transponder replies are encoded with altitude information, all avionics shops check to see that encoded replies are correct for a series of different altitudes. This altitude testing protocol is so close to the tests required on the altimeter in 91.411 that experienced shops combine the tests. These tests may be conducted on a test bench but almost all modern shops use portable test equipment to test the equipment as installed in the customer’s airplane. These tests must be conducted by a certified repair station holding a radio rating, Class III or a limited rating.
The last time I took my Comanche in to Don Dominguez at the San Luis Avionics (in San Luis Obispo, Calif.) for recertification, he did the testing on the ramp. My transponder/encoder worked fine, but the altimeter failed the barometer scale error. Dominguez said that was due to internal friction. He had warned me during his before-testing check that he thought it was “out.”
He signed off the 91.413 transponder part of the test and told me that the static system was leak-free but that I couldn’t legally fly IFR until I got the altimeter recertified. I sent it off.
I reinstalled it (I am a certificated airframe mechanic), and I found the static system leakage rate to be within the allowable range after checking it with my portable test box.
The static system and altimeter/encoder checks are important because they serve to put all the airplanes in the airspace where they belong, altitude-wise.
Happy flying.
Know your FAR/AIM and check with your mechanic before starting any work.
Steve Ells has been an A&P/IA for 38 years and is a commercial pilot with Instrument and Multi-Engine ratings. Ells also loves utility and bush-style airplanes and operations. He served as Associate Editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation and the proud owner of a 1960 Piper Comanche. He lives in Paso Robles, Calif. with his wife Audrey. Send questions or comments to editor@www.piperflyer.com.
RESOURCES
San Luis Avionics Inc.
sanluisav.com
