Aviation From My Rocking Chair: What, Another Instrument Column?

For all the wonderful new digital panels available now, there are still a lot of “Steam Gauges” around, and these need handling and testing from time to time. This month, with the help of Mr. Ron Rucker and “Aircraft Maintenance Technology,” I’d like to cover some handling and testing precautions for aircraft instruments. Remember the first time you looked at an aircraft instrument panel and wondered what all those gauges were used for? And after you knew how they worked, were you amazed at how reliable they could he? I was too. Starting with gyros, let’s see if we can keep them in operation, even if the TV sets go dim.

Gyros:

(1) A large percentage of problems are caused by shipping/handling accidents or contaminates in the component. A very small drop of only 1/4” onto a hard surface approximates 2G. 6G will damage most modern gyros. The term “Handle like eggs” comes to mind.

(2) For vacuum gyros, keep all system and gyro filters clean.

(3) Don’t plumb vacuum gyros in series. This reduces pressure to all units in the system. (4) Don’t automatically lubricate fittings on vacuum gyros. The reduced friction may lead to over torquing, with damage to fittings or backplate ports.

(5) Plug all ports when shipping. This will reduce the possibility of damage or contamination within the instrument.

(6) Always allow gyros to ‘spin down’ and stop before handling. This will prevent damage to bearings and gimbals due to tumbling. Note: Modern gyros have a bank limit of approx. 100-110 degrees and a pitch limit of approx. 60-70 degrees before they will tumble (Non-aerobatic).

(7) When gyros have not been used for a couple of months, or have been in storage, power them up for 30 minutes to allow bearings to be exercised and lubricated.

Pressure instruments:

(1) Manifold Pressure Indicator - Check the accuracy by comparing it with Altimeter baro settings. (2) Altimeters- Some have vibrators installed to help reduce inherent friction in the instrument. Make certain they are working.

(3) Airspeed Indicator- The biggest problem is the capillary tube inside. If it is cracked, bent, or broken, the instrument should be considered beyond repair.

O.k., let’s go over some things to watch while handling, removing, or replacing instruments. (1) When sending components in for repair, be very specific about malfunctions: (a) You can’t give TOO much detail about the failure.

(b) Tag the instrument.

(c) Write discrepancies on the packing sheet.

(d) Don’t write on the glass face. Many glasses are coated and replacement costs can be high. (2) Simple checks for troubleshooting components still installed in the aircraft. (a) Static instruments (Altimeter, Rate of Climb, etc.) which are undergoing pitot/static tests, often show leaks. This could be caused by improperly sealed test fittings. Make sure this is not the problem before rejecting a part.

(b) Resist the urge to tap on the instrument face. I know this looks cool, but needles won’t swing through crushed glass! If you feel a need to tap on something, use the panel.

(c) If dual instruments are installed, swap connections and see if the problem follows.

(d) Water finds it’s way into static systems from time to time. Send any instrument to the repair shop immediately if the possibility of or actual water is found in it.

(e) On instruments with plastic cases, such as; Airspeeds, Altimeters, or Rate of Climb, don’t over tighten the fittings. If the seal between the threaded inserts and the case is broken, the case will require replacement. (f) Blown diaphragms in the Altimeters and Airspeed can occur while testing pitot/static systems. Be aware that dumping pressure testers too fast, without venting the instruments first can cause this. And finally, DON’T EVER BLOW INTO THE PITOT TUBES! It is extremely easy to damage your instruments, and it just looks WRONG.

Well, Troopers, I’ve used up my limited gray matter once again, so be safe out there and since I write this on Mothers Day, bless all the Mothers, Mothers in Law, Grandmothers, and Great Grandmothers out there. Thanks for giving us life and flight.

Rick Vaux TC4130