Aircraft Instruments: Knowing how to handle is as critical as how to repair

Aircraft Instruments

Knowing how to handle is as critical as how to repair

By Ron Rucker

September 1999

Instruments have, for many years, been the stepchild of avionics. Although instruments don't always contain avionics, they have historically been lumped in the same category. Nonetheless, they are extremely critical to flight and navigation and have a history that dates back to the early 1900's "seat of the pants" flyers.

It is also critical that maintenance personnel understand how to handle and maintain instruments. Although many repairs to instruments must be done in an approved shop under controlled conditions, there are many opportunities to damage instruments during the installation and removal of this often sensitive equipment. Consider that a drop of six inches to a typical instrument can subject the internal components to as much as 2 gs of force!

Following are some specific tips for handling of common instrumentation:

Gyros
Gyros (not to be mistaken for Greek fast-food) are possibly your most troublesome and fragile instrument. Handle them like your mother-in-law's favorite serving dish.

• A great percentage of problems occur in shipping/handling and contaminates in the mechanism. A very small drop, something less than 1/4-in. is equivalent to approximately 2 gs and 6 gs will damage most modern gyros.

• On vacuum gyros, always keep system and gyro filters clean and don't plumb gyros in series. This tends to reduce vacuum pressure to the units.
• Don't automatically lubricate fittings on vacuum gyros. This tends to reduce friction when tightening and can lead to over torquing and damage the fitting and the back plate ports.
•Always plug all ports when shipping to reduce possibility of damage or contamination to the unit. • Always allow gyros to "run-down" before handling to prevent damage to the bearings and/or the gimbals.
• If gyros are in storage or are not used for periods of two to three months, they need to be "powered-up" and operated for perhaps a half hour to allow the bearings to be exercised and lubricated.

Pressure Instruments
Manifold Pressure Indicator
• If you need to check the accuracy, compare it with the Altimeter's baro setting. Altimeters
• Some units have vibrators that help reduce inherent friction in the instrument, which helps reduce inaccurate readings. Always make certain the vibrator is operational and doing its job.

True Airspeed Indicator
•The most troublesome problem is the capillary tube. If it is cracked, bent or broken the instrument is usually considered beyond repair.

General Instrument care and feeding
The following are "care and feeding" tips offered to help you in the removal, installation, and handling of instruments.

1. When sending an instrument or an accessory 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 information on the packing sheet.
d. Don't write on the glass. (Many glasses are coated and costs on some are quite high.)