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

2. When troubleshooting units in the aircraft, follow some simple checks:
a. Static instruments that are under pitot static tests often indicate leaks. This could be the result of improperly sealed fittings. Please make certain this is not your problem before sending in for repair.
b. Don't tap (pound) directly on the glass of any instrument to try and correct a reading. If you must, tap (pound) on something, tap the panel.
c. If it's a Dual Instrument, switch the connections to see if the problem follows.
d. On instruments with plastic cases, such as Airspeeds, Altimeters, or Rate of Climb, don't over tighten the fittings. This can break the seal between the insert and the case; and cost you the price of another case.
e. When using Teflon® tape to provide a seal, be judicious in its use so as to not allow any excess to either block the port or have the possibility of entering the instrument chamber.
f. Water in static systems is a problem that occurs from time to time. Any instrument that either has a possibility or actually has water in them should be sent in for repair immediately. If any instruments are installed as a dual system, send them in for repair/calibration as a set.
g. Blown diaphragms in Airspeeds and Altimeters occur quite frequently while testing the pitot/static system. One way to ensure this does not happen is to be aware not to "dump" the pressure too fast without venting the instruments. The sector gears can also be damaged during this same procedure. Keep in mind, both of these parts are expensive to replace.

How a Conventional Gyro Works
(From AC65-15A)
In a typical vacuum driven attitude gyro system, air is sucked through the filter, then through passages in the rear pivot and inner gimbal ring, then into the housing where it is directed against the rotor vanes through two openings on opposite sides of the rotor. The air then passes through four equally spaced ports in the lower part of the rotor housing and is sucked out into the vacuum pump or venturi.

The chamber containing the ports is the erecting device that returns the spin axis to its vertical alignment whenever a precessing force, such as bearing friction, displaces the rotor from its horizontal plane. Four exhaust ports are each half-covered by a pendulous vane, which allows discharge of equal volumes of air through each port when the rotor is properly erected. Any tilting of the rotor disturbs the total balance of the pendulous vanes, tending to close one vane of an opposite pair while the opposite vane opens a corresponding amount. The increase in air volume through the opening port exerts a precessing force on the rotor housing to erect the gyro, and the pendulous vanes return to a balanced condition.

The limits of the attitude indicator specified in the manufacturer's instructions refer to the maximum rotation of the gimbals beyond which the gyro will tumble. The bank limits of a typical vacuum driven attitude indicator are from approximately 100 to 110 degrees, and the pitch limits vary from approximately 60 to 70 degrees, depending on the design of a specific unit. If, for example, the pitch limits are 60 degrees with the gyro normally erected, the rotor will tumble when the aircraft climb or dive angle exceeds 60 degrees. As the rotor gimbal hits the stops, the rotor precesses abruptly, causing excessive friction and wear on the gimbals. The rotor will normally precess back to the horizontal plane at a rate of approximately eight degrees per minute.

Many gyros include a caging device, used to erect the rotor to its normal operating position prior to flight or after tumbling, and a flag to indicate that the gyro must be uncaged before use. Turning the caging knob prevents rotation of the gimbals and locks the rotor spin axis in its vertical position.

Electrical Instruments
EGT's and ITT's
• If you are testing on a Jetcal® make certain you are using the resistance leads required by the specs.

EGT's
• Usually provides only a relative indication on recip engines. Turbine Indicators, however, are quite accurate. If the unit is tied to the Buss Voltage, don't check with a multi-meter, it degrades the IC's in the unit. One of the biggest problems in this unit is its susceptibility to high resistance on connectors.

Voltammeters/ Ammeters-
• Fairly simple instruments, but the meters are mostly 50 millivolt types and can not tolerate buss voltage. It will "smoke" the ammeter portion of the unit.

Fuel and Torque Transmitters
•A somewhat frequent problem that occurs in torque transmitters is the presence of oil in the vent tube that eventually ends up in the vent chamber thereby causing the transmitter to operate sluggishly. Modifying the transmitter with a larger diameter vent tube usually prevents this problem.
• Another frequent problem on Fuel Transmitters that can be costly is the method of removal of the transmitters from the aircraft. Some transmitters have a protective shield that either gets bent or cracked during removal. An expensive shield replacement can be prevented by placing your wrench at the base, which is stronger, rather than across the shield, which is weaker.
• Transmitters, due to their location in the aircraft, have a tendency for the connectors to become corroded or contaminated. This condition causes problems that make the transmitter appear to be the culprit. Some simple preventative maintenance can cure this situation and save shipping and time-consuming costs. A small, stainless steel brush and alcohol can be used to remove the offending contamination.