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

Sept. 1, 1999

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:

Take care not to overtighten the fittings or you will "spin" them and possibly crack the case.

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.)

Airspeeds are susceptible to leaks at the port fittings. Do not overtighten.

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.

Action of pendulous vanes.

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.

True Airspeed capillary tubes are a problem due to cracking, bending or breaking.

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.

• 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.

Fuel Probes
The majority of problems inherent with probes are caused by contamination and corrosion internally and particularly on the connector pins. You can save yourself a trip to the shipping department and possibly your customer some money by making certain the connectors are clean and making good contact.

Plastic back gyros are susceptible to cracking

1. Certain prop and percent types "click and clack" during start-up. If you hear this, it is generally not a problem — it is merely the start mechanism.
2. Electronic tachometers are much more sophisticated and different in your techniques for troubleshooting. Due to the fact the unit has IC's that control its operation, they should not be checked with a standard multimeter.
3. Tachometers that have a synchroscope function usually have problems with the jewel bearings wearing on the shaft; thereby, causing the synchroscope to move sluggishly or not at all.

These points regarding the "care and feeding" of aviation instruments have hopefully provided some insight that will help prolong your instrument and possibly be less costly to you.