Altimeters measure the pressure of the air that surrounds us and our airplanes. That measurement shows the pilot his altitude in feet (or meters) above sea level. The altimeter is a precision instrument that must be recertified every two years to maintain accuracy. During this recertification test, each unit goes through a complete inspection. If it fails, the unit should be disassembled, cleaned, aligned and calibrated to ensure that all parts are in proper operating condition.
It is our experience that more than half of the units tested also require some sort of repair. Here are the most common repairs found during the recertification process.
Alignment and /or calibration
When a unit is out of alignment or calibration, it gives an incorrect altitude reading to the pilot. An alignment is necessary if the pointer is off the same amount throughout the entire range of the scale. A calibration is necessary when the pointer is off at varying amounts during the scale error test.
An alignment can be a simple, external adjustment to the baro adjustment screw, but calibration requires the altimeter to be disassembled and internally adjusted. Either operation needs to be done in a certified repair shop, such as Duncan Aviation.
Sticky or jumpy pointer movements will cause the pilot to also be weary of the accuracy of the indication. This can be caused by contaminated or dirty gears, pivots and jewels or incorrect meshing of the gears inside the unit.
This should alert the pilot to the potential for incorrect readings of the unit or its output, such as encoding or other output voltages. The flag may be indicating that something in the unit is not operating correctly, that some input to the altimeter is not correct or that the flag itself is not functioning correctly.
An encoder that is not working correctly will give incorrect altitude readings to air traffic control, causing the controller's need to see the correct altitude of the aircraft.
Other problems may include leaks and lighting. Most all altimeters leak. A very small leak won't hurt anything, but a large leak will make the unit inaccurate or not able to work at all. It is also not unusual for the bezel lighting or an indicator lamp to be inop.
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