An aircraft hydraulic power unit (HPU) consists of many working parts. In order to ensure an HPU is working to the best of its ability and can be deemed reliable and accurate, routine calibration should be performed. Within an HPU there are three particular gauges that require regular calibration. These gauges consist of the system pressure gauge, the analog pressure gauge and the temperature gauge.
To give you a better idea of how each gauge is calibrated, we will discuss step-by-step instructions of how the calibration process is performed. Please be advised that these instructions are to further your knowledge on the calibration process and NOT for you to perform on your own. Calibration of all instruments should always be performed by a professional. If not done by a professional it could lead to serious harm or injury.
SYSTEM PRESSURE GAUGE
The system pressure gauge can be calibrated by using to two different methods. Each method is based upon the installation of a calibration port. For these step-by-step instructions we will focus on the most common method which includes an equipped calibration port.
A technician will shut off the HPU and disconnect it from the aircraft. When disconnected, the technician will then close the calibration port shut-off valve and attach the calibration gauge.
After the technician attaches the calibration gauge, the HPU will have to be set up starting with setting the reservoir selector valve.
Next the technician will open the bypass valve, close the pressure ball valves and close the return ball valves.
Once all the valves are correctly set, the technician will then start the HPU and reopen the calibration port shut-off valve.
The technician will then build pressure by closing the bypass valve. After pressure is built for a set amount of time, the technician will then shut off the bypass valve along with the HPU. At this time, the calibration port shut-off valve should also be re-closed and the calibration gauge removed.
After all five steps have been completed, the technician will calculate the pressure exerted from the HPU and measure uncertainties.
ANALOG PRESSURE GAUGE
During aircraft HPU calibration, the analog pressure gauge can also be calibrated by two different methods. The most common way of calibration is as follows:
A technician will start by acquiring an accurate pressure calibration gauge. Once the instrument is acquired the technician will turn off HPU and disconnect it from its power source.
Next the technician will remove the hydraulic panel and disconnect the pressure hose.
After all pieces have been removed, the technician will attach the accurate pressure gauge to the equipment. From here the technician will record the gauge values at pre-specified increments.
When all steps have been completed, the technician will then take the results and calculate uncertainties.
ANALOG TEMPERATURE GAUGE
The analog temperature gauge, also referred to as the pyrometer, only has one method for calibration. This method has very precise requirements and requires an accurate temperature calibration gauge.
To begin, a technician will remove the HPU top panel to access the pyrometer bulb. Once the HPU top panel is removed, the technician can then remove the pyrometer bulb.
Once all pieces have been moved, the technician will then connect the calibration temperature gauge to the pyrometer bulb. To ensure the analog temperature gauge is functioning accurately, the the Pyrometer temperature display should be at 160 ° F. To ensure the analog temperature gauge is functioning properly, the temperature should never read lower than 159 ° F or higher than 161 ° F.
For proper functionality of an aircraft HPU, all three gauges have to be accurate. The only way to ensure accuracy and reliability of the equipment is to schedule regular times for HPU calibration. HPU malfunctions could lead to a variety of risk, some even could cause severe damage to an aircraft. In order to avoid these situations, be sure to send your HPU to be calibrated frequently!