SL-GMS Improves Speed and Accuracy of APU Tests
The maintenance facility at San Francisco has six jet engine test cells. Two are dedicated to engines for wide body aircraft, such as the McDonnell Douglas DC-10 or Boeing 747 and 767. Two other cells test the engines from narrow body aircraft – Boeing’s 727 and 737 type aircraft. The remain ing two test cells are for Auxiliary Power Units (APUs).
The APU is a jet engine that supplies the airplane’s electrical power and air conditioning requirements while it is on the ground, and also air for the main engine starters. The APU is also a redundant source of electrical power in-flight in some aircraft. Just like the main engines, each APU must be periodically torn clown to piece parts and each piece part must be refurbished. The APU is reassembled and tested to insure compliance with the manufacturer’s specifications. The tests include running the APU at full power while measuring its electrical and air flow capacities.
Before United developed the automated test cell utilizing SL-GMS software, testing was conducted manually. The operator would mount the APU in a fixture in a sound proof room and connect the wiring, piping and fuel lines used to run the APU during the test. Then, the operator would go to a second room containing the test cell’s console. The old console had every gauge, meter, switch and light needed to test every aspect of six different engine types. Parameters tested included pressures and temperature, airflow, generator loading, vibration, speeds and electrical power.
This arrangement had serious limitations. First, the operator followed a test script, throwing switches and twisting dials that would operate the APU and step it through a sequence of events, loading and unloading the various systems. This sequence took an average of thirty minutes. However, the timing and sequence was left to the operator’s discretion. The test results were often compromised due to the operator’s inability to read the many dials and gauges simultaneously at points throughout the test sequence.
Further, the operator would manually record the results on preprinted forms and a log sheet. This was a time consuming process.
The test cell also lacked flexibility. If a new type of engine was added to the fleet, holes had to be drilled in the console for the additional dials and switches. Similarly, if engineers wanted to test additional parameters on an existing APU, more gauges would have to be installed.
United decided to develop a new, automated test system for APUs that would eliminate variability from test results. Also, the new system would be required to test all current engine types. Plus, it would be flexible enough to be easily adapted to the two or three new fleets targeted for acquisition in the 1990’s. Finally, it would have to be capable of communicating with many of United’s existing systems.