Systems and pinpointing malfunctions in hydraulic

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systems and pinpointing malfunctions in hydraulic systems of aircraft. With ultrasound detectors, airframe mechanics listen for the high frequency, inaudible sounds created by air molecules escaping from leaks even within wings and listen for the ultrasound of hydraulic fluid bypassing dysfunctional valves. Previous techniques of detecting aircraft fuel system leaks involving detector probes often provided the information only on the existence of a leak, not on its precise location. The time required for tracer fluids to permeate fuel systems was also a problem. Most aircraft use bleed air to supply air to subsystems designated as high heat systems. The temperature of the bleed air depends on when the bleed air is removed from the engine. Bleed air temperature can be from 385 to 774 °C (725 to 1425 °F) and at pressures of 21 to 75 kPa (3 to 10 lb f ·in. –2 gage). Airborne ultrasound is used to identify bleed air leaks between the aircraft engine and these subsystems. Progressive aircraft reworking calls for overall examination of fuel systems on some aircraft after specified periods of flight time. During progressive reworking, the aircraft’s entire system of fuel tanks and lines is pressurized to 24 kPa (3.5 lb f ·in. –2 gage) with air. With the ultrasound detection leak testing instruments in hand, the inspector guides the probe along the surface of tanks and lines exposed for testing as well as along wing surfaces. When the detector emits a hissing sound, the inspector aims the probe in the direction of the sound’s greatest intensity. Marking this leak, the inspector proceeds over the entire system. At 24 kPa (3.5 lb f ·in. –2 gage) pressure, the detector can pinpoint a leak smaller than 0.07 mm (0.003 in.) diameter from 450 mm (18 in.) away. Aircraft Oxygen System Inspection A low cost, portable ultrasound detection system has reduced the time required to inspect aircraft emergency oxygen systems by 50 percent, according to maintenance officials of commercial airlines. The ultrasound detector is used during overhaul and tests aboard all aircraft. To reveal leakage during overhaul, the passenger aircraft oxygen system is 204 Acoustic Emission Testing F IGURE 17. Detecting oxygen leaks in aircraft panels.
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brought to its emergency operational mode. In some aircraft, the distribution manifolds throughout the aircraft and cabin are under constant pressure, permitting inspection without this preliminary step. With the ultrasound leak detector, airframe inspectors at the maintenance facility listen for leaks, even within cabin panels (see Fig. 17). The ultrasonic system takes about 3 h to inspect an aircraft’s entire oxygen system. Because of the short wavelengths of ultrasonic energy, the sonic waves from a leak, even from tubing having only low pressures, penetrate the cabin panels.
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  • Fall '19
  • Acoustic Emission

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