F/A-18 GCU Fault Detection System Paying Off for FRCSW

From Navair News: http://www.navair.navy.mil/index.cfm?fuseaction=home.NAVAIRNewsStory&id=6561

A Fleet Readiness Center Southwest (FRCSW) investment one year ago is paying dividends in the command’s efforts to provide dependable, mission-ready components to the fleet.

The Intermittent Fault Detection and Isolation System (IFDIS), purchased by FRCSW to troubleshoot the F/A-18 Hornet airframe’s Generator Converter Unit (GCU) chassis, recently resolved an issue with a GCU that had consistently befuddled fleet maintainers.

The GCU is used in the powering of the aircraft’s electrical systems. A faulty GCU can result in an aircraft losing power and cockpit pressurization problems.

“This problematic GCU had been removed for repair from various aircraft seven times in the recent past,” said Brett Gardner, Avionics Advanced Technologies Investment (ATI) Team Lead. “The fleet was so frustrated that they could not repair the unit that they sent it to FRCSW for an engineering investigation (EI).”

The IFDIS checks the connection points in the GCU harness, ensuring that all circuitry lines are free of intermittent shorts or opens.

In this case, IFDIS resolved the problem by identifying an intermittent pushed pin issue with the GCU wiring harness.

A visual inspection revealed that the component had not sustained any physical damage and the unit had passed a test program instruction check.

Afterward, the component was analyzed using an Aircraft Engine Components Test Stand (AECTS) which found no discrepancies in the unit’s basic operational test, performance test, and cold oil test at both high and low speeds.

The GCU’s main chassis wire harness was removed and tested on IFDIS where it passed on the X and Y axis, but failed on the Z axis at minus 18 degrees Celsius and three g vibrations.

IFDIS features an environmental simulation compartment that emulates typical F/A-18 flight stresses and conditions.

The simulator can vary temperatures from 350 F to minus 100 F and produce vibration levels to more than 2,000 pounds of force.

Analysis revealed that two pins of the GCU main wire harness connector were not properly engaged. Once reset the GCU passed in the Z axis portion of the test.

“From the results of the EI testing we can also see that our own depot AECTS test set was not capable of detecting the intermittent. The harness had to be installed on IFDIS to detect the failure,” Gardner noted.

The loose pins were most likely caused by aircraft vibration, resulting in an intermittent break in electrical contact. They were sent to the FRCSW materials lab for further testing.

IFDIS uses internal and external connection points to the GCU chassis and is controlled through a central computer with monitor displays that inform the operator what points are being checked and when an intermittent failure is detected.

“In my experience there have been many EIs where we have suspected intermittent connections as being the root cause failure mode, but did not have an effective tool for easily testing, identifying and isolating intermittent circuits,” Gardner said.

“As a result, we spent many man hours manipulating our test equipment trying to get the intermittent to reveal itself, either by removing cooling air during testing or tapping gently on the side of the unit while under power to induce the intermittent. We were lucky enough to get a few of the intermittents to reveal themselves using these crude techniques but the success stories were few and far between.”

In addition to the GCU wire harness chassis, IFDIS is also capable of testing other weapons replaceable assemblies, or “boxes” that hold circuit cards that comprise an avionic function, like radar or certain cockpit displays.