NTSB Identification: NYC00IA116.
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Incident occurred Sunday, April 02, 2000 in PINE KNOT, KY
Probable Cause Approval Date: 05/01/2003
Aircraft: Israel Aircraft Industries 1125, registration: N511WA
Injuries: 4 Uninjured.

NTSB investigators used data provided by various sources and may have traveled in support of this investigation to prepare this aircraft incident report.

While in cruise flight, the flightcrew selected "GPS" on the pilot's Electronic Horizontal Situation Indicator (EHSI). The number one inverter then began to malfunction and produce lower than normal voltage. As the voltage decreased, the vertical gyroscopes began to slow down and the displays on the Electronic Attitude Display Indicators (EADI) "blacked out," without any warning from the master caution panel. Shortly thereafter, heading warning flags appeared and the compass cards locked in position on both EHSI's. Due to the decreased voltage, the motors of the directional gyroscopes drew higher-than-normal amperage, and blew their respective internal 0.75-amp fuses. When the fuses blew, the load was decreased on the number one inverter, and the voltage output increased back to normal levels; however, because of their inaccessibility, the flightcrew could not reset the fuses. Additionally, a sensing relay, pre-set to de-energize at about 40-volts AC, did not activate to illuminate an "AC FAIL" light on the master caution annunciator panel. With the normal voltage levels restored, the vertical gyroscopes began to accelerate, and approximately 8 to 10 minutes later, the EADI displays returned. The flight landed uneventfully. After the incident, the airplane manufacturer issued an Alert Service Bulletin providing instructions to replace existing low voltage sensing relays with higher accuracy relays capable of providing a warning at higher voltages and to replace the 3-amp Directional Gyros circuit breakers on the overhead circuit breaker panel, with 0.5-amp circuit breakers, enabling disconnection of power to the EHSI's prior to their internal 0.75-amp blowing.

The National Transportation Safety Board determines the probable cause(s) of this incident to be:

The partial failure of an inverter and the inadequate design of the electrical circuit breaker system.

Full narrative available

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