PINE KNOT, KY, USA
N511WA
Israel Aircraft Industries 1125
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.
On April 2, 2000, about 2030 eastern daylight time, an Israel Aircraft Industries 1125, N511WA, experienced a total loss of it's gyroscopic reference system while in cruise flight over Pine Knot, Kentucky. The certificated airline transport crew and two passengers were not injured. Visual meteorological conditions prevailed and an instrument flight rules flight plan was filed for the executive/corporate flight conducted under 14 CFR Part 91. According to the operator of the airplane, while at 41,000 feet in level flight, the crew 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." 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 for the EHSI's 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. With the normal voltage levels restored, the vertical gyroscopes began to accelerate, and approximately 8 to 10 minutes later, the EADI displays returned. The master caution panel did not alert the crew of the low voltage condition. Additionally, when the internal fuses for the EHSI's blew, the flightcrew was unable to reset them. The flight landed at the Cincinnati Municipal Airport Lunken Field, Cincinnati, Ohio, without incident. According to the airplane's manufacturer, the airplane was equipped with two 1,000-amp inverters, each capable of providing 400 cycle 115-volts AC, and 26-volts AC power to the entire airplane, with reserve. The inverters also had an internal "fault" mode that detected conditions to include thermal and over/under current conditions. The "AC FAIL" warning annunciator system, installed on both the number 1 and number 2 electrical systems, consisted of a circuit breaker connected to a 115-volts AC bus, and an AC coil relay. With a respective AC bus normally powered, the AC coil relay would also be powered. If power was removed from the AC bus, or lowered to about 40-volts AC, the AC coil relay would then de-energize, and provide a ground signal to the opposite-powered AC bus, simultaneously an "AC FAIL" light would illuminate on the master caution annunciator panel. The electrical system also included transfer relays. If the EHSI were selected to a long-range navigation source, the number 2 transfer relay would have activated and transferred the number 2 electrical bus consumers to the number 1 bus, requiring the number 1 inverter to supply all power to the airplane. The airplane manufacturer issued an Alert Service Bulletin, number 1125-24A-246, which provided instructions to replace existing low voltage sensing relays with higher accuracy relays capable of providing a warning at approximately 97.5-volts AC, 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 partial failure of an inverter and the inadequate design of the electrical circuit breaker system.
Source: NTSB Aviation Accident Database
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