Summary
We propose to adopt a new airworthiness directive (AD) for the products listed above. This proposed AD results from mandatory continuing airworthiness information (MCAI) originated by an aviation authority of another country to identify and correct an unsafe condition on an aviation product. The MCAI describes the unsafe condition as:
It was detected by the STC holder that in earlier installations of the ACASII system there were no isolation diodes installed in the Heading and Attitude Valid lines. The absence of an isolation diode in the valid lines can prevent the valid flag to come up even if a gyro fault exists. The problem has only been detected for Heading Valid lines but could equally affect the Altitude Valid lines.
With installation of the ACASII, the heading and attitude valid lines have to be connected to the TPU67A. On valid state, the signals are +28VDC. On invalid, the signals are open. This condition of direct connection (without an isolation diode installed) of the valid lines to the TPU67A, if not corrected, could cause the TPU67A to feed current into the open stated valid lines. This prevents the flag to appear even if the gyro is invalid, providing the flight crew with erroneous navigation information.
The proposed AD would require actions that are intended to address the unsafe condition described in the MCAI.