Written by Larry Troyer, Avionics Tech Rep
I had a Challenger 600 customer call this morning with this squawk:
The EFIS computer is displaying a "FD Fail" message and no flight director modes will engage. After installing a known, function FZ-500 flight director computer, it displays the same symptoms.
It was quickly determined something in the aircraft took out both computers, but what?
I recalled a similar situation with this computer where the aircraft had a short to ground on the FD VALID logic output pin that was taking it out and causing the same symptoms. After instructing the technician to check this wire in the aircraft, he found it was indeed shorted to ground. If left unfixed, this would again take out the flight director computer.
This is the second instance where I have seen this scenario occur.
This pertains to only aircraft with Honeywell SPZ-500 ( ) systems installed, which does cover a good share of corporate, twin turboprops and corporate jets of the 1980s and 1990s.