An AT-72 flight crew experienced two momentary losses of most flight instruments and the autopilot when landing lights were turned off.
Synopsis
An AT-72 flight crew experienced two momentary losses of most flight instruments and the autopilot when landing lights were turned off.
Narrative
After reaching our cruising altitude I turned off the landing lights as part of the cruise checklist and simultaneously we lost all of our primary flight instruments for a few seconds. All four EFIS screens went black. We lost both ADC's to include both altimeters; airspeed indicators; and had digital horizontal dashes on the TAT; SAT; TAS panel. Also; as an associated condition; the auto pilot kicked off; the ADU went to standby and the AFCS Invalid message appeared on the ADU.Everything returned to normal shortly after we were able to reset everything and turn the autopilot back on. We continued the flight to DFW as everything had returned to normal. The flight attendants did not notice anything out of the ordinary. Later in the flight; after discussing what we saw; I inadvertently recreated the exact same thing when I selected the landing lights to the off position again. In range we contacted Maintenance and they met us at the gate to check the landing lights before shut down. I was later advised the maintenance corrective action for this write-up was to MEL a landing light that was found inoperative at our arrival time.I would feel more comfortable if Maintenance had taken some action to correct the electrical problem; or at least determined why the landing light switches can affect the flight instruments in a way that is not covered in our systems knowledge.
Second reporter narrative
The Crew Chief said he had never has seen this before and decided to MEL a landing light; replace the other light and put the aircraft back in service. After checking with Maintenance Control no solution has been found to my knowledge.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.