B777-300 Captain reported a long delay due to (MFD) Multi-function Flight Display faults; caused by the headset adapter block being placed in close proximity to the nose landing gear air/ground sensor. This led to the aircraft cycling rapidly in and out of Air Mode; which triggered the MFDs to malfunction.

Narrative

On Day 0; Aircraft X; ZZZ - ZZZZ; was at the gate in ZZZ on APU power; without ground power connected. Just prior to departure; all of our MFDs (Multi-function Flight Displays) began to flash; then fail; and numerous cockpit lights flashed along with the main cabin lights. We were unable to control the MFDs and until it stopped; the status page had 20+ Faults. This issue caused an 8-hour delay to clear the faults and replace some components that failed because of it. Both the original FAs (Flight Attendant) and pilots timed out; the flight had to be re-crewed; and departed late with another aircraft.When a Maintenance team initially got to the cockpit; they told us that they did not know what caused the problem and proceeded to work on clearing the faults. I explained to the team that clearing the faults was not good enough. I needed to know what caused the problem and that it would not happen in flight. Eventually; the team was joined by another Mechanic who said he knew what caused the problem; because he had seen it about 5 years ago on another 777. The team returned us to APU power. The Mechanic went down to the nose gear and repeated the light show we saw with the fault. He did this by bringing the headset adapter block nearby the air/ground sensor on the nose gear. Apparently; the sensor works with an electromagnetic field and the headset adapter; when hung too close to it; can trigger the aircraft into Air Mode." In our case; in and out of Air Mode rapidly.How is it even possible that we allow that headset anywhere near a $300 million jet? If we are that foolish; how do we not let every single Mechanic/Ramper/pilot who works with the 777 know that this is a real danger? Perhaps if we explained to the CFO how much this simple error cost air carrier X; he/she would ban that adapter from use on the 777. Oh what a wonderful world that would be."