Hangar flying story.
Many, many, years ago, when I was a 747-100 F/E, my company got a great deal on some engine washing solvent. Apparently, this solvent corroded something in the fire loop circuitry on the 747. On the engineers panel, there were two fire/overheat temp gauges for each engine. Zero, green, would indicate a stopped engine. Normal indications were two to three for a normal engine. A ten would set off the fire alarm. If one of the fire loops became inop, the checklist called for cutting that loop out of the warning circuit. At the time, this was not that unusual a circumstance. Then the other loop was relied upon to provide fire warning.
On this particular day, the aircraft arrived in Taiwan with a writeup on the # 4 engine A loop. It had been signed off normally. Looking back for the last week in the logbook (paper, at that time), There had been four writeups of a single fire loop (loop A or B) in the last five days (same engine). We discussed the matter as a crew, Including how to respond to a "twitchy" temp indicator. Sure enough, about five hours into the flight, The B loop on the #4 engine started giving funky readings, while the A loop, which had been written up by the previous crew, remained steady. The temp indicator actually started bouncing between zero and max, setting off the fire alarm disagree light each time. Ran the checklist, cutting out the B loop. We discussed what we would do if the A loop, which had numerous previous writeups, decided to indicate a fire. We agreed that if the next loop indicated a fire, that we would shut down the engine (light weight, daylight, good weather). Then, another fire loop on #2 engine also failed, indicating a fire. Same checklist, same actions, now decided to NOT shutdown an engine without additional confirmatory evidence of a fire.
On descent, all fire loop temperature indications, indicated a rise, which was normal for this airplane, but the indications usually stabilized at 4 or 5. The A loop on number 4 engine (which had been written up by the previous crew) continued a slow rise. I advised the Captain, and told him that I was morally certain that we were going to have a false fire warning before landing; but that of course I could not be absolutely certain that the warning would be false. We got the fire warning bell at 500', and the Captain elected to not do anything about the warning. The warning bell turned out to be intermittent, sounding for about three seconds every ten seconds. The Captain elected to shut down the engine after clearing the runway, but the temp indicator continued to bounce from the bottom to the top of the gauge, setting off the warning bell every ten seconds on taxi in.
Maintenance could find no evidence of a fire or overheat. As long as A/C power remained on the aircraft, the fire warning continued to sound intermittently until the mechanic pulled some plugs.
Joe