What are the criteria for the replacement of a magnetron on X-band and S-band radars?
I believe the choice is between following maker’s replacement recommendation or waiting once an issue is reported. I would go with the latter while always having a spare magnetron onboard. @AndreasTravlos do you agree?
Usually, Radar makers’ recommend a replacement at about 4,000 running hours and relevant notification pops up in radar screen. However, as per experience the actual life time can reach up to 8,000 to 10,000 running hours.
Considering that performance test and radar magnetron current is in line with makers’ limits, I would suggest to replace the magnetrons when performance is deteriorated. A set of magnetrons should be always available onboard as mentioned.
In that respect, I agree with @Philip.Argyropoulos.
A magnetron can be considered consumed and due for replacement when its current at the longest available pulse length has dropped to about half the value it had when new.
Regarding inspections from 3rd parties, a radar should be considered in good order when all requirements of IMO RESOLUTION MSC.192(79) - especially those of table 2 - are satisfied. The total operating hours of the magnetron or any other component are completely irrelevant, in the same way that the time since last oil change is irrelevant to the outcome of your car’s annual technical inspection.