A B747-400 Captain reports his concerns about Mechanics and Maintenance Supervisors attempting to coerce flight crews to ignore serious defects; is ongoing and continuous and becoming more commonplace.

Narrative

During my cockpit preflight; I noticed that window L2 had a delamination in the lower left corner which was also affected by a milky white film; indicating moisture ingress. Also; the exterior weather stripping was separating from the window. While both L2 and L3 windows had been previously deferred for 18-inch delaminations; there was no mention of the milky white film. I entered the defect into the maintenance CPU system and we advised maintenance. A Mechanic came out and stated that it was 'Already deferred.' I explained that my write-up was for a different defect; for which he stated it was 'Within limits.' He offered up the Maintenance Manual for my review. I had experienced this defect previously; and directed him to page 56-11-00 which clearly states that the window must be replaced if the delamination is affected by a milky white film. He called his Supervisor; who came up and stated that 'We have deferral limits' that would allow this to continue. I asked to see this in writing; but he could not provide it. At this point; it was very apparent to me and my crew that maintenance was intentionally trying to bypass the Maintenance Manual's directive to replace the window. I called Maintenance Control on the phone and he confirmed my concern; that the window would have to be replaced. My concern is that this practice of our mechanics attempting to coerce flight crews to ignore serious defects is ongoing and continuous; and becoming more commonplace. Flight crews need to be able to completely trust Line Maintenance to give us truthful and accurate assessments of potential defects. We do not have the time; training; or resources to second-guess the information provided by Line Maintenance. I am very concerned that I have experienced several occasions recently where maintenance has provided erroneous and misleading maintenance guidance regarding major safety defects; in an apparent attempt to improve dispatch reliability at the expense of flight safety. Had I not taken the time and initiative to contact Maintenance Control; this flight would have been dispatched with a no-go maintenance defect. Callback conversation with Reporter revealed the following information: Reporter stated he called their Maintenance Control; to have them talk with the Maintenance Supervisor; and explain to the Supervisor why the L-2 cockpit window with the 18-inch delamination that was previously deferred; required replacement due to moisture ingress. Reporter stated he should not have to be the one to explain to Maintenance about damage limits that they should know exists; if they would just take the time to investigate the problem.