QUESTION: Will using windshield heat consistently reduce the chances of my windshield cracking?
ANSWER: King Air windshields are constructed of two layers of glass separated by a polyvinyl inner layer. Contrary to popular belief, turning on the windshield heat does not
cause or prevent the windshield from cracking. Most windshield crack when the
heat is turned on because of pre-existing physical damage to the outer ply. Turning on the heat causes a stress fracture and the windshield cracks. A study conducted of King
Air pilots in England where they are required to consistently use windshield heat for a
bird-proofing reveal maintenance reports that do not differ much from US reports. In other words, their windshield crack at approximately the same frequency as ours, even though we have no consensus on when to operate the windshield heat. So what is causing the failures?
According to PPG, (they manufacture most windshields) it is the penetration of water through the primary weather seal. The primary weather seal is area just outside of the
transparent part of the window. The hydraulic pressure of rain, snow or ice
crystals can erode a poorly maintained weather seal and work its way into the vinyl interlayer of the windshield. Once there, delamination can occur. I’ve seen numerous King Air’s with yellow delamination spots dotting the windshield. These spots are the warning signs that water may have penetrated the interlayer. This leads to heating system failure and possible shattering of the outboard window glass. Proper maintenance can slow down or even stop moisture caused deterioration. Schedule seal maintenance during you normal phase inspections because the sealant usually requires 48 hours to cure. Of course,
some pilots recommend turning on the heat just prior to take off and turning
off after landing. There is nothing wrong with this procedure, but it is not