Summary
We propose to adopt a new airworthiness directive (AD) for certain The Boeing Company Model 737-100, -200, -200C, -300, -400, and -500 series airplanes. This proposed AD was prompted by reports indicating that the pitot heat switch is not always set to ON, which could result in misleading air data. This proposed AD would require replacement of pitot anti-icing system components, installation of a junction box and wiring provisions, repetitive testing of the anti- icing system, and applicable on-condition actions. We are proposing this AD to address the unsafe condition on these products.