The FAA is superseding Airworthiness Directive (AD) 96-20-07, which applies to certain B-Series Combustion Heaters Models B1500, B2030, B3040, and B4050 that are installed on airplanes. AD 96-20-07 currently requires you to repetitively test (pressure decay) the combustion tube and overhaul any heater that does not pass any test.
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