Summary
The FAA proposes to adopt a new airworthiness directive (AD) for certain Bombardier Model CL-600-2B19 (Regional Jet Series 100 and 440) airplanes. This proposed AD would require modification of the Auxiliary Power Unit (APU) cooling air exhaust. This proposed AD is prompted by reports of incomplete drainage of the APU enclosure. We are proposing this AD to prevent a negative pressure condition from developing in the APU enclosure when the APU is operating on the ground, which could create a potential fire hazard if flammable liquid leakage occurs inside the APU enclosure and cannot be drained overboard.