Airworthiness Directives; Learjet Model 23, 24, 24A, 24B, 24B-A, 24C, 24D, 24D-A, 24E, 24F, 24F-A, 25, 25A, 25B, 25C, 25D, 25F, 28, 29, 31, 31A, 35, 35A (C-21A), 36, 36A, 55, 55B, and 55C Airplanes

The FAA is adopting a new airworthiness directive (AD) for certain Learjet Model 23, 24, 24A, 24B, 24B-A, 24C, 24D, 24D-A, 24E, 24F, 24F-A, 25, 25A, 25B, 25C, 25D, 25F, 28, 29, 31, 31A, 35, 35A (C- 21A), 36, 36A, 55, 55B, and 55C airplanes. This AD requires modifying the left- and right-hand standby fuel pump switches. This AD also requires revising the Emergency and Abnormal Procedures sections of the airplane flight manual to advise the flightcrew of the proper procedures to follow in the event of failure of the standby fuel pump to shut off. This AD results from a report of inadvertent operation of a standby fuel pump due to an electrical system malfunction. We are issuing this AD to prevent this inadvertent operation, which could result in inadvertent fuel transfer by the left or right wing fuel system and subsequent over-limit fuel imbalance between the left and right wing fuel loads. This imbalance could affect lateral control of the airplane which could result in reduced controllability.