Document
OAS-43 (4/18)
Department of the Interior
Accident Prevention Bulletin
No. DOI APB 26-01 February 9, 2026 Page 1 of 2 Subject: Fleet Aircraft Alterations Area of Concern: Unapproved Aircraft Alterations Distribution: All Fleet Operations and Maintenance Discussion: In recent years, there has been a noticeable increase in Department of the Interior (DOI)-owned fleet aircraft identified with unapproved alterations. These alterations have been discovered during inspections or following adverse flight events in which safety was compromised. In several cases, these unapproved changes were undocumented, non-standard, and introduced additional hazards that rendered the aircraft unairworthy.
SAFECOM #26-0094 . During a Top Cub preflight inspection for a maintenance check flight, an improvised side window latching mechanism was observed (Figure 1). This alteration was not entered into any work order or OAS-2 logbook and did not use approved parts or installation methods. While likely intended to prevent window movement in flight, the modification introduced new hazards, including the potential for impeded egress and impact injury risks. This undocumented “work-around” was not compliant with FAA or OAS maintenance requirements and resulted in an unairworthy aircraft.
Figure 2. Top Cub Improvised Figure 1. Top Cub Window Latch SAFECOM #22-0049 . An aircraft operating in cold conditions experienced a crankshaft oil seal failure and significant in-flight oil loss. The failure resulted from breather whistle holes being taped over with aluminum tape. When the breather tube subsequently froze, oil system over-pressurization occurred. More recently, aluminum tape was again discovered covering a fleet aircraft breather tube hole during a carding inspection (Figure 2). This unauthorized “work- around,” intended to reduce oil splatter in the engine compartment, is not a manufacturer-approved Figure 2. Oil Breather Tube Hole Taped Over No. DOI APB 26-01 February 9, 2026 Page 2 of 2 procedure and presents a serious risk of catastrophic engine power loss.
SAFECOM 25-0885 . During an operation on floats, the throttle became stuck in the full power position. Fortunately, the pilot was able to shut the aircraft down. Post flight inspection revealed the rear seat throttle was physically stuck in an (unapproved) throttle panel cut out notch/modification (Figure 3). Both the front and rear panels had been modified by an unknown individual and these alterations were undocumented.
Figure 3. Notch in Throttle Panel Safety Implications: These examples demonstrate how unapproved alterations compromise safety and airworthiness. The FAA defines aircraft airworthiness as conformity to the approved type design and in a condition for safe operation.
Unauthorized changes invalidate aircraft conformity, introduce unassessed hazards, and may significantly increase risk.
Recommendations: • Ensure compliance with DOI aircraft maintenance policies and FAA standards.
• All DOI fleet aircraft alterations shall be performed in accordance with 351 DM 2, 2.4A(6). 351 DM 2- Aircraft Equipment and Maintenance • Alaska Region personnel coordinate all maintenance through Fleet Services in accordance with OPM-03 . Modifications are to be performed at the Alaska OAS Repair Station or as directed by the Regional Director in accordance with OPM-14 .
/s/ Keith C. Raley Keith C. Raley Chief, Aviation Safety, Training, Program Evaluation, and Quality Management DOI, Office of Aviation Services