New service bulletin requires verification of passenger oxygen mask stringer

Effectivity: Learjet 70/75 aircraft 

ATA: ​35 - Oxygen​ 

By: TJ Will – Customer Support Engineering 

The passenger oxygen distribution system on the Learjet 70 and Learjet 75 aircraft supplies oxygen to passenger masks in the cabin and lavatory areas. Masks are stowed in dual passenger mask containers (two masks per container) which are located in the headliners. When the oxygen control valve is opened, masks drop from these containers.  

In some cabin configurations, the masks include a stringer assembly attached to the mask, which provides additional length to acquire the mask. In fielded airplanes, masks may have been removed and replaced with a mask that did not include the stringer assembly.   

Recommended Service Bulletins SB70-35-02 and SB75-35-02, released on March 13, 2023, provide inspection instructions to verify the requirement of stringers; and if so, how to confirm stringers are installed at each mask location. The stringer requirement is noted by a placard installed on the inside of the cover of the oxygen mask container. The placard will say: STRINGER ASSY MUST BE SECURED TO THE OXYGEN MASK AS DEFINED IN DRAWING 452520XXXX.   

If this placard is installed, all passenger masks must have the stringer assembly installed. If this placard is not installed, the engineering configuration does not require stringers.   

Stringer assembly part number 289-680-1 was added to the Learjet 70 and Learjet 75 aircraft IPC in the Nov. 30, 2022 revision. In this same revision, Maintenance Manual 35-20-03 was revised to include instructions for stringer removal and installation, as well as repacking passenger oxygen masks with stringers into the containers.  

For additional information or questions, please contact your regional Field Service Representative.