Learjet 60 CAB ALT HI Red Annunciator

At Duncan Aviation, we have recently been made aware of an issue on Learjet 60s that have complied with Service Bulletin (SB) 60-31-2 and have the ice detector option installed with the ice detector annunciators in the Glareshield.

Flashing Master Warn Lights

According to the airframe manual—AFM-123, Change 37, Normal Procedures, 24.f.(2), page 2-12: when testing the CAB ALT position on the rotary test switch, the CAB ALT HI red annunciator should illuminate and therefore trigger the MASTER WARN lights to flash.

Also when preforming the Cabin Pressure Warning System FT per the aircraft maintenance manual—(AMM -103) 21-30-00, the same steps that trigger the CAB ALT HI red annunciator should also trigger the MASTER WARN lights to flash.

It appears that when these checks are being performed, the MASTER WARN lights do not always trigger. When this issue is being explored, we have discovered that the more the CAB ALT HI annunciator is illuminated, the less often the MASTER WARN will trigger to the point of not triggering at all.

Duncan Aviation has been working on defining the scope and resolution in this matter. If you are experiencing this issue, please feel free to contact your Duncan Aviation Learjet tech reps anytime.