What was found
OPERATOR HAS EXPERIENCED A RECURRING L BLEED AIR LEAK EICAS MESSAGE ACROSS A FLEET OF FIVE LEARJET 45XR AIRCRAFT EQUIPPED WITH HONEYWELL TFE731-20BR ENGINES. THE DISCREPANCY IS INTERMITTENT AND HAS NOT BEEN REPRODUCIBLE DURING GROUND TROUBLESHOOTING, MAKING ROOT CAUSE IDENTIFICATION DIFFICULT. FOR THIS EVENT, THE DISCREPANCY OCCURRED OVER A PERIOD OF SEVERAL WEEKS. INITIAL REPORTS INDICATED THE L BLEED AIR LEAK MESSAGE WOULD ILLUMINATE INTERMITTENTLY DURING FLIGHT AND EXTINGUISH WITHOUT MAINTENANCE INTERVENTION. IN ONE OCCURRENCE, THE MESSAGE APPEARED DURING DESCENT PASSING APPROXIMATELY FL120. ADVANCING THE THRUST LEVERS CAUSED THE MESSAGE TO EXTINGUISH AND IT DID NOT REOCCUR DURING THE REMAINDER OF THE FLIGHT. MAINTENANCE ACCOMPLISHED EXTENSIVE TROUBLESHOOTING OF THE BLEED AIR LEAK DETECTION SYSTEM AND ASSOCIATED DUCTING. INSPECTIONS INCLUDED BLEED AIR PLUMBING, SENSING LINES, CONNECTORS, THERMAL BLANKETS, JOINTS, AND DUCT ASSEMBLIES THROUGHOUT THE BAGGAGE COMPARTMENT, BELLY, AND ENGINE PYLON AREAS. MULTIPLE CORRECTIVE ACTIONS WERE PERFORMED, INSPECTION, CLEANING, AND RESEATING OF BLEED AIR LEAK DETECTOR CONNECTORS. REPLACEMENT OF LP DUCT SLIP-JOINT SEALS ADJACENT TO LEAK DETECTION SENSING LINES. REPLACEMENT OF HP DUCT SEALS ADJACENT TO LEAK DETECTION SENSING LINES. REPLACEMENT OF RH LP DUCT ASSEMBLY SEALS PER LEARJET TECHNICAL SUPPORT RECOMMENDATION. REPLACEMENT OF LH ECS SHUTOFF VALVE WITH A SERVICEABLE UNIT. REPLACEMENT OF ECS CONTROLLER. INSPECTION AND TIGHTENING OF BLEED AIR LEAK DETECTION SENSING LINES. FOLLOWING EACH MAINTENANCE ACTION, GROUND RUNS AT VARYING POWER SETTINGS WERE SATISFACTORY. MULTIPLE MAINTENANCE CHECK FLIGHTS WERE CONDUCTED ATTEMPTING TO DUPLICATE THE REPORTED CONDITIONS. MOST CHECK FLIGHTS WERE SATISFACTORY AND THE AIRCRAFT WAS REPEATEDLY RETURNED TO SERVICE. HOWEVER, THE DISCREPANCY CONTINUED TO RECUR DURING SUBSEQUENT REVENUE FLIGHTS, INCLUDING BOTH CLIMB AND CRUISE PHASES OF FLIGHT. IN ONE INSTANCE, THE DISCREPANCY RETURNED APPROXIMATELY TWO DAYS AFTER MAINTENANCE ACTION; IN ANOTHER, IT OCCURRED DURING CLIMB THROUGH APPROXIMATELY 8,000 FEET MSL. THROUGHOUT THE TROUBLESHOOTING PROCESS, NO ACTUAL BLEED AIR LEAK OR OVERHEAT CONDITION COULD BE POSITIVELY IDENTIFIED. THE DISCREPANCY REMAINS INTERMITTENT AND FLIGHT-CONDITION DEPENDENT. THE INABILITY TO REPRODUCE THE EVENT ON THE GROUND HAS SIGNIFICANTLY COMPLICATED TROUBLESHOOTING EFFORTS AND RESULTED IN REPEATED AIRCRAFT DOWNTIME.