What was found
SHORTLY AFTER LEVEL OFF AT FL380, AT MACH.78 CRUISE, EXPERIENCED A LT 14TH DUCT RED CAS MSG. AND 14TH STAGE DUCT FAIL RED WARNING ON THE OVERHEAD PANEL & AUDIBLE BLEED AIR DUCT . QRH PROCEDURES FOLLOWED AND AT A POWER SETTING OF APPROX. 81%N1, WARNING REMAINED OUT. SEVERAL POWER REDUCTIONS UNTIL REACHING 81%. COULD NOT MAINTAIN ALTITUDE & AIRSPEED AT REDUCED POWER. ELECTED TO RETURN IN REDUCED POWER CONFIG, NO ISSUES NOTED. PERFORMED MDC DOWNLOADS IAW AMM 45-45-00. BLD AIR LEAK DETECT SYS OPS TEST IAW AMM 36-21-00. WORKING PROPERLY. INSPECTED LT 14TH STAGE MANIFOLD SENSING ELEMENT & FOUND ELEMENT CLOSE TO DUCT BTWN HELL HOLE & PYLON. REPOSITIONED IAW AMM 36-21-00. PERFORMED OPS TEST & WORKING GOOD.