Narrative:

We were at FL350 in cruise, WX conditions were IMC in high/stratus/cirrus clouds and the RAT gauge read (-17 C). The pms (performance management system) said our optimum altitude was 35.6 so our gross weight was low enough to maintain our flight level. There were no visible icing conditions, ie, on windshields or wiper blades. Had just climbed from FL310, 20 minutes earlier. No initial abnormal engine or flight control indications at FL310 or 350. I had just selected performance cruise on the automatic-throttle system and the aircraft performed perfectly. About 5-10 mins later, the following events occurred: EPR gauge split. Automatic-throttle disconnected (indication of icing), aircraft decelerated from M .754 to M .710. Mct was selected with engine anti-ice on. We regained airspeed with no altitude loss. In approximately 10 mins repeat of above. In approximately 10 mins, again repeat of above. Mct was selected at .72 M. Both airfoil and engine anti-ice were selected. The aircraft continued to lose airspeed. We lost 300' altitude. We called and received from msp center permission to descend back to FL310. The aircraft decelerated to .668 M and after 4 calls we received the clearance. At FL310 the power was sufficient and operations were again normal. Conclusions: I suspect the same as a manufacturer safety circular suggests (black ice). A super cooled wing arrives on the ground. Warm AVGAS is added and the wings sweats extensively. You then make an expeditious climb back to a high altitude and the potential for wing root icing is there with no other icing visible. If a fast climb is anticipated, in other words, no intermediate level off, then the wings must be preheated with airfoil anti-ice in the climb.

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Original NASA ASRS Text

Title: MLG AT FL350 UNABLE TO MAINTAIN ALT WITHOUT USE OF ENGINE ANTI ICE. REPORTER ALSO SUSPECTS BLACK ICE ON WING ROOTS.

Narrative: WE WERE AT FL350 IN CRUISE, WX CONDITIONS WERE IMC IN HIGH/STRATUS/CIRRUS CLOUDS AND THE RAT GAUGE READ (-17 C). THE PMS (PERFORMANCE MGMNT SYSTEM) SAID OUR OPTIMUM ALT WAS 35.6 SO OUR GROSS WEIGHT WAS LOW ENOUGH TO MAINTAIN OUR FLT LEVEL. THERE WERE NO VISIBLE ICING CONDITIONS, IE, ON WINDSHIELDS OR WIPER BLADES. HAD JUST CLIMBED FROM FL310, 20 MINUTES EARLIER. NO INITIAL ABNORMAL ENGINE OR FLT CTL INDICATIONS AT FL310 OR 350. I HAD JUST SELECTED PERFORMANCE CRUISE ON THE AUTO-THROTTLE SYSTEM AND THE ACFT PERFORMED PERFECTLY. ABOUT 5-10 MINS LATER, THE FOLLOWING EVENTS OCCURRED: EPR GAUGE SPLIT. AUTO-THROTTLE DISCONNECTED (INDICATION OF ICING), ACFT DECELERATED FROM M .754 TO M .710. MCT WAS SELECTED WITH ENGINE ANTI-ICE ON. WE REGAINED AIRSPEED WITH NO ALT LOSS. IN APPROX 10 MINS REPEAT OF ABOVE. IN APPROX 10 MINS, AGAIN REPEAT OF ABOVE. MCT WAS SELECTED AT .72 M. BOTH AIRFOIL AND ENGINE ANTI-ICE WERE SELECTED. THE ACFT CONTINUED TO LOSE AIRSPEED. WE LOST 300' ALT. WE CALLED AND RECEIVED FROM MSP CENTER PERMISSION TO DSND BACK TO FL310. THE ACFT DECELERATED TO .668 M AND AFTER 4 CALLS WE RECEIVED THE CLRNC. AT FL310 THE POWER WAS SUFFICIENT AND OPERATIONS WERE AGAIN NORMAL. CONCLUSIONS: I SUSPECT THE SAME AS A MANUFACTURER SAFETY CIRCULAR SUGGESTS (BLACK ICE). A SUPER COOLED WING ARRIVES ON THE GND. WARM AVGAS IS ADDED AND THE WINGS SWEATS EXTENSIVELY. YOU THEN MAKE AN EXPEDITIOUS CLIMB BACK TO A HIGH ALT AND THE POTENTIAL FOR WING ROOT ICING IS THERE WITH NO OTHER ICING VISIBLE. IF A FAST CLIMB IS ANTICIPATED, IN OTHER WORDS, NO INTERMEDIATE LEVEL OFF, THEN THE WINGS MUST BE PREHEATED WITH AIRFOIL ANTI-ICE IN THE CLIMB.

Data retrieved from NASA's ASRS site as of August 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.