Narrative:

During cruise flight at FL350; cavu over an undercast; speed mach .81 assigned by ATC approximately 10 miles in trail of B747 at FL360. Zero IRS wind indicated. Encountered wake turbulence from B747 and aircraft initially rolled gently left; then it rolled rapidly right to approximately 30 degrees of bank. Received 'bank angle' aural warning; low speed 'foot' moved up to actual speed (FMC computed speed); stick shaker activated. Disconnected autoplt; recovered to level flight and resumed flight at cleared altitude FL350 after losing approximately 1200 ft during the upset and recovery. Advised ATC and requested offset to avoid further encounters. We had been in trail of the B747 for approximately 20 to 30 mins but there had been a light wind from the north which was moving the wake off the route. As we noted the absence of wind at our cruise altitude the upset occurred.supplemental information from acn 774741: aircraft Y was at FL320. Aircraft X was at FL350 and trailing aircraft Y by 4.5 NM. Aircraft Y requested climb to FL360. Controller issued speed restr to fly mach .81 to aircraft X in order to achieve 5 NM longitudinal separation which would then allow him to accommodate the climb request. The controller then issued climb to FL360 to aircraft Y. As aircraft Y climbed through FL350 he was separated 5.5 NM longitudinally from aircraft X. However; aircraft X reported that he had 'stalled due to wake turbulence.' aircraft X descended 1100 ft to FL339 before regaining control of the aircraft and climbing back to FL350. Although the controller clearance was 'legal;' it clearly placed aircraft X in a precarious position with regard to in-flight stability. Moreover; the pilot did not recognize this potential situation prior to accepting the speed restr. In this instance; no other aircraft were in close proximity so no loss of separation occurred. Furthermore; since this was a cargo flight; no passenger or flight attendants were injured. To my knowledge; no crew members were injured. My recommendation regarding possible solutions are two-fold. It appears that more study should be done regarding longitudinal separation standards to determine if existing standards (5 NM) are sufficient when heavy jets are involved. Secondly; pilots should be briefed regarding this incident; with emphasis on accepting speed restrs which may place them in an unsafe position within their flight profile.

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Original NASA ASRS Text

Title: MD11 CREW RPTS AN ARTCC RADAR CTLR ALLOWED A B747 TO CLB THROUGH THEIR FLT PATH 5 MILES AHEAD CAUSING A STALL.

Narrative: DURING CRUISE FLT AT FL350; CAVU OVER AN UNDERCAST; SPEED MACH .81 ASSIGNED BY ATC APPROX 10 MILES IN TRAIL OF B747 AT FL360. ZERO IRS WIND INDICATED. ENCOUNTERED WAKE TURBULENCE FROM B747 AND ACFT INITIALLY ROLLED GENTLY L; THEN IT ROLLED RAPIDLY R TO APPROX 30 DEGS OF BANK. RECEIVED 'BANK ANGLE' AURAL WARNING; LOW SPEED 'FOOT' MOVED UP TO ACTUAL SPEED (FMC COMPUTED SPEED); STICK SHAKER ACTIVATED. DISCONNECTED AUTOPLT; RECOVERED TO LEVEL FLT AND RESUMED FLT AT CLRED ALTITUDE FL350 AFTER LOSING APPROX 1200 FT DURING THE UPSET AND RECOVERY. ADVISED ATC AND REQUESTED OFFSET TO AVOID FURTHER ENCOUNTERS. WE HAD BEEN IN TRAIL OF THE B747 FOR APPROX 20 TO 30 MINS BUT THERE HAD BEEN A LIGHT WIND FROM THE N WHICH WAS MOVING THE WAKE OFF THE RTE. AS WE NOTED THE ABSENCE OF WIND AT OUR CRUISE ALTITUDE THE UPSET OCCURRED.SUPPLEMENTAL INFO FROM ACN 774741: ACFT Y WAS AT FL320. ACFT X WAS AT FL350 AND TRAILING ACFT Y BY 4.5 NM. ACFT Y REQUESTED CLIMB TO FL360. CTLR ISSUED SPEED RESTR TO FLY MACH .81 TO ACFT X IN ORDER TO ACHIEVE 5 NM LONGITUDINAL SEPARATION WHICH WOULD THEN ALLOW HIM TO ACCOMMODATE THE CLIMB REQUEST. THE CTLR THEN ISSUED CLIMB TO FL360 TO ACFT Y. AS ACFT Y CLIMBED THROUGH FL350 HE WAS SEPARATED 5.5 NM LONGITUDINALLY FROM ACFT X. HOWEVER; ACFT X RPTED THAT HE HAD 'STALLED DUE TO WAKE TURBULENCE.' ACFT X DESCENDED 1100 FT TO FL339 BEFORE REGAINING CONTROL OF THE ACFT AND CLIMBING BACK TO FL350. ALTHOUGH THE CTLR CLRNC WAS 'LEGAL;' IT CLEARLY PLACED ACFT X IN A PRECARIOUS POSITION WITH REGARD TO IN-FLIGHT STABILITY. MOREOVER; THE PLT DID NOT RECOGNIZE THIS POTENTIAL SITUATION PRIOR TO ACCEPTING THE SPEED RESTR. IN THIS INSTANCE; NO OTHER ACFT WERE IN CLOSE PROXIMITY SO NO LOSS OF SEPARATION OCCURRED. FURTHERMORE; SINCE THIS WAS A CARGO FLT; NO PAX OR FLT ATTENDANTS WERE INJURED. TO MY KNOWLEDGE; NO CREW MEMBERS WERE INJURED. MY RECOMMENDATION REGARDING POSSIBLE SOLUTIONS ARE TWO-FOLD. IT APPEARS THAT MORE STUDY SHOULD BE DONE REGARDING LONGITUDINAL SEPARATION STANDARDS TO DETERMINE IF EXISTING STANDARDS (5 NM) ARE SUFFICIENT WHEN HEAVY JETS ARE INVOLVED. SECONDLY; PLTS SHOULD BE BRIEFED REGARDING THIS INCIDENT; WITH EMPHASIS ON ACCEPTING SPEED RESTRS WHICH MAY PLACE THEM IN AN UNSAFE POSITION WITHIN THEIR FLT PROFILE.

Data retrieved from NASA's ASRS site as of January 2009 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.