Narrative:

My flight was to depart off runway 31L with the idlewild climb (SID) which is a right turn to 090 degrees and climb to 2000 ft. The wind was out of the north at 10 KTS, I was the captain and it was my leg to fly. We were to depart behind a heavy B747 which was to turn left to a southwest heading after takeoff. With the wind out of the north I was not concerned about wake turbulence. Separation time as a right turn to the north and east would keep us out of their wake. A min after the heavy's takeoff we were cleared for takeoff with instructions to begin our right turn after 500 ft. Immediately after takeoff, tower told us to now turn left to a southwesterly heading. I told my first officer to tell tower we were unable because of wake separation. Tower again said to turn left to the southwest. I came on the radios and said 'we can't do that.' tower then asked if we could climb straight out and maintain 500 ft. The SID he wanted to put us on has a climb to 5000 ft. We were originally assigned a climb limit of 2000 ft. I continued to climb to avoid the wake off the B747 which was 2 mi ahead of us climbing above us. The 2000 ft came and went and the altitude alert went off. My first officer asked what altitude was I climbing to. I replied I was climbing to avoid the wake. At 4000 ft, tower said to climb to 5000 ft and contact departure. We did and the flight proceeded without further incident. Tower never said why the last and immediate change on departure nor why the disregard for wake turbulence separation behind the heavy. I don't know what caused the problem. Possibly asking for the 2 min wake separation would have precluded the tower's scramble to get us out, which I will always do from now on, regardless of departure SID.

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Original NASA ASRS Text

Title: MDT CLRED AT 1 MIN BEHIND A B747 ON TKOF. CLRNC CHANGED WHEN AIRBORNE TO TURN SW INSTEAD OF N AND E. WAKE TURB SEPARATION NOT ADEQUATE.

Narrative: MY FLT WAS TO DEPART OFF RWY 31L WITH THE IDLEWILD CLB (SID) WHICH IS A R TURN TO 090 DEGS AND CLB TO 2000 FT. THE WIND WAS OUT OF THE N AT 10 KTS, I WAS THE CAPT AND IT WAS MY LEG TO FLY. WE WERE TO DEPART BEHIND A HVY B747 WHICH WAS TO TURN L TO A SW HDG AFTER TKOF. WITH THE WIND OUT OF THE N I WAS NOT CONCERNED ABOUT WAKE TURB. SEPARATION TIME AS A R TURN TO THE N AND E WOULD KEEP US OUT OF THEIR WAKE. A MIN AFTER THE HVY'S TKOF WE WERE CLRED FOR TKOF WITH INSTRUCTIONS TO BEGIN OUR R TURN AFTER 500 FT. IMMEDIATELY AFTER TKOF, TWR TOLD US TO NOW TURN L TO A SOUTHWESTERLY HDG. I TOLD MY FO TO TELL TWR WE WERE UNABLE BECAUSE OF WAKE SEPARATION. TWR AGAIN SAID TO TURN L TO THE SW. I CAME ON THE RADIOS AND SAID 'WE CAN'T DO THAT.' TWR THEN ASKED IF WE COULD CLB STRAIGHT OUT AND MAINTAIN 500 FT. THE SID HE WANTED TO PUT US ON HAS A CLB TO 5000 FT. WE WERE ORIGINALLY ASSIGNED A CLB LIMIT OF 2000 FT. I CONTINUED TO CLB TO AVOID THE WAKE OFF THE B747 WHICH WAS 2 MI AHEAD OF US CLBING ABOVE US. THE 2000 FT CAME AND WENT AND THE ALT ALERT WENT OFF. MY FO ASKED WHAT ALT WAS I CLBING TO. I REPLIED I WAS CLBING TO AVOID THE WAKE. AT 4000 FT, TWR SAID TO CLB TO 5000 FT AND CONTACT DEP. WE DID AND THE FLT PROCEEDED WITHOUT FURTHER INCIDENT. TWR NEVER SAID WHY THE LAST AND IMMEDIATE CHANGE ON DEP NOR WHY THE DISREGARD FOR WAKE TURB SEPARATION BEHIND THE HVY. I DON'T KNOW WHAT CAUSED THE PROB. POSSIBLY ASKING FOR THE 2 MIN WAKE SEPARATION WOULD HAVE PRECLUDED THE TWR'S SCRAMBLE TO GET US OUT, WHICH I WILL ALWAYS DO FROM NOW ON, REGARDLESS OF DEP SID.

Data retrieved from NASA's ASRS site as of July 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.