Narrative:

I issued a crossing restr to aircraft X, who was inbound to boston, ma, to cross scupp at 11000 ft at time XA09Z, with the altimeter setting of 29.19. Aircraft X was level at FL340 when I issued this restr. He had opposite direction traffic at FL350 who was also equipped with rvsm equipment. This aircraft, aircraft Y, a F900/Q reported an aircraft in close proximity of 300 ft so I turned him 30 degrees right, away from traffic. His traffic was aircraft X, whose altitude now indicated he was at FL347. I asked aircraft X to verify leaving FL340. He reported at FL340 and said he had a pilot's discretion descent. I believe what happened is the pilot of aircraft X set his altimeter to 29.19 when I first issued the altimeter. This would account for the 700 ft he climbed from FL340 to FL347. Aircraft X type aircraft was a B747-400/west.

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Original NASA ASRS Text

Title: B747 AT FL350 CHANGES TO BOS ALT FOR DSCNT, CLBS 700 FT, AND CONFLICTED WITH ENRTE F900.

Narrative: I ISSUED A XING RESTR TO ACFT X, WHO WAS INBOUND TO BOSTON, MA, TO CROSS SCUPP AT 11000 FT AT TIME XA09Z, WITH THE ALTIMETER SETTING OF 29.19. ACFT X WAS LEVEL AT FL340 WHEN I ISSUED THIS RESTR. HE HAD OPPOSITE DIRECTION TFC AT FL350 WHO WAS ALSO EQUIPPED WITH RVSM EQUIP. THIS ACFT, ACFT Y, A F900/Q RPTED AN ACFT IN CLOSE PROX OF 300 FT SO I TURNED HIM 30 DEGS R, AWAY FROM TFC. HIS TFC WAS ACFT X, WHOSE ALT NOW INDICATED HE WAS AT FL347. I ASKED ACFT X TO VERIFY LEAVING FL340. HE RPTED AT FL340 AND SAID HE HAD A PLT'S DISCRETION DSCNT. I BELIEVE WHAT HAPPENED IS THE PLT OF ACFT X SET HIS ALTIMETER TO 29.19 WHEN I FIRST ISSUED THE ALTIMETER. THIS WOULD ACCOUNT FOR THE 700 FT HE CLBED FROM FL340 TO FL347. ACFT X TYPE ACFT WAS A B747-400/W.

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.