Narrative:

Prc has a set of parallel runways. Full stop traffic from the northwest is brought over the top at 500 ft above the pattern to enter left downwind; descend; then land runway 21L. I was bringing an aircraft inbound with this technique due to several touch-and-go aircraft on runway 21R. The aircraft was restr to maintain at or above 6500 ft and just overflying right downwind traffic in the pattern that showed 6500 ft on his mode C. I had the other local controller verify altitude of the downwind aircraft and we discovered that they had the wrong altimeter setting which made their altitude 6500 ft instead of 6000 ft. Luckily; the aircraft inbound kept his altitude higher than normal and passed over the other aircraft at 6900 ft instead of 6500 ft.

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Original NASA ASRS Text

Title: PRC LCL CTLR DESCRIBED POTENTIAL CONFLICT BTWN 2 PATTERN ACFT BECAUSE 1 ACFT HAD WRONG ALTIMETER SETTING.

Narrative: PRC HAS A SET OF PARALLEL RWYS. FULL STOP TFC FROM THE NW IS BROUGHT OVER THE TOP AT 500 FT ABOVE THE PATTERN TO ENTER L DOWNWIND; DSND; THEN LAND RWY 21L. I WAS BRINGING AN ACFT INBOUND WITH THIS TECHNIQUE DUE TO SEVERAL TOUCH-AND-GO ACFT ON RWY 21R. THE ACFT WAS RESTR TO MAINTAIN AT OR ABOVE 6500 FT AND JUST OVERFLYING R DOWNWIND TFC IN THE PATTERN THAT SHOWED 6500 FT ON HIS MODE C. I HAD THE OTHER LCL CTLR VERIFY ALT OF THE DOWNWIND ACFT AND WE DISCOVERED THAT THEY HAD THE WRONG ALTIMETER SETTING WHICH MADE THEIR ALT 6500 FT INSTEAD OF 6000 FT. LUCKILY; THE ACFT INBOUND KEPT HIS ALT HIGHER THAN NORMAL AND PASSED OVER THE OTHER ACFT AT 6900 FT INSTEAD OF 6500 FT.

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.