ACR X DSCNT TO OCCUPIED ALT (WRONG ALT SETTING ISSUED) HAD LESS LTSS FROM ACR Y. SYS ERROR.

Narrative

ACR X CLRED TO DSND TO 110. ALTIMETER INCORRECTLY GIVEN AS 30.49 (ACTUALLY 29.49). ACR Y CLRED TO CLB TO 110 AND ACR X CLRED TO AMEND ALT TO 120. ACR X ACKNOWLEDGED MODE C. ACR X SHOWED 116. CTLR ASKED ACR X TO VERIFY ASSIGNED ALT 120. ACR X RPTED 126 FOR 120. ACR Y LEVEL 110 PASSED 3.5 KM BEHIND ACR X AT 110. AT FIRST THOUGHT MODE C ERROR. BUT VOICE TAPE AT CTR CLRLY INDICATED CTLR GAVE ERRONEOUS ALTIMETER SETTING. SUPPLEMENTAL INFO FROM ACN 200427: WHILE DSNDING TO AN ASSIGNED ALT OF 15000 FT I CHKED IN ON A NEW FREQ WITH CTR. I WAS ISSUED AND READ BACK AN ALTIMETER SETTING OF 30.46. I WAS SHORTLY THEREAFTER INSTRUCTED TO DSND TO 11000 FT WHICH CTR AMENDED TO 12000 FT. AS WE WERE ABOUT TO LEVEL AT 12000 FT; CTR ASKED OUR ALT I TOLD THE CTLR WE WERE LEVELING AT 12000 FT AND HE SAID OUR ALT INDICATED 11200 FT. I THEN XMITTED OUR ALT INDICATED 12000 FT WITH AN ALTIMETER OF 30.46. WITH NO RESPONSE FROM CTR I OVERHEARD ANOTHER ACFT XMIT THAT SOMEONE READBACK A WRONG ALTIMETER. KNOWING THAT SOMETHING WAS SERIOUSLY WRONG; I ATTEMPTED TO CONTACT CTR FOR INSTRUCTIONS AS I BELIEVE THERE WAS AN ACFT AT 11000 FT ABOUT 1 MI BEHIND ME. I DISCOVERED THE CORRECT ALTIMETER SETTING TO BE 29.47 INSTEAD OF 30.46 WHICH WE SET IN AND INITIATED A CLB BACK TO 12000 FT. AT ABOUT 11500 CTR INSTRUCTED ME TO DSND TO 11000.