Narrative:

On a flight from kansas city, mo (mci) to ypsilanti, mi (yip), our aircraft was cleared in descent by ZAU radar to cross 40 mi west of ft wayne VOR at and/or maintain FL290. We complied with this altitude clearance. Communications traffic on the ZAU frequency was heavy. The ATC controller and flight crews were using short rapid fire commands and acknowledgements. We were subsequently told to maintain 'two five 0'. I interpreted this command as a clearance to leave FL290 and descend to FL250. FL250 was entered in the altitude alerter. I acknowledged the command with 'maintain two five 0/call sign'. While at FL275 we were told to return to FL290 by center, and questioned on previously given altitude clearance instructions. ATC had issued instructions to maintain 250 KTS. I offer two suggestions which may preclude this situation from occurring again. 1) all cockpit crew members should use headset equipment during all phases of flight. A careful listening watch, especially during periods of high traffic levels, will insure proper interpretation of ATC instructions by flight crew members. 2) during periods of high traffic levels, ATC controllers and pilots should strive to use clear, concise, and standard FAA communications phraseology in the issuance of altitude clearance instructions and airspeed restrictions. Correct interpretation of instructions given to pilots should not automatically be assumed by controllers.

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Original NASA ASRS Text

Title: ACR MISINTERPRETED AIRSPEED RESTRICTION FOR AN ALT RESTRICTION.

Narrative: ON A FLT FROM KANSAS CITY, MO (MCI) TO YPSILANTI, MI (YIP), OUR ACFT WAS CLRED IN DSCNT BY ZAU RADAR TO CROSS 40 MI W OF FT WAYNE VOR AT AND/OR MAINTAIN FL290. WE COMPLIED WITH THIS ALT CLRNC. COMS TFC ON THE ZAU FREQ WAS HEAVY. THE ATC CTLR AND FLT CREWS WERE USING SHORT RAPID FIRE COMMANDS AND ACKNOWLEDGEMENTS. WE WERE SUBSEQUENTLY TOLD TO MAINTAIN 'TWO FIVE 0'. I INTERPRETED THIS COMMAND AS A CLRNC TO LEAVE FL290 AND DSND TO FL250. FL250 WAS ENTERED IN THE ALT ALERTER. I ACKNOWLEDGED THE COMMAND WITH 'MAINTAIN TWO FIVE 0/CALL SIGN'. WHILE AT FL275 WE WERE TOLD TO RETURN TO FL290 BY CENTER, AND QUESTIONED ON PREVIOUSLY GIVEN ALT CLRNC INSTRUCTIONS. ATC HAD ISSUED INSTRUCTIONS TO MAINTAIN 250 KTS. I OFFER TWO SUGGESTIONS WHICH MAY PRECLUDE THIS SITUATION FROM OCCURRING AGAIN. 1) ALL COCKPIT CREW MEMBERS SHOULD USE HEADSET EQUIPMENT DURING ALL PHASES OF FLT. A CAREFUL LISTENING WATCH, ESPECIALLY DURING PERIODS OF HIGH TFC LEVELS, WILL INSURE PROPER INTERPRETATION OF ATC INSTRUCTIONS BY FLT CREW MEMBERS. 2) DURING PERIODS OF HIGH TFC LEVELS, ATC CTLRS AND PLTS SHOULD STRIVE TO USE CLEAR, CONCISE, AND STANDARD FAA COMS PHRASEOLOGY IN THE ISSUANCE OF ALT CLRNC INSTRUCTIONS AND AIRSPEED RESTRICTIONS. CORRECT INTERPRETATION OF INSTRUCTIONS GIVEN TO PLTS SHOULD NOT AUTOMATICALLY BE ASSUMED BY CTLRS.

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