Narrative:

Small aircraft X departed lbe airport, swbound, assigned runway heading and climbing to 4000 ft. Small transport Y was nwbound, level at 5000 ft, deviating around WX. Shortly after X was radar idented, he requested direct mgw. The controller issued direct mgw and stated 'maintain 5000 ft.' this altitude error was missed when the pilot read back 'direct mgw, 5000 ft.' the data block associated with X displayed an interim altitude of 4000 ft. For this reason, the conflict alert was not activated until separation was lost. Just before this time, X began to question his clearance to 5000 ft. The controller took immediate action by reassigning 4000 ft. However, separation was lost. Recommendation: error was the result of the controller transposing assigned altitudes. Aircraft was told to 'maintain 5000,' not 'climb and maintain 5000 ft.' aircraft read back '5000 ft,' then commenced to climb and maintain 5000 ft. Controller erred, thinking he assigned the aircraft to maintain 4000 ft. The information in the data block and posted on the strip reinforced this belief. Solution: say what you intend to say. Be sure to 'hear' what is said when listening to readbacks.

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Original NASA ASRS Text

Title: SMA X CLB TO OCCUPIED ALT HAD LTSS FROM SMT Y. SYS ERROR.

Narrative: SMA X DEPARTED LBE ARPT, SWBOUND, ASSIGNED RWY HDG AND CLBING TO 4000 FT. SMT Y WAS NWBOUND, LEVEL AT 5000 FT, DEVIATING AROUND WX. SHORTLY AFTER X WAS RADAR IDENTED, HE REQUESTED DIRECT MGW. THE CTLR ISSUED DIRECT MGW AND STATED 'MAINTAIN 5000 FT.' THIS ALT ERROR WAS MISSED WHEN THE PLT READ BACK 'DIRECT MGW, 5000 FT.' THE DATA BLOCK ASSOCIATED WITH X DISPLAYED AN INTERIM ALT OF 4000 FT. FOR THIS REASON, THE CONFLICT ALERT WAS NOT ACTIVATED UNTIL SEPARATION WAS LOST. JUST BEFORE THIS TIME, X BEGAN TO QUESTION HIS CLRNC TO 5000 FT. THE CTLR TOOK IMMEDIATE ACTION BY REASSIGNING 4000 FT. HOWEVER, SEPARATION WAS LOST. RECOMMENDATION: ERROR WAS THE RESULT OF THE CTLR TRANSPOSING ASSIGNED ALTS. ACFT WAS TOLD TO 'MAINTAIN 5000,' NOT 'CLB AND MAINTAIN 5000 FT.' ACFT READ BACK '5000 FT,' THEN COMMENCED TO CLB AND MAINTAIN 5000 FT. CTLR ERRED, THINKING HE ASSIGNED THE ACFT TO MAINTAIN 4000 FT. THE INFO IN THE DATA BLOCK AND POSTED ON THE STRIP REINFORCED THIS BELIEF. SOLUTION: SAY WHAT YOU INTEND TO SAY. BE SURE TO 'HEAR' WHAT IS SAID WHEN LISTENING TO READBACKS.

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.