Attempted to advise controller aircraft had copied cleared altitude wrong.

Attributes
ACN	184688
Time
Date	199107
Day	Sun
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : smo airport : lax
State Reference	CA
Altitude	msl bound lower : 7000 msl bound upper : 7000
Environment
Flight Conditions	Mixed
Light	Night
Aircraft 1
Controlling Facilities	tracon : lax
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent other
Route In Use	arrival : profile descent
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent other
Route In Use	arrival : profile descent
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 250 flight time total : 10000
ASRS Report	184688
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	other anomaly other
Independent Detector	other flight crewa
Resolutory Action	none taken : unable other
Consequence	Other
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	other

Narrative:

During very busy period of IFR arrs approach control issued a clearance to an aircraft (unknown call sign) to 'descend to 2500 ft'. The other airplane read back 'cleared to 1500 ft'. Approach controller missed the error as he was overloaded. I attempted to tell approach controller but transmission was blocked by other xmissions. I was not sure who the wrong clearance was for and wrongfully assumed that mode C readouts would keep everyone ok. I knew there was ceiling of around 1000 ft, and rationalized that a tragedy would not occur. However, the controller did not notice the altitude error until 1500 ft, when the offending air carrier was told to go back to 2500 ft. I learned 2 things. I should have insisted on relaying the error to approach, even if it was me who misunderstood. Also the air carrier didn't read back correctly, terminology wise. ('1500' approach would've probably noticed.)

Original NASA ASRS Text

Title: ATTEMPTED TO ADVISE CTLR ACFT HAD COPIED CLRED ALT WRONG.

Narrative: DURING VERY BUSY PERIOD OF IFR ARRS APCH CTL ISSUED A CLRNC TO AN ACFT (UNKNOWN CALL SIGN) TO 'DSND TO 2500 FT'. THE OTHER AIRPLANE READ BACK 'CLRED TO 1500 FT'. APCH CTLR MISSED THE ERROR AS HE WAS OVERLOADED. I ATTEMPTED TO TELL APCH CTLR BUT XMISSION WAS BLOCKED BY OTHER XMISSIONS. I WAS NOT SURE WHO THE WRONG CLRNC WAS FOR AND WRONGFULLY ASSUMED THAT MODE C READOUTS WOULD KEEP EVERYONE OK. I KNEW THERE WAS CEILING OF AROUND 1000 FT, AND RATIONALIZED THAT A TRAGEDY WOULD NOT OCCUR. HOWEVER, THE CTLR DID NOT NOTICE THE ALT ERROR UNTIL 1500 FT, WHEN THE OFFENDING ACR WAS TOLD TO GO BACK TO 2500 FT. I LEARNED 2 THINGS. I SHOULD HAVE INSISTED ON RELAYING THE ERROR TO APCH, EVEN IF IT WAS ME WHO MISUNDERSTOOD. ALSO THE ACR DIDN'T READ BACK CORRECTLY, TERMINOLOGY WISE. ('1500' APCH WOULD'VE PROBABLY NOTICED.)

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.