Air carrier misinterpreted clearance and started descent to an altitude occupied by another aircraft...

Attributes
ACN	127276
Time
Date	198911
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : sea
State Reference	WA
Altitude	msl bound lower : 10000 msl bound upper : 11000
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	tower : ltt
Operator	common carrier : air carrier
Make Model Name	Widebody, Low Wing, 3 Turbojet Eng
Navigation In Use	Other
Flight Phase	descent other
Route In Use	arrival other enroute airway : sea
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Heavy Transport, Low Wing, 4 Turbojet Eng
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : handoff position
Qualification	controller : radar
Experience	controller radar : 20
ASRS Report	127276
Person 2
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : clearance non adherence : required legal separation
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 9000 vertical : 300
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Air carrier X assigned and acknowledged 11000', in level flight. Air carrier Y, opp direction level 10000'. I observed air carrier X descending through 10800' and told controller. Controller instructed air carrier X, 'maintain 11000'.' pilot said, 'roger, out of 11 for 10000.' controller: 'negative, climb and maintain 11000, traffic 12 O'clock...air carrier X, turn left--air carrier y--turn left!!!' pilot given hell over the air and replied, 'when I read back a wrong altitude, I expect feedback.' controller, trnee and handoff scared and mad, informed supervisor. Air carrier X called supervisor and whole event covered over because 'it wouldn't look good chewing out a pilot on the air' (good grief). No explanation from supervisor as to why pilot started descent.

Original NASA ASRS Text

Title: ACR MISINTERPRETED CLRNC AND STARTED DESCENT TO AN ALT OCCUPIED BY ANOTHER ACFT.

Narrative: ACR X ASSIGNED AND ACKNOWLEDGED 11000', IN LEVEL FLT. ACR Y, OPP DIRECTION LEVEL 10000'. I OBSERVED ACR X DSNDING THROUGH 10800' AND TOLD CTLR. CTLR INSTRUCTED ACR X, 'MAINTAIN 11000'.' PLT SAID, 'ROGER, OUT OF 11 FOR 10000.' CTLR: 'NEGATIVE, CLB AND MAINTAIN 11000, TFC 12 O'CLOCK...ACR X, TURN LEFT--ACR Y--TURN LEFT!!!' PLT GIVEN HELL OVER THE AIR AND REPLIED, 'WHEN I READ BACK A WRONG ALT, I EXPECT FEEDBACK.' CTLR, TRNEE AND HDOF SCARED AND MAD, INFORMED SUPVR. ACR X CALLED SUPVR AND WHOLE EVENT COVERED OVER BECAUSE 'IT WOULDN'T LOOK GOOD CHEWING OUT A PLT ON THE AIR' (GOOD GRIEF). NO EXPLANATION FROM SUPVR AS TO WHY PLT STARTED DSCNT.

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.