Air carrier first officer mistakenly hears 10000' altitude assignment instead of the 11000' that was actually issued. Captain, PF, descends to 10000' because he could not hear radio communication due to aircraft radio equipment problem.

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Attributes
ACN	136928
Time
Date	199002
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : eca
State Reference	CA
Altitude	msl bound lower : 10000 msl bound upper : 11000
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Controlling Facilities	tower : cos
Operator	common carrier : air carrier
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
Navigation In Use	other vortac
Flight Phase	descent other
Route In Use	enroute airway : v585
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 55 flight time total : 23600 flight time type : 55
ASRS Report	136928
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : cfi pilot : commercial pilot : instrument
Experience	flight time last 90 days : 40 flight time total : 1250 flight time type : 25
ASRS Report	136455
Events
Anomaly	altitude deviation : overshoot non adherence : clearance
Independent Detector	other controllera
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Ambiguous
Air Traffic Incident	Pilot Deviation

Narrative:

Communications problem in acf. Pilot unable to hear coplts xmissions. Able to hear intercom but weak. Received clearance to descend en route northwest on V-585 approaching manteca from 14000' by stockton approach. Copilot acknowledged call. I asked him to put new altitude on 'altitude alert' readout and he dialed in 10000'. I descended to 10000'. Stockton approach asked me to verify that my altitude was 11000'. Copilot stated that our clearance was to 10000' and that was the altitude at which we now were. Approach said 'negative - that we should be at 11000'' and we immediately climbed to 11000'. No known traffic conflict. Mechanic had been asked to lower sensitivity of PA to eliminate squeal during transmission. Had lowered sensitivity of all radio and communication circuits in aircraft including intercom. Copilot was new to aircraft (approximately 30 hours) and to this type of operation. G & I had only 55 hours in this aircraft also. This is a new startup operation and the shakedown period is rough.

Original NASA ASRS Text

Title: ACR FO MISTAKENLY HEARS 10000' ALT ASSIGNMENT INSTEAD OF THE 11000' THAT WAS ACTUALLY ISSUED. CAPT, PF, DESCENDS TO 10000' BECAUSE HE COULD NOT HEAR RADIO COM DUE TO ACFT RADIO EQUIPMENT PROBLEM.

Narrative: COMS PROB IN ACF. PLT UNABLE TO HEAR COPLTS XMISSIONS. ABLE TO HEAR INTERCOM BUT WEAK. RECEIVED CLRNC TO DSND ENRTE NW ON V-585 APCHING MANTECA FROM 14000' BY STOCKTON APCH. COPLT ACKNOWLEDGED CALL. I ASKED HIM TO PUT NEW ALT ON 'ALT ALERT' READOUT AND HE DIALED IN 10000'. I DSNDED TO 10000'. STOCKTON APCH ASKED ME TO VERIFY THAT MY ALT WAS 11000'. COPLT STATED THAT OUR CLRNC WAS TO 10000' AND THAT WAS THE ALT AT WHICH WE NOW WERE. APCH SAID 'NEGATIVE - THAT WE SHOULD BE AT 11000'' AND WE IMMEDIATELY CLBED TO 11000'. NO KNOWN TFC CONFLICT. MECH HAD BEEN ASKED TO LOWER SENSITIVITY OF PA TO ELIMINATE SQUEAL DURING XMISSION. HAD LOWERED SENSITIVITY OF ALL RADIO AND COM CIRCUITS IN ACFT INCLUDING INTERCOM. COPLT WAS NEW TO ACFT (APPROX 30 HRS) AND TO THIS TYPE OF OPERATION. G & I HAD ONLY 55 HRS IN THIS ACFT ALSO. THIS IS A NEW STARTUP OPERATION AND THE SHAKEDOWN PERIOD IS ROUGH.

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.