Small transport descended below assigned IFR altitude while approaching airport.

Attributes
ACN	107477
Time
Date	198903
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : fat
State Reference	CA
Altitude	msl bound lower : 2000 msl bound upper : 2000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : fat
Operator	common carrier : air carrier
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
Flight Phase	descent other
Route In Use	arrival other
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Experience	flight time last 90 days : 225 flight time total : 2800 flight time type : 1200
ASRS Report	107477
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 6500 flight time type : 3300
ASRS Report	107419
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 other
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 12000 vertical : 300
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

North of clovis VORTAC, we contacted fat approach control. We were instructed to turn right heading 140 degrees to intercept localizer backcourse to runway 11L and maintain 3000'. Subsequently, we were cleared for the localizer backcourse runway 11L approach. The captain read back clearance, at which time I requested progressive instructions. I descended to 2000' once established on localizer backcourse inbound in accordance to profile depiction on approach chart. After approximately 2 to 3 mins at 2000', approach instructed us to make an immediate right turn to a heading of 180 degrees. We were told that we had not been cleared for the approach and that we were to cross morla at 3000'. We were then given a new heading to intercept the localizer backcourse and cleared for the approach. After landing we were instructed to call TRACON. The captain phoned TRACON, informed of the other aircraft, and told that a system error had occurred. If we had not been cleared for the approach ATC should have corrected the captain when he read back, 'cleared for localizer backcourse 11 approach.' if we were cleared for the approach subsequent to altitude restriction, altitude restriction should have ben reiterated to avoid any confusion.

Original NASA ASRS Text

Title: SMT DESCENDED BELOW ASSIGNED IFR ALT WHILE APCHING ARPT.

Narrative: N OF CLOVIS VORTAC, WE CONTACTED FAT APCH CTL. WE WERE INSTRUCTED TO TURN RIGHT HDG 140 DEGS TO INTERCEPT LOC BACKCOURSE TO RWY 11L AND MAINTAIN 3000'. SUBSEQUENTLY, WE WERE CLRED FOR THE LOC BACKCOURSE RWY 11L APCH. THE CAPT READ BACK CLRNC, AT WHICH TIME I REQUESTED PROGRESSIVE INSTRUCTIONS. I DSNDED TO 2000' ONCE ESTABLISHED ON LOC BACKCOURSE INBND IN ACCORDANCE TO PROFILE DEPICTION ON APCH CHART. AFTER APPROX 2 TO 3 MINS AT 2000', APCH INSTRUCTED US TO MAKE AN IMMEDIATE RIGHT TURN TO A HDG OF 180 DEGS. WE WERE TOLD THAT WE HAD NOT BEEN CLRED FOR THE APCH AND THAT WE WERE TO CROSS MORLA AT 3000'. WE WERE THEN GIVEN A NEW HDG TO INTERCEPT THE LOC BACKCOURSE AND CLRED FOR THE APCH. AFTER LNDG WE WERE INSTRUCTED TO CALL TRACON. THE CAPT PHONED TRACON, INFORMED OF THE OTHER ACFT, AND TOLD THAT A SYS ERROR HAD OCCURRED. IF WE HAD NOT BEEN CLRED FOR THE APCH ATC SHOULD HAVE CORRECTED THE CAPT WHEN HE READ BACK, 'CLRED FOR LOC BACKCOURSE 11 APCH.' IF WE WERE CLRED FOR THE APCH SUBSEQUENT TO ALT RESTRICTION, ALT RESTRICTION SHOULD HAVE BEN REITERATED TO AVOID ANY CONFUSION.

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.