IFR small aircraft descended below assigned altitude and conflicted with an air carrier on approach.

Attributes
ACN	153281
Time
Date	199008
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : sbp
State Reference	CA
Altitude	msl bound lower : 3800 msl bound upper : 4600
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zla
Operator	general aviation : personal
Make Model Name	Small Aircraft, High Wing, 1 Eng, Fixed Gear
Flight Phase	descent other other
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
Flight Phase	descent : approach descent other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Experience	controller radar : 8
ASRS Report	153281
Person 2
Affiliation	Other
Function	instruction : instructor
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 1710 flight time type : 100
ASRS Report	153020
Events
Anomaly	altitude deviation : overshoot non adherence : clearance
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance
Consequence	Other
Miss Distance	horizontal : 12000 vertical : 400
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Small aircraft X had been cleared to hold at crepe intersection at 5000'. Air carrier Y had been cleared for a localizer runway 11 approach, radar service terminated and frequency changed to tower. Small aircraft X entered the holding pattern. I noticed small aircraft X indicating 4600'. I told the pilot to maintain 5000' and to say his altitude. Air carrier Y mode C indicated 3800'. Supplemental information from acn 153020: my student and I were on an INS training from montgomery field, san diego, to san luis obispo, ca. When we were near san marcos VOR on V386, ZLA gave us a clearance (cleared to crepe, direct morro bay R27 frams intersection, crepe direct). When we were approaching frams intersection ZLA told us to descend to 9000'. A couple of mins later we were at frams intersection turning to 048 degree heading to intercept san luis obispo's localizer for runway 11 and to go direct to crepe intersection. A min later ZLA told us to descend to and maintain 5000'. I was watching for traffic since we were in VMC conditions. The next time I looked up at the altimeter he was at 4700'. I told the student to get back to 5000' at the same time ZLA told us to maintain 5000'. My student looked at the approach plate and thought he could descend to the minimum 4000'. The main fact is we went 300' below our assigned altitude.

Original NASA ASRS Text

Title: IFR SMA DESCENDED BELOW ASSIGNED ALT AND CONFLICTED WITH AN ACR ON APCH.

Narrative: SMA X HAD BEEN CLRED TO HOLD AT CREPE INTXN AT 5000'. ACR Y HAD BEEN CLRED FOR A LOC RWY 11 APCH, RADAR SVC TERMINATED AND FREQ CHANGED TO TWR. SMA X ENTERED THE HOLDING PATTERN. I NOTICED SMA X INDICATING 4600'. I TOLD THE PLT TO MAINTAIN 5000' AND TO SAY HIS ALT. ACR Y MODE C INDICATED 3800'. SUPPLEMENTAL INFO FROM ACN 153020: MY STUDENT AND I WERE ON AN INS TRNING FROM MONTGOMERY FIELD, SAN DIEGO, TO SAN LUIS OBISPO, CA. WHEN WE WERE NEAR SAN MARCOS VOR ON V386, ZLA GAVE US A CLRNC (CLRED TO CREPE, DIRECT MORRO BAY R27 FRAMS INTXN, CREPE DIRECT). WHEN WE WERE APCHING FRAMS INTXN ZLA TOLD US TO DSND TO 9000'. A COUPLE OF MINS LATER WE WERE AT FRAMS INTXN TURNING TO 048 DEG HDG TO INTERCEPT SAN LUIS OBISPO'S LOC FOR RWY 11 AND TO GO DIRECT TO CREPE INTXN. A MIN LATER ZLA TOLD US TO DSND TO AND MAINTAIN 5000'. I WAS WATCHING FOR TFC SINCE WE WERE IN VMC CONDITIONS. THE NEXT TIME I LOOKED UP AT THE ALTIMETER HE WAS AT 4700'. I TOLD THE STUDENT TO GET BACK TO 5000' AT THE SAME TIME ZLA TOLD US TO MAINTAIN 5000'. MY STUDENT LOOKED AT THE APCH PLATE AND THOUGHT HE COULD DSND TO THE MINIMUM 4000'. THE MAIN FACT IS WE WENT 300' BELOW OUR ASSIGNED ALT.

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.