Sct TRACON controller and a ce-560XL flight crew reported a communication breakdown resulting in the...

Attributes
ACN	1518655
Time
Date	201802
Local Time Of Day	1201-1800
Place
Locale Reference	SCT.TRACON
State Reference	CA
Environment
Flight Conditions	Marginal
Light	Daylight
Aircraft 1
Make Model Name	Citation Excel (C560XL)
Operating Under FAR Part	Part 91
Flight Phase	Descent
Route In Use	Vectors
Flight Plan	IFR
Person 1
Function	Approach
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 2
Person 2
Function	Captain Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	ATC Issue All Types Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Published Material / Policy Deviation - Procedural Clearance Inflight Event / Encounter CFTT / CFIT

Narrative:

Aircraft X checked in going to crq. I instructed him to proceed direct to the escon IAF and descend and maintain 9;000 ft. As he was reading back the instructions; another aircraft checked in on another frequency. I thought I heard aircraft X say 9;000 ft so I moved on and gave the other aircraft control instructions. As I scanned back to aircraft X I noticed the altitude was 7;500 ft descending and he was in an 8;700 ft MVA. I instructed aircraft X to check his altitude and gave him the current altimeter; then instructed him to maintain 9;000 ft. He responded asking me to verify I want him to climb back up to 9;000 ft. At this point he was in the 6;000 ft MVA so I instructed him to just descend and maintain 6;000 ft but that his previous instruction was 9;000 ft.even if I think I heard the right read back; [I must still] verify it if there are any distractions.

Original NASA ASRS Text

Title: SCT TRACON Controller and a CE-560XL flight crew reported a communication breakdown resulting in the aircraft descending below the MVA.

Narrative: Aircraft X checked in going to CRQ. I instructed him to proceed direct to the ESCON IAF and descend and maintain 9;000 ft. As he was reading back the instructions; another aircraft checked in on another frequency. I thought I heard Aircraft X say 9;000 ft so I moved on and gave the other aircraft control instructions. As I scanned back to Aircraft X I noticed the altitude was 7;500 ft descending and he was in an 8;700 ft MVA. I instructed Aircraft X to check his altitude and gave him the current altimeter; then instructed him to maintain 9;000 ft. He responded asking me to verify I want him to climb back up to 9;000 ft. At this point he was in the 6;000 ft MVA so I instructed him to just descend and maintain 6;000 ft but that his previous instruction was 9;000 ft.Even if I think I heard the right read back; [I must still] verify it if there are any distractions.

Data retrieved from NASA's ASRS site 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.

Sct TRACON controller and a ce-560XL flight crew reported a communication breakdown resulting in the aircraft descending below the MVA.