ATC controller reported allowing an aircraft to remain on the previous (combined) frequency; then lo...

Narrative:

I was working ontario arrivals sector. It was east traffic. Flows are different than normal west configuration. I was getting arrivals from center on frequency 127.0. I should have switched aircraft over to 125.5 as they transitioned my airspace. I had a few arrivals go in without the frequency switch so I felt that it was okay leaving the aircraft on 127.0. Aircraft X was given a turn to base heading 360 to maintain 3000 feet. The pilot read it back and made the turn. About 30 seconds later I gave aircraft X a turn to final and the approach clearance. Aircraft X does not respond. I tried multiple times but could not get through. I was able to use an aircraft to aircraft relay to get aircraft X on a turn away from terrain and to have them change to my frequency 125.5. When I was able to reach aircraft X I issued a low altitude alert and turned them south to resequence. I need to make sure to switch aircraft as they are transitioning from sector to sector even when I am working combined. I will work on having a better knowledge of gaps in the frequencies and where the problem spots are. I will also review east traffic procedures so that I am more familiar with them when we do go east.

Original NASA ASRS Text

Title: ATC Controller reported allowing an aircraft to remain on the previous (combined) frequency; then lost communications as the aircraft flew below the Minimum Vectoring Altitude.

Narrative: I was working Ontario arrivals sector. It was east traffic. Flows are different than normal west configuration. I was getting arrivals from Center on frequency 127.0. I should have switched aircraft over to 125.5 as they transitioned my airspace. I had a few arrivals go in without the frequency switch so I felt that it was okay leaving the aircraft on 127.0. Aircraft X was given a turn to base heading 360 to maintain 3000 feet. The Pilot read it back and made the turn. About 30 seconds later I gave Aircraft X a turn to final and the approach clearance. Aircraft X does not respond. I tried multiple times but could not get through. I was able to use an aircraft to aircraft relay to get Aircraft X on a turn away from terrain and to have them change to my frequency 125.5. When I was able to reach Aircraft X I issued a low altitude alert and turned them south to resequence. I need to make sure to switch aircraft as they are transitioning from sector to sector even when I am working combined. I will work on having a better knowledge of gaps in the frequencies and where the problem spots are. I will also review east traffic procedures so that I am more familiar with them when we do go east.

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.

Attributes
ACN	1501439
Time
Date	201712
Local Time Of Day	0001-0600
Place
Locale Reference	SCT.TRACON
State Reference	CA
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Make Model Name	B747 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Person 1
Function	Approach
Qualification	Air Traffic Control Developmental
Events
Anomaly	ATC Issue All Types Deviation - Procedural Published Material / Policy Inflight Event / Encounter CFTT / CFIT

ATC controller reported allowing an aircraft to remain on the previous (combined) frequency; then lost communications as the aircraft flew below the minimum vectoring altitude.