Narrative:

While in cruise flight on an IFR to VFR on top flight; I was in communication with la center (frequency 132.85) when I received a call advising us of an aircraft rapidly closing from behind. The callout also advised us that they were not talking to the overtaking aircraft and that we should change altitude if necessary. The callout voice was not that of the controller handling us at the time (male callout; female controller)in less than 10 seconds we located the aircraft; a t-tail twin turboprop on a path which would result in a collision. When first sighted that aircraft was at our 10 o'clock. We pitched up hard and to the left in an attempt to pass over and behind the other aircraft. That aircraft executed a hard left descending turn which appeared to be almost 180 degree course reversal for it.the callout voice then said that another agency was talking to the twin turboprop. We continued on our prior course. [Three days later] I contacted la quality assurance and asked a representative to look into the situation.the explanation received was that two controlling agencies operated in the same airspace between the altitudes of 13000 feet and 14000 feet MSL. The representative believes that socal was controlling the turbo prop while la center was handling my aircraft. They went on to explain that in this event it was likely that the turbo prop was climbing fast enough that its location was initially blocked by the radars operating system so that its location was not available on my controllers screen until such time as the callout was made to me.my concern; aside from the general dislike of the idea of falling in a flaming mass of debris onto one of the world's great cities; is that there is nothing myself or the controller handling my aircraft could do differently. Without the callout voice (an instructor at la center I'm told) it's very possible this could have resulted in collision. The pilot of the twin turbo prop as the overtaking converging pilot might have been able to see and avoid sooner but I suspect he also was counting on some notice from the agency controlling them.without changes in the overlap of airspace and perhaps to the operating systems on the controllers screens this could happen again. It seems that a procedural change may be required here.

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Original NASA ASRS Text

Title: Pilot reported a traffic call from an unknown ATC Instructor voice. Pilot observed traffic and executed an avoidance maneuver and observed the other aircraft doing the same. Reporter thought this to be an internal issue between SCT and ZLA and a procedural problem that needs to be address.

Narrative: While in cruise flight on an IFR to VFR on top flight; I was in communication with LA Center (Frequency 132.85) when I received a call advising us of an aircraft rapidly closing from behind. The callout also advised us that they were not talking to the overtaking aircraft and that we should change altitude if necessary. The callout voice was not that of the controller handling us at the time (male callout; female controller)In less than 10 seconds we located the aircraft; a T-tail twin turboprop on a path which would result in a collision. When first sighted that aircraft was at our 10 o'clock. We pitched up hard and to the left in an attempt to pass over and behind the other aircraft. That aircraft executed a hard left descending turn which appeared to be almost 180 degree course reversal for it.The callout voice then said that another agency was talking to the twin turboprop. We continued on our prior course. [Three days later] I contacted LA quality assurance and asked a representative to look into the situation.The explanation received was that two controlling agencies operated in the same airspace between the altitudes of 13000 feet and 14000 feet MSL. The representative believes that SoCal was controlling the turbo prop while LA center was handling my aircraft. They went on to explain that in this event it was likely that the turbo prop was climbing fast enough that its location was initially blocked by the radars operating system so that its location was not available on my controllers screen until such time as the callout was made to me.My concern; aside from the general dislike of the idea of falling in a flaming mass of debris onto one of the world's great cities; is that there is nothing myself or the controller handling my aircraft could do differently. Without the callout voice (an instructor at LA center I'm told) it's very possible this could have resulted in collision. The pilot of the twin turbo prop as the overtaking converging pilot might have been able to see and avoid sooner but I suspect he also was counting on some notice from the agency controlling them.Without changes in the overlap of airspace and perhaps to the operating systems on the controllers screens this could happen again. It seems that a procedural change may be required here.

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.