Military pilot reported a near midair collision because they missed an ATC restriction.

Attributes
ACN	1567527
Time
Date	201808
Local Time Of Day	1201-1800
Place
Locale Reference	SAN.Airport
State Reference	CA
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Mentor/Turbo Mentor (T-34)
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	VFR
Aircraft 2
Make Model Name	Helicopter
Person 1
Function	Pilot Flying Single Pilot
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Flight Instructor
Experience	Flight Crew Last 90 Days 90 Flight Crew Total 1560 Flight Crew Type 238
Events
Anomaly	Conflict NMAC Deviation - Altitude Crossing Restriction Not Met Deviation - Altitude Undershoot Deviation - Procedural Clearance
Miss Distance	Horizontal 500 Vertical 200

Narrative:

Aircraft X was executing a VFR transition from south to north with approval from lindbergh tower. There were two other aircraft doing the same thing; another [military trainer aircraft] with [same squadron] callsign and a helicopter. This was in addition to the regular tower communication for their normal operations (takeoff and landing). Tower cleared aircraft X into the bravo airspace; but I think he gave me a restriction. I rogered up the clearance but missed the restriction. I was about to come up and ask but at that time had a near- midair with a helicopter coming opposite direction. After that; I heard the tower talking to someone on VHF about my missed restriction of 1;000' AGL. I'm very sorry about this. I will brief my [team] during our [review] so that everyone is aware of this possibility of happening and the importance of using good CRM with everyone on freq.

Original NASA ASRS Text

Title: Military Pilot reported a NMAC because they missed an ATC restriction.

Narrative: Aircraft X was executing a VFR transition from south to north with approval from Lindbergh Tower. There were two other aircraft doing the same thing; another [military trainer aircraft] with [same squadron] callsign and a helicopter. This was in addition to the regular tower communication for their normal operations (takeoff and landing). Tower cleared Aircraft X into the Bravo airspace; but I think he gave me a restriction. I rogered up the clearance but missed the restriction. I was about to come up and ask but at that time had a near- midair with a helicopter coming opposite direction. After that; I heard the tower talking to someone on VHF about my missed restriction of 1;000' AGL. I'm very sorry about this. I will brief my [team] during our [review] so that everyone is aware of this possibility of happening and the importance of using good CRM with everyone on freq.

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.