Tower controller described a conflict between a VFR pipeline patrol aircraft and another VFR transit...

Narrative:

A pipeline cessna was flying south over the airport VFR at 2;500. As the aircraft approached the airport; the C172 departed a satellite airport 6 NM northwest; and asked for flight following to the west. I gave the C172 a VFR code. When the aircraft tagged up his altitude was 2;100 and climbing directly at the pipeline aircraft. I told the C172 radar contact; issued traffic; and suggested the aircraft descend back to 2;000. However; the C172 continued climbing; and the pipeline aircraft descended to 2;300 without warning. I noticed both aircraft at 2;300 and the same position. I then issued a traffic alert to the C172; then to the pipeline aircraft. The aircraft eventually saw each other; and the pipeline aircraft said they were about 100 ft apart. I reviewed the voice and radar data with my supervisor. The data showed 0 ft vertical; 0.15 horizontal; and 13 seconds from when the aircraft tagged up and the issuance of the safety alert. We discussed traffic alert phraseology.

Original NASA ASRS Text

Title: Tower Controller described a conflict between a VFR pipeline patrol aircraft and another VFR transitioning aircraft that failed to adhere a suggested altitude.

Narrative: A pipeline Cessna was flying south over the airport VFR at 2;500. As the aircraft approached the airport; the C172 departed a satellite airport 6 NM northwest; and asked for flight following to the west. I gave the C172 a VFR code. When the aircraft tagged up his altitude was 2;100 and climbing directly at the pipeline aircraft. I told the C172 RADAR contact; issued traffic; and suggested the aircraft descend back to 2;000. However; the C172 continued climbing; and the pipeline aircraft descended to 2;300 without warning. I noticed both aircraft at 2;300 and the same position. I then issued a traffic alert to the C172; then to the pipeline aircraft. The aircraft eventually saw each other; and the pipeline aircraft said they were about 100 FT apart. I reviewed the voice and RADAR data with my Supervisor. The data showed 0 FT vertical; 0.15 horizontal; and 13 seconds from when the aircraft tagged up and the issuance of the safety alert. We discussed traffic alert phraseology.

Data retrieved from NASA's ASRS site as of April 2012 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	944790
Time
Date	201104
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.TRACON
State Reference	US
Aircraft 1
Make Model Name	Skyhawk 172/Cutlass 172
Flight Phase	Climb
Aircraft 2
Make Model Name	Cessna Aircraft Undifferentiated or Other Model
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Person 1
Function	Approach Departure
Qualification	Air Traffic Control Developmental
Events
Anomaly	ATC Issue All Types Conflict NMAC Deviation - Altitude Excursion From Assigned Altitude

Tower controller described a conflict between a VFR pipeline patrol aircraft and another VFR transitioning aircraft that failed to adhere a suggested altitude.