GRB Controller described a confused flight plan event when a departure strip indicated VFR; the pilot insisting the aircraft was on an IFR flight; the reporter noted the distraction of using CENRAP contributed to the confusion.

Narrative

GRB was in full CENRAP Mode. I had taken the R1 (GRB Arrival/Departure) position. At the start of the session traffic activity was light and increased to moderate about 40 minutes later. Complexity was increased above the norm due to CENRAP procedures. There were no primary RADAR targets and different separation rules were in effect. Targets of departure aircraft would acquire about 5 miles from the airport and RADAR contact would be lost on aircraft once they descended to 3;000 FT MSL and below. The Tower dropped down a hand written VFR Flight Strip; normal for VFR departures; for a Jet Commander. The strip indicated the aircraft ID; no code (box 4 where a transponder code normally would be indicated was crossed through); direction of flight (westbound); the standard symbol for 'maintain VFR at or below 3;000 FT'; and 'OC' for an on course departure. At first I wondered aloud why the Tower even gave me a VFR aircraft since I most likely would not be able to provide traffic advisories. I believe it was the Supervisor who responded that maybe the aircraft would be climbing higher. The Jet Commander made his initial call. I advised him; 'Cancel altitude restriction'; (standard for our VFR departures) and reiterated proceed on course. A couple of minutes later I observed the aircraft tagging up on a transponder code. I realized then that this aircraft was the Jet Commander that had arrived earlier on an IFR flight plan. Even though the flight strip indicated no code was given I thought the aircraft forgot to Squawk 1200 and had just departed with his previous inbound code. I was not aware how long he had actually been on the ground and it seemed plausible that his code may have still been in the Coast List. I asked the Jet Commander to IDENT and RADAR identified him 6 miles west of GRB airport. My mind was mostly focused on my other traffic that were all IFR and the increased separation standards needed under CENRAP. Some of the tags and virgules were not staying together and there were no primary RADAR targets. After I assessed what else was happening and made a few transmissions I returned to the Jet Commander. I believe he was about 10-12 miles west and just below the ceiling of our airspace which is 13;000 FT MSL. I advised him that I thought he was squawking his previous inbound code. The Jet Commander advised he was on an IFR clearance. I did not fully comprehend this as it made no sense as I only had a VFR flight strip. I felt that there must have been a misunderstanding. Sometimes aircraft change their minds and ask for a VFR departure even though they had an IFR on file. I thought maybe the Tower had given him the transponder code from a proposed IFR flight plan and advised him to maintain VFR. At GRB we will do this and usually send down the IFR flight strip with a large VFR written on it. These were all split second thoughts. I observed the altitude readout as passing through 13;200 and the aircraft was about 13 miles west now. I told the Jet Commander he was leaving my airspace; RADAR services terminated; and Squawk 1200. The pilot again insisted he was on an IFR flight plan. I quickly looked around my console to see if there was another flight strip I had missed for the Jet Commander. I told the pilot all I had was a VFR Flight Strip. He seemed adamant about being on an IFR clearance. I was going to call the Tower and ask them what they had issued but realized the aircraft was climbing fast and there was no time to inquire. I decided to take the pilot at his word but needed to stop his climb and call ZMP to coordinate. I tried to quickly assess what ZMP had for traffic and issued the Jet Commander an altitude of 16;000 FT. I flashed the target to ZMP and then called ZMP to advise them of the situation. They told me to issue 17;000 FT for the Jet Commander and stated he was RADAR contact. I issued the Jet Commander the new altitude and advised him to contact ZMP on 124.4. The Supervisor was sitting next to me during this time and was able to hear what was happening as I was using a handset. All transmissions were heard over the loudspeaker.