Date: 201411
Local time of day: 1801-2400
Location: ISP.Airport, NY
Flight conditions: VMC
Weather elements / visibility: 15
Light: Daylight
Aircraft: Small Aircraft
Operator: Personal
Operating under: Part 91
Flight plan: VFR
Mission: Personal
Flight phase: Taxi
ATC / advisory: Tower ISP
Second aircraft: Small Aircraft
Reporter organization: Government
Reporter function: Ground; Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness; Communication Breakdown; Confusion
Second reporter function: Pilot Flying; Single Pilot
Anomaly: Conflict NMAC; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Detector: Person Flight Crew; Person Air Traffic Control
When detected: Taxi
Contributing factors: Procedure; Human Factors
Primary problem: Human Factors

Narrative: Aircraft X asked to taxi to runway 24 full length; issued: 'Aircraft X taxi to runway 24 via alpha; sierra; B4; cross RWY 33L join taxiway echo. Stay to your right for opposite direction small aircraft inbound to the ramp.'  Aircraft X responded 'You want me to do my run-up on A1?' I said 'Negative!' and then repeated the taxi instructions issued before; very slowly. Aircraft X responded 'Roger.'  At this time I cleared another aircraft for take-off from the mid-field intersection; advising him the prior touch & go would be in right traffic. I then cleared another aircraft to land on runway 24 at which point a small aircraft on taxiway S saw Aircraft X enter runway 24 and announced it on the frequency. I observed Aircraft X had not followed his instructions and proceeded straight out via A1 and turned right onto runway 24. Aircraft X veered to the right when he observed the small aircraft departure opposite direction. The small aircraft also veered to the right and passed Aircraft X at an altitude of approximately 300 feet. Aircraft X was advised to exit the runway and was issued a Brasher warning.Even if ground had been open; I believe this event would have occurred.


Second narrative: I received taxi clearance and proceeded to A1 intersection. This is the taxi clearance used during the construction on Sierra and Alpha taxiways. Upon turning onto runway 24 for back taxi; I realized an airplane was taking off on runway 24; and did an immediate 180 degree turn back to A1. Ground control contacted me with the corrected clearance; which I followed. It was Sierra; B4; cross 33L; Echo; and hold short of 33R. With a crossing clearance of 33L; I was holding short of runway 24 at Echo for run up. Ground control contacted me and told me to contact the tower on landing by phone. I thought they meant to contact the tower when landing back at ISP; not at my destination of GON. I left my plane at GON for maintenance; and called the ISP tower after the drive back to ISP. I was told to contact the ISP tower supervisor the next morning.I think the problem was caused by misunderstanding the initial taxi clearance; and not giving back a proper read back on initial call up to ground. My performance was affected by being used to the construction taxi clearance; and not listening properly with a read back to ground control.


Synopsis: Controller and pilot report a runway incursion that happened due to communication; leading to a NMAC with departing traffic.