Date: 201202
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Aircraft: Bonanza 33
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Flight phase: Initial Climb
Airspace: Class D ZZZ
ATC / advisory: Tower ZZZ
Second aircraft: B737 Undifferentiated or Other Model
Reporter organization: Government
Reporter function: Local; Instructor
Reporter qualification: Air Traffic Control Fully Certified
Anomaly: ATC Issue All Types; Deviation - Altitude Overshoot
Detector: Person Air Traffic Control
Result: General None Reported / Taken
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: The Bonanza departed Runway 15 with an assignment to climb to 020 and turn left to heading 050 after departure. These instructions were assigned by the Flight Data position; and the pilot read back the instructions. No other departure instructions were given to the pilot. Upon commencing the left turn; the person I was training issued traffic to the Bonanza on a low level helicopter about 3 miles from his position. Subsequently; the altitude read out on the RADAR was unavailable for a moment for the Bonanza. So; the trainee asked the Bonanza to say altitude to ensure safety between the Bonanza and the helicopter prior to switching the Bonanza to the Departure Controller. The pilot said 'passing through two thousand' (which he had not yet done according to the altitude read out that came back up on our screen). I believed the pilot had said 'to' not 'through'; however; upon listening to the tapes; it is clear that the pilot stated the word 'through'. The Controller I was training switched the aircraft to the Departure Controller with the Bonanza indicating 019. We observed the Bonanza indicating 021. We observed the target of the Bonanza merge Data Tags with an aircraft flying over our airspace at 030. We were unable to read any of the Data Tags until they unmerged; at which point the Bonanza was indicating 023. I thought it was possible for the Departure Controller to have climbed the Bonanza; because at that point they had passed the position of the over-flying jet. So; I did not realize that an unsafe event had actually occurred. Upon review of RADAR feed; the Bonanza climbed to within 400 FT of the over-flying jet before beginning a descent. Phraseology change to differentiate between two almost identically sounding words could prevent a re-occurrence; to vs. through. We could have asked the pilot to 'Say again'. Additionally; the pilot could have maintained the altitude that he was assigned by the Flight Data Controller; and the event would have never occurred.


Second narrative: Before being switched to departure; a Bonanza stated that he was climbing through 020 MSL; and then was switched. The aircraft was given an altitude assignment of 020 by Clearance Delivery; and the pilot read that back with the call sign. After being switched to departure; the aircraft climbed to 026 MSL; deviating from his altitude; getting close to a B737 at 030 MSL. To and through sound very close when heard with all the static on the headset. Everybody thought that the pilot said to; but when reviewing the tapes you can hear clearly that he said through. It is my understandings that nothing will happen to this pilot; and the controllers are the only ones being assigned liability. We need to stop assigning controllers with total liability and doing absolutely nothing to the pilots. Most of the time; pilots are the ones who create the conditions for these mistakes; and they continue because nobody does anything to discourage them. This is not an error in my opinion because we did not assign him that altitude. He simply decided to deviate from previous instructions and told us he was doing it.


Synopsis: Tower Controller described a loss of separation event when a departing aircraft assigned 20 continued to climb and conflicted with overflight traffic; the misunderstanding of the words 'to' and 'through' listed as contributing factors.