A pilot reported taking off without a takeoff clearance from the tower.

Narrative:

I flew into ZZZ for the first time in three years. I reviewed the departures for departure details to refresh myself. I got the ATIS information and then called ZZZ tower. The west waterway was primarily being used for takeoff and landing. I was parked at the northwest corner of ZZZ water. I asked tower if they were allowing takeoffs to the east; and he responded that they were too busy to do that. He gave me clearance to taxi to the east for a west takeoff. When I started to taxi to the slow taxi channel (north); the tower asked if I was prepared to make a 'step turn to the east' I responded that I could and he told me to let him know when I was ready and I could do so after the next departing traffic. I let him know that I could and he told me to make a step turn to the east. I got on step; went down lake and took off towards the east. Tower contacted me and asked what my altitude was as I continued east and I responded 700 ft. And continued to climb to the 900 ft. Ceiling published in the departure procedure.he asked me to report [the] overpass; which I did and shortly after he cleared me to turn south. He asked me when I would be able to write down a phone number to call ZZZ tower when I landed; and I responded to the affirmative when I got a pen. I called tower after I landed and found out that I had not been cleared to take off to the east; but had been cleared to do a high speed step taxi to the east for a westerly takeoff. It was my error. Having just asked tower if I could get a takeoff to the east; I had thought the situation had changed and misunderstood the high speed taxi instructions to be an approval to takeoff. After talking to ZZZ tower on the land phone after landing; realized that I had not heard the words 'cleared for takeoff'. After 54 years of flying without an incident; I put myself and potentially other pilots at risk by taking off into landing traffic. A lesson I'll never forget. My apologies to the tower operator that had to deal with my error.

Original NASA ASRS Text

Title: A pilot reported taking off without a takeoff clearance from the Tower.

Narrative: I flew into ZZZ for the first time in three years. I reviewed the departures for departure details to refresh myself. I got the ATIS information and then called ZZZ Tower. The west waterway was primarily being used for takeoff and landing. I was parked at the northwest corner of ZZZ water. I asked Tower if they were allowing takeoffs to the east; and he responded that they were too busy to do that. He gave me clearance to taxi to the east for a west takeoff. When I started to taxi to the slow taxi channel (north); the Tower asked if I was prepared to make a 'step turn to the east' I responded that I could and he told me to let him know when I was ready and I could do so after the next departing traffic. I let him know that I could and he told me to make a step turn to the east. I got on step; went down lake and took off towards the east. Tower contacted me and asked what my altitude was as I continued east and I responded 700 ft. and continued to climb to the 900 ft. ceiling published in the departure procedure.He asked me to report [the] overpass; which I did and shortly after he cleared me to turn south. He asked me when I would be able to write down a phone number to call ZZZ Tower when I landed; and I responded to the affirmative when I got a pen. I called Tower after I landed and found out that I had not been cleared to take off to the east; but had been cleared to do a high speed step taxi to the east for a westerly takeoff. It was my error. Having just asked Tower if I could get a takeoff to the east; I had thought the situation had changed and misunderstood the high speed taxi instructions to be an approval to takeoff. After talking to ZZZ Tower on the land phone after landing; realized that I had not heard the words 'cleared for takeoff'. After 54 years of flying without an incident; I put myself and potentially other pilots at risk by taking off into landing traffic. A lesson I'll never forget. My apologies to the Tower Operator that had to deal with my error.

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.

Attributes
ACN	1677935
Time
Date	201908
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Skywagon 185
Operating Under FAR Part	Part 91
Flight Phase	Takeoff
Flight Plan	VFR
Person 1
Function	Single Pilot Pilot Flying
Qualification	Flight Crew Multiengine Flight Crew Commercial Flight Crew Instrument Flight Crew Sea
Experience	Flight Crew Last 90 Days 35 Flight Crew Total 5000 Flight Crew Type 1000
Events
Anomaly	Airspace Violation All Types Deviation - Procedural Clearance Deviation - Track / Heading All Types Ground Incursion Runway