Date: 201907
Local time of day: 1201-1800
Location: DSM.Airport, IA
Flight conditions: VMC
Light: Daylight
Aircraft: Small Aircraft
Operator: Personal
Operating under: Part 91
Flight plan: VFR
Mission: Personal
Flight phase: Takeoff / Launch
Airspace: Class C DSM
ATC / advisory: Tower DSM
Reporter organization: Government
Reporter function: Supervisor / CIC; Local; Ground
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness
Second reporter function: Pilot Flying; Single Pilot
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Taxiway
Detector: Person Air Traffic Control
When detected: In-flight
Result: Flight Crew Became Reoriented
Contributing factors: Airport; Chart Or Publication; Human Factors
Primary problem: Human Factors

Narrative: I was working Local; Ground and CIC (Controller in Charge) combined. Aircraft X was in the run up pad at the end of Taxiway Papa performing a run up before takeoff. Aircraft Y checked in on approximately a 6 mile right base for Runway 05. After clearing Aircraft Y to land; Aircraft X called ready for takeoff and appeared to [be] taxiing from the run up pad to the runway. I cleared Aircraft X for takeoff and informed them of the arriving aircraft on the 6 mile base. I then sent a transmission to Aircraft Y to inform them of traffic (Aircraft X) to depart runway prior to their arrival. I then continued to do a check for the runway and airport environment for potential conflicts for the departing aircraft. As I scanned back to Aircraft X starting their takeoff role it appeared that they might have lined up on a taxiway rather than the runway. Due to the distance between the tower and the approach end of the runway I reached for binoculars to verify which surface Aircraft X was beginning their takeoff roll on. After confirming that Aircraft X was conducting takeoff roll on the taxiway I attempted to cancel the takeoff clearance when Aircraft X was abeam Papa 6. There was no response from the pilot and they had lifted off of the taxiway. Aircraft X then appeared to sidestep over to the runway and continued with the departure. No other aircraft or vehicles were present on Taxiway Papa and there were no workers or equipment in the closed sections of the taxiway at the time. The distance from the approach end of the runway and the tower is the biggest issue with this event. The distance made it so that I felt it was necessary to check to ensure the aircraft was on the taxiway to insure that I wasn't cancelling a takeoff clearance unnecessarily and potentially creating a high stress and potentially dangerous event without cause.


Second narrative: I am relatively new in the area; having flown out of DSM several times. The airport is undergoing extensive maintenance with one of the runways shutdown and numerous portions of taxiways out of service. Most of my flying time has been out of smaller airports. I had taxied via Papa to the run-up area at the southwest end of Runway 05 near P7 and completed my run-up checks there. The run-up area is adjacent to a taxiway rather than the runway as it is at many airports. Ground traffic at the field had been very light during taxing. I called Tower requesting takeoff clearance; which I was given. At the same time the Tower gave clearance to landing traffic for my runway. With that in mind; I expeditiously left the run-up area and turned right for takeoff; mistaking a taxiway for the runway. The Tower did not inform me otherwise until I was airborne and climbing out. So far as I could see there was no conflicting traffic on Taxiway P; the runway heading was as expected; and the width of the runway was similar to many of the airports I primarily use. I did not notice my error until the Tower informed me as I was climbing out and requested that I telephone on my return. I have never done anything remotely like this in years of flying. I attribute this circumstance to my haste to takeoff in advance of landing traffic to in order to avoid that potential conflict. As it happened; there was no real conflict with the landing aircraft and I have no details on the circumstances of that aircraft. However; I had the impression it was commercial jet traffic. This could have been avoided by slowing down and proceeding in a more stepwise manner.


Synopsis: DSM Tower Controller and pilot reported the pilot took off from a taxiway.