Aw-139 helicopter pilot reported an near midair collision when they took off without clearance.

Attributes
ACN	1343602
Time
Date	201603
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Agusta Undifferentiated or Other Model
Operating Under FAR Part	Part 91
Flight Phase	Initial Climb
Route In Use	Direct
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	Initial Climb
Person 1
Function	Pilot Flying Captain
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP) Flight Crew Flight Instructor
Experience	Flight Crew Last 90 Days 25 Flight Crew Total 3200 Flight Crew Type 200
Events
Anomaly	Conflict NMAC Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy

Narrative:

Near miss occurred when departing from the hangar at a tower controlled airport. Sequence of events leading up to the near miss are as follows: aircraft requested taxi and departure to the northeast. Tower acknowledged and cleared the aircraft to taxi as requested. Aircraft taxied out; completed takeoff checks and departed. After takeoff ATC tower made immediate evasive maneuver calls to prevent a collision with other traffic that had recently departed.this close call was a clear case of a pilot requesting a clearance and hearing the clearance that he was expecting; not the clearance that was actually given. Tower gave a taxi clearance and a taxi clearance only. At no time did the tower say 'cleared for takeoff'. The pilot was expecting the tower to issue a taxi and takeoff clearance in one transmission; but only a taxi clearance was given. [Aircraft] took off without ever hearing or receiving a 'cleared for takeoff' in their clearance.

Original NASA ASRS Text

Title: AW-139 helicopter pilot reported an NMAC when they took off without clearance.

Narrative: Near miss occurred when departing from the hangar at a tower controlled airport. Sequence of events leading up to the near miss are as follows: aircraft requested taxi and departure to the NE. Tower acknowledged and cleared the aircraft to taxi as requested. Aircraft taxied out; completed takeoff checks and departed. After takeoff ATC Tower made immediate evasive maneuver calls to prevent a collision with other traffic that had recently departed.This close call was a clear case of a pilot requesting a clearance and hearing the clearance that he was expecting; not the clearance that was actually given. Tower gave a taxi clearance and a taxi clearance only. At no time did the tower say 'cleared for takeoff'. The pilot was expecting the Tower to issue a taxi and takeoff clearance in one transmission; but only a taxi clearance was given. [Aircraft] took off without ever hearing or receiving a 'cleared for takeoff' in their clearance.

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.