HISTORY OF FLIGHT On November 17, 1999 about 1210 Pacific standard time, a Bell 206L-3, N206AC, registered to and operated by Aero-Copters, Inc., as a 14CFR91 commercial flight, and a Bell 206B, N5735A, registered to WPXI, Inc. and operated by KIRO Television as a 14CFR91 commercial flight, collided in mid-air about seven miles north of Boeing Field Airport, Seattle, Washington. Visual meteorological conditions prevailed and no flight plan was filed for N5735A; however, there was a flight plan on file for N206AC. Both aircraft received substantial damage. The commercial pilot of N5735A sustained minor injuries, and his passenger was uninjured. The commercial pilot, the sole occupant of N206AC, was not injured. N206AC departed Boeing Field approximately five minutes prior to the accident with a planned destination of Vancouver, British Columbia, Canada. N5735A departed Boeing Field approximately 36 minutes prior to the accident for a local electronic news gathering flight. In a written statement, the pilot of N5735A reported that he was transitioning the area of Lake Union in a south-southwesterly direction when the two aircraft collided. He stated that "...I simultaneously saw the other aircraft and felt the impact". The passenger of N5735A, a news photographer, stated that he "...heard a loud bang and felt the helicopter shake". After the collision the pilot determined the aircraft was still controllable and initiated a descent to a rooftop helipad located approximately a 1/2 mile from where the helicopters collided. The pilot landed the aircraft on the helipad without further incident. After landing at the helipad, the pilot and photographer determined that the other aircraft involved in the collision was operated by Aero-Copters. They contacted Aero-Copters and learned that the other helicopter involved in the collision landed at a nearby helipad. The pilot of N5735A reported that prior to the collision he was monitoring the common traffic advisory frequency (CTAF) for Lake Union (122.90). He also stated that just before transitioning the area he reported his position, altitude and heading on 122.90. During a telephone interview and subsequent written statement, the pilot of N206AC reported that he departed Boeing Field, Seattle, Washington, and was proceeding northbound. He stated that he had just cleared Boeing's Delta airspace and switched to Seattle Radio (122.50) when he "...experienced a loud bang and vibration in the main rotor system". The pilot initiated a descent to a nearby helipad at the University of Washington and was able to land without further incident. The pilot reported that after landing he discovered damage to the main rotor blades and suspected that he had collided with a weather balloon. Neither pilot reported experiencing any mechanical failures or malfunctions prior to the collision. DAMAGE TO AIRCRAFT Both aircraft sustained substantial damage. The leading edge of the main rotor blade of N206AC had a large gouge, with paint transfer, approximately twelve inches inboard from the rotor tip. The gouge was approximately 5 inches in length with scratch marks radiating aft on both the top and bottom surfaces of the rotor blade. Just above the leading edge damage, a five-inch slash in the honeycomb material was noted. The slash was approximately 1/4 inch deep and ran perpendicular to the leading edge. A small puncture type hole was also noted to the fuselage aft of the rear door (refer to photograph #2). The helicopter operated by WPXI, Inc., N5735A, sustained substantial damage to the main rotor blades and fuselage. A large slash was noted to the forward engine/transmission cowling, measuring approximately 18 inches in length. The helicopter's wire cutter and support brackets, located just forward of the engine cowling, were fractured. Both the right and left windshields were broken, and numerous fragments of Plexiglas were found in the cockpit area. Approximately 6 inches outboard of the main rotor blade trim tab, a gouge in the lower surface of the rotor blade was noted. The gouge paralleled the chord line and was approximately 6 inches in length (refer to attached photos). METEOROLOGICAL INFORMATION At 1153 Pacific Standard Time, the hourly observation (METAR) for Boeing Field, seven miles south of the accident site, reported winds from 360 degrees at 4 knots; visibility 10 miles; broken clouds at 1,700 feet AGL; temperature 51 degrees F; dew point temperature 44 degrees F; altimeter 29.97 inches hg. ADDITIONAL DATA Preliminary radar data provided by Seattle-Tacoma Air Traffic Control Tower, Seattle, Washington, indicated that N5735A was tracking a heading of 227 degrees magnetic, with an airspeed of 70 knots and an altitude of 1,000 feet above mean sea level prior to the collision; N206AC was tracking a heading of 342 degrees magnetic with an airspeed of 118 knots and an altitude of 1,000 feet above mean sea level. Hard copies of the radar data from Seattle-Tacoma Air Traffic Control Tower were not available to the National Transportation Safety Board. Both aircraft were released to the operators on November 17, 1999.