On August 7, 1999, about 0930 Alaska daylight time, a float equipped deHavilland DHC-6-300 airplane, N288SA, sustained substantial damage while docked at Ketchikan, Alaska, when it was struck by another float equipped deHavilland DHC-6-300 airplane, N224SA. Both airplanes were being operated as visual flight rules (VFR) local area sightseeing flights under Title 14, CFR Part 121, when the accident occurred. Both airplanes were operated by Seaborne Aviation Inc., Ketchikan. The captain, the first officer, and the 15 passengers on N288SA, were not injured. The captain, the first officer, and the 15 passengers on N224SA, were not injured. Visual meteorological conditions prevailed. VFR company flight following procedures were in effect. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on August 9, 1999, the Director of Operations for the company reported the crew of N288SA was beginning a passenger briefing to passengers before departing on a sightseeing flight. The passengers were on board the airplane, but the engines had not been started. N224SA, was also tied to the Ketchikan dock, parked behind N288SA. Each airplane was separated by about eight feet. The director of operations said the crew of N224SA had completed their briefing to the passengers on board the airplane, and had started the engines. The airplane was tethered to the dock by ropes attached to the floats at the forward and aft end of the airplane. Each rope was tended by a company dock handler. Due to a slight change in the manner the passengers arrived from a cruise ship, the passengers had not received a briefing booklet before boarding N224SA. A company representative approached the forward dock handler to hand the briefing booklets to the airplane crew. The aft dock handler observed this activity and mistook its meaning as a "thumbs up" signal. He released the docking rope, allowing N224SA to begin moving forward under idle power. The nose of N224SA collided with the tail of N288SA. N288SA received damage to the rudder and trim tab assembly. N224SA received minor damage to the left pitot tube, and the left side of the nose.