Background
An airline transport pilot was attempting to take off from a lake in a float-equipped airplane with a load of passengers that he had flown to the lake earlier that day. The pilot calculated takeoff distances for the water run and over a 50-foot obstacle as 1,050 feet and 2,210 feet, respectively. He did not add a safety margin to these calculations. The approximate shore-to-shore distance of the takeoff path was 1,800 feet.
Takeoff Attempt
During taxi, the pilot retracted the wing flaps, and they remained in that position for the takeoff. GPS data showed that the airplane attained a speed of about 49 knots before impacting terrain just beyond the shoreline. The airplane's stall speed with flaps retracted was about 52 knots.
Postaccident Examination
Postaccident examination revealed that the left wing flap was in the fully retracted position, while the right wing flap assembly was damaged. The airplane flight manual takeoff checklist stated that flaps were to be selected to the "TAKE-OFF" position before takeoff. Furthermore, the takeoff performance data contained in the flight manual depended on the use of "TAKE-OFF" flaps and did not account for no-flaps takeoffs. Even if the pilot had used the correct flap setting for takeoff, the calculated takeoff distances were near the available takeoff distance, and it is likely that the airplane would still not have been able to avoid a collision with terrain.
Pilot Statement
The pilot stated that there was no mechanical malfunction or failure with the airplane, and he should have "done the right thing," which was to conduct two flights, each with a half load of passengers.