Several small forest fires had erupted near Cranbrook and the Air Spray Electra Tanker 86 was called in to fight the fires. Accompanied by a Turbo Commander the Electra took off from Cranbrook at 12:10. The Turbo Commander assessed the appropriate aircraft flight path profiles and established the most suitable fire-retardant delivery program for the ground fire. Following the flight path demonstrations by the Turbo Commander aircraft, Tanker 86 proceeded to carry out the retardant drop on the fire. After delivering the specified retardant load, Tanker 86 was seen to turn right initially then entered a turn to the left. In a left angle of bank of about 70 degrees and nose-low the Electra then struck the terrain on the side of a steep ridge at about 3900 feet above sea level. The aircraft exploded on impact. FINDINGS AS TO CAUSES AND CONTRIBUTING FACTORS: 1. For undetermined reasons, the Electra did not climb sufficiently to avoid striking the rising terrain. 2. Given the flight path and the rate of climb chosen, a collision with the terrain was unavoidable. 3. The characteristics of the terrain were deceptive, making it difficult for the pilots to perceive their proximity and rate of closure to the rising ground in sufficient time to avoid it.