History of Flight
On 18 December 2009 at approximately 1330Z, a Beech 95-B55 (registration ZS-NKB) departed from runway 29 at FAWB (Wonderboom Airport) for a type conversion training flight in the general flying area. The aircraft carried an instructor and a student who held a valid private pilot licence. Multiple witnesses observed that the aircraft failed to gain height after takeoff and was flying very low. The pilot transmitted a MAYDAY call on frequency 120.6 MHz but did not specify the nature of the emergency. Shortly after takeoff, the aircraft collided with vegetation and terrain, coming to rest in the Apies River approximately 3.5 km from the airport. Both occupants sustained fatal injuries during the accident sequence.
Aircraft and Crew Information
The aircraft was a Beech 95-B55, serial number TC-1213, manufactured in 1969. It was powered by two Continental IO-470 L engines. The instructor held an Airline Transport Pilot Licence with a Grade II Instructor rating, had 2,759.4 total flying hours, and 42 hours on type. The student held a Private Pilot Licence and had 611.8 total flying hours, with 2 hours on type. The student had flown earlier that morning for two hours with another instructor without recorded problems.
Meteorological Conditions
Meteorological information from the South African Weather Service indicated fine weather conditions at the accident time: wind from 340° true north at 5 knots, temperature 34°C, dew point 11°C, visibility 10 km or more, few clouds at 4500 feet, and density altitude at FAWB of 7326 feet.
Wreckage and Impact
The left wing collided with vegetation prior to impact in the river bed. The aircraft then crashed into the river against a retainer wall. The aircraft was destroyed by the impact and a subsequent fire. Post-impact fire damaged most of the aircraft; the empennage, submerged in water, did not sustain fire damage. Minor heat damage occurred to surrounding vegetation, and a substantial amount of fuel was spilled into the river.
Tests and Examination
Examination at the accident site revealed that the right-hand engine had been running on the right magneto only and the left-hand engine on the left magneto only. The fuel boost pumps were found selected in the high position. An engine examination found that top spark plugs and some bottom spark plugs on the right-hand engine were fouled to the extent that they failed to fire under pressure on a spark plug tester. Flight testing in a similar aircraft type established that with full rich mixture and fuel boost pumps in the high position, the aircraft descended at a rate of 300+ feet per minute at full power with landing gear and flaps retracted.
Probable Cause and Contributing Factors
The probable cause of the accident was that the aircraft failed to gain height after takeoff and collided with vegetation and terrain. Contributing factors included fouled spark plugs on the right-hand engine; the fuel boost pumps being switched to high, resulting in a negative rate of climb and an over-rich mixture setting on both engines, causing them to fail to produce full power; the instructor's view of the fuel boost pump switch position being obstructed by the control column; lack of clear instructions in the Pilot's Operating Handbook on the use of the fuel boost pumps; and the limited flight experience of both the instructor and student on the aircraft type.