Flight History
On 22 April 2014 at approximately 0716Z, a commercial pilot accompanied by a passenger departed Cape Town International Airport (FACT) on an instrument flight rules (IFR) flight to Swartwater in the Limpopo Province. About 16 minutes after takeoff, while the aircraft was climbing through an altitude of 13,500 feet to 17,000 feet, the air traffic controller (ATC) advised the pilot that the aircraft's Mode C transponder was transmitting erroneous altitude data, indicating that the aircraft was descending whereas the pilot believed it was ascending. The pilot informed ATC that the aircraft was not descending and attempted to rectify the problem by recycling the Mode C transponder, but the issue persisted.
During the IFR flight to Swartwater, the transponder information remained intermittent. ATC requested the pilot to descend to the visual flight rules (VFR) flight level FL 135. Subsequently, the pilot requested approval from Area West to ascend to flight level (FL) 195, which was granted. The pilot was apparently unaware that the pitot-static tube system, which supplies both pitot and static air pressure for the airspeed indicator, altimeter, and triple indicator, was most probably blocked by dust or sand.
Aircraft Breakup
The aircraft exceeded its maximum structural cruising speed (VNO) of 173 knots and its calibrated never-exceed speed (VNE) of 203 knots. The VNO exceedance lasted 1 hour and 44 minutes, and the VNE exceedance lasted 9 minutes. This overspeed condition resulted in a catastrophic inflight breakup of the aircraft. The wreckage was found scattered over a 1.58-kilometer path in mountainous terrain. Both occupants on board sustained fatal injuries.
Probable Cause
The official investigation determined the probable cause as: "The aircraft exceeded the Maximum Structural Cruising Speed (VNO) and Calibrated Never Exceed Speed VNE airspeed due to the fact that erroneous airspeed and altitude data information indicated on the cockpit instruments as a result of blockage of the pitot tube by dust and sand. The fact that the pilot switched off the transponder was considered as a contributory factor."