While climbing to 19,000 feet, the pilots received an ‘altitude mismatch’ message and returned to Exeter Airport. An inspection after landing found a small white crystalline deposit covering three of the four static pressure holes on the left primary static probe. Primary static probes are used to measure pressure, which along with other data allows the Air Data Units to calculate parameters such as altitude, indicated and true airspeed and temperature.
The investigation found that a non-approved product was probably used to improve the seal between a testing device and the probe during maintenance immediately before the flight, and this may have resulted in the blockage of the static holes, leading to the altitude mismatch message. Two Safety Recommendations have been made to prevent similar incidents. The maintenance organisation has taken safety action to introduce tighter controls on the test kit equipment.