British investigators say ice crystals are the likely culprit in several 2012 incidents where operators received erroneous airspeed or altitude readings.

In the first incident, pilots of a British Airways (BA) Airbus A321 on initial approach to London Heathrow Airport on April 20, 2012, after a flight from Stockholm Arlanda Airport, received wildly fluctuating air speeds on primary and backup flight displays when entering cloud tops at 14,000 ft.

The erroneous readings lasted between 10 sec. and 2 min.

Along with causing the fly-by-wire system to transition out of normal mode, the errors also caused a false traffic and collision alerting system resolution advisory.

In June 2012, the same aircraft, while climbing through “the top of a dome of cloud” at 26,500 ft., caused the pilot and co-pilot’s airspeed indicators to reduce “towards zero” twice, then return to normal.

The Air Accidents Investigation Branch (AAIB), in its report, says a BA A321 in August 2012 also experienced similar air data behavior for a “slightly longer period” at 26,800 ft.

The AAIB notes that while the pitot tubes used to compute airspeed are heated, “conditions can be encountered in which the heat removed from the probe due to the environmental conditions exceeds the ability of the heating system” and ice accumulates on the probe.

Airbus tells the AAIB that the incidents are “not associated with any fault on the aircraft”, but are consistent with their studies of at least two incidents where pitot probes were obstructed by ice crystals.

While European Aviation Safety Agency (EASA) certification rules define altitude and temperature envelopes for continuous and intermittent maximum icing conditions for supercooled liquid droplets, there currently are no requirements for ice crystals, as ice crystals are “not considered to be as hazardous as liquid,” says the AAIB.

EASA has proposed revising the certification rules for ice crystals, but AAIB notes that the new rules would not cover the conditions likely experienced during at least one of the incidents.

Airbus, which has its own internal ice crystal testing requirements, says it is developing expanded envelopes that will be included in updated EASA requirements. “Airbus considers that the current EASA and Airbus requirements need to be improved to better address pitot probe icing,” says the AAIB.