European safety regulators are ordering operators of Airbus A330s and A340s to incorporate a new emergency procedure to cope with blocked angle-of-attack sensors.
The order follows an incident involving an A330 which suffered blockage of all angle-of-attack probes during climb.
This resulted in autopilot disconnection and activation of the angle-of-attack protection when the aircraft's Mach number increased.
In an emergency directive the European Aviation Safety Agency says that, under this combination of the protection and Mach increase, the flight-control laws order the aircraft to pitch down.
If this occurs, it adds, the flight crew "may not be able to counteract with a sidestick deflection, even in the full backward position".
Inquiries into the A330 event indicate that the aircraft involved was equipped with angle-of-attack probes with conic plates.
"It is suspected that these plates might have contributed to the event," says EASA. "Investigations are ongoing to determine the root cause of this angle-of-attack probes blockage."
The conic plates can be incorporated during production on both Thales and Goodrich probes, says EASA.
EASA says the condition could result in "reduced control" of the aircraft. It says Airbus has devised an emergency procedure to address the issue and EASA is ordering that this be included in flight manuals from 6 December.
It adds that the directive covers both the A330 and A340 as each can be fitted with the conic plates. EASA says that the order is an "interim" measure and further action could follow.
EASA recently proposed that Airbus A300 and A310 aircraft be fitted with new conic plates to improve ice protection of their angle-of-attack sensors.