A year after a fatal crash involving a stall during icing conditinos, US operators of Saab 340 must replace the turboprop's stall warning computer after regulators found omissions in the existing alert system.
The Federal Aviation Administration says it will issue an airworthiness directive on 10 December mandating the hardware changes within 24 months.
The FAA estimates the final rule will cost US operators a total of $6.42 million, or $39,600 per aircraft, to complete the modifications.
New stall warning computers are necessary because the current system has failed several times to warn crews of natural stall events in icing conditions, the FAA says.
A Saab 340 operated by Sol Lineas Aereas crashed last year after stalling in heavy icing conditions, killing all 22 people aboard. In 2006, an American Eagle Saab 340 also experienced a stall upset event, which prompted a recommendation by the National Transportation Safety Board for the FAA to require an upgrade to the stall warning computer.
The airworthiness directive finally was issued by the FAA several months after Saab recommended that operators install a new computer with improved stall warning logic.
The new computer will provide an artificial stall warning to the crews. When the crews select the "engine anti-ice" function, the stall warning computer will automatically switch to a mode customised for detecting loss of airflow over the wing during icing conditions.