Embraer plans to begin retrofitting the Phenom 100s already delivered in the USA with new flap control unit logic and other upgrades from April to correct problems at the centre of a Brazilian emergency airworthiness directive levied on the twinjet last month, the first for the new design.

The US Federal Aviation Administration on 27 February issued a complementary "final" AD prohibiting operators from using full flaps on landing unless flying at recommended higher airspeeds that are called for in icing conditions.

Embraer learned of problems with the Eaton Aerospace-built flap control unit after US deliveries began in early January, says Mauricio Almeida, vice-president of Embraer's executive jets programmes.

Ph100 cockpit 
 © John Croft/Flight International

Based on the operation of the flap-level position sensor, engineers have determined that the system could improperly detect that the flaps have failed based on a spurious output of the flap lever sensor. The Phenom 100 has four pre-set flap levels at 0°, 10°, 26° and 36°.

Although no problems have occurred in practice and in more than 500 go-arounds in certification and in-service reliability testing, Embraer says pilots trying to raise flaps from the fully extended ("4", or landing position) to the go-around ("2") position could be faced with an unexpected stick pusher activation.

The Phenom has an aural stall warning alert and a stick pusher. The former activates as the aircraft approaches an aerodynamic stall to alert the pilot to the issue. The latter activates if the aircraft continues to approach a stall, physically pushing the control column forward. Embraer officials earlier told Flight International that the stick pusher was originally included in the design to reduce landing distance by allowing the aircraft to fly at the maximum lift coefficient angle of attach (AOA) of the wing at any flap setting.

© John Croft/Flight International

As built, the Phenom 100 anti-stall logic includes a safety feature whereby the stick pusher threshold occurs at a lower AOA if the system detects a flap failure, whether spurious or real, or if the aircraft is flying in icing conditions, both of which can reduce the AOA where a stall occurs.

The fix, being developed by Embraer and Eaton, will include an improvement to the logic that monitors the flap lever sensor to eliminate false "flap failure" indications, as well as a change in the stall warning computer software to add stick pusher thresholds for each flap setting rather than a single global threshold as currently designed.

Almeida says Embraer expects to certificate the new system by mid-March and will begin retrofits on the delivered fleet and on the assembly lines by the end of March.

Source: Flight International