Loss of control risk behind proposed 737 wiring change

Washington DC
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Three reports of pilots failing to manually activate air data sensor heating systems on Next Generation Boeing 737 aircraft have prompted the US Federal Aviation Administration to propose a fix to more than 1,000 aircraft in the US-registered fleet.

Air data sensors include pitot tubes, which if partially or fully blocked by ice, provide erroneous airspeed information to the flight deck.

"The affected airplanes do not have an automatic activation of the air data sensor heating system," said the FAA. "Pilots activate the system manually as a pre-takeoff checklist item. Failure to activate the air data sensor hearing system could result in ice formation on air data system sensors, which could lead to misleading airspeed data or loss of all airspeed indicating systems, and loss of control of the airplane."

If finalised as written, the proposed airworthiness directive (AD), to be published on 28 February, will require operators within two years to make overhead panel and wiring changes that will in effect provide power to the pitot tubes, angle-of-attack sensors and total air temperature probes at all times during flight.

On the overhead panel, the traditional "on" and "off" probe heat toggle switch positions will be replaced by "on" and "auto" positions.

With the switch set to "auto", electrical heat will be provided to the sensors any time either of the aircraft's engines is operating.

The FAA estimated that the changes, as defined in a 16 November 2011 Boeing service bulletin, would take as many as 80 hours per aircraft to implement, with an associated cost of nearly $15 million for the estimated 1,025 affected Boeing 737-600/700/700C/800/900/900ER aircraft.