UPDATE 1-Boeing fix will prevent repeated activation of anti-stall system -sources

A Boeing Co software fix for the grounded 737 MAX will prevent repeated operation of an anti-stall system at the centre of safety concerns and deactivate it altogether if two sensors disagree too much, two people familiar with pilot briefings said. The anti-stall system - known as MCAS, or Maneuvering Characteristics Augmentation System - is under scrutiny after two recent fatal crashes including a Lion Air crash in October.

Boeing has said it will outline a software fix in coming days aimed at addressing the situation faced by Lion Air pilots. Airline briefings started on Saturday. Pilots have been told that the MCAS system - which forces the nose downwards to avoid a stall, or loss of lift - will only operate one time for each event rather than impose repeated corrections like those believed to have pushed the Lion Air jet into a dive, the people familiar with the briefings said.

Additionally, MCAS will be disabled whenever two sensors that measure the 'angle of attack' - a parameter that determines how close a plane is to an aerodynamic stall - differ too widely. This is a change from the previous set-up which only linked MCAS to one sensor at a time, ignoring the other, and which may have resulted in a single point of failure on Lion Air 610.

The pilot will be able to deduce that MCAS is no longer working in the background because the system will show a warning message labelled "AOA disagree", indicating the two sensors are producing values that differ by an excessive margin. Accordingly, the AOA disagree warning light, which was previously optional, will now be a standard feature.

(Reporting by Eric M. Johnson in Seattle and Tim Hepher in London; Editing by Lisa Shumaker)