Boeing 737 Max operators are to be instructed to update engine-control software to address a loss of thrust issue, attributed to icing, on the type’s CFM International Leap-1B powerplants.

At least two occurrences have been investigated which Leap-1Bs suffered temporary loss of thrust control as a result of icing in engine pressure lines, the European Union Aviation Safety Agency states.

This is despite a previous update to the Leap-1B software which was introduced after a similar problem affected the Leap-1A engine for the Airbus A320neo family.

Instances of Leap-1As being unable to accelerate to target thrust during take-off had prompted analysis which determined that water could accumulate and freeze inside the pressure sensor lines or system, possibly resulting in erroneous pressure readings by the full-authority digital engine control.

CFM developed new electronic engine control software to handle erroneous readings more effectively, and this was applied not just to the Leap-1A but also the similar -1B.

But EASA says icing-related loss of thrust control has occurred on Leap-1Bs which had already been updated with the revised software.

CFM has since revised the software to “improve detection and accommodation” of erroneous pressure readings, and EASA is to require 737 Max operators to modify the operating software configuration.

The change is among a series of updates to the airworthiness limitations section of the Leap-1B engine shop manual.

This revision enhances inspection of stage-two high-pressure turbine rotor discs, as a precaution against uncontained failure, after discovery of a sub-surface anomaly in a part manufactured from the same disc alloy.

It also adapts instructions for high-pressure turbine static structure inspections after incidents relating to fuel-nozzle coking and “thermal distress”, says EASA, and updates inspection and in-service limits to address radial drive shaft bearing cage failures.