Indonesian officials are providing the strongest hints yet that a faulty airspeed indicator played a role in the deadly crash of a Lion Air jet into the Java Sea.

Investigators said Monday that one of the so-called black boxes showed that the airspeed indicator on the Boeing jet malfunctioned on its last four flights, including the Oct. 29 crash that killed all 189 people on board.

Airspeed indicators have been around for decades to tell pilots how fast they are flying. They are paired with separate indicators measuring the degree to which the nose is pointed up, down or level.

Modern jetliners have redundant measurements to help pilots spot and disregard a single reading that looks unlikely and possibly erroneous.

Speed-measuring systems consist of tubes and sensors that measure air pressure generated by the plane’s movement and compare it with surrounding air pressure. They fail occasionally, especially in bad weather at high altitude, when the tubes located under the plane’s nose can become jammed with ice, preventing air from reaching the sensors. The Oct. 29 Lion Air flight took off in good weather.

Frozen pitot tubes were blamed for the 2009 crash of Air France Flight 447, which killed all 228 people on board. The year before, the U.S. Air Force said moisture in sensors caused the 2008 crash of a B-2 stealth bomber on Guam; both pilots ejected safely. In 2015, a wasp nest plugged the sensors on an Allegiant Air jet leaving St. Petersburg, Florida, forcing pilots to cut the flight short and land in Orlando.

Pilots train in simulators to learn how to notice potentially faulty readings and work around them. They learn the normal power settings and attitude, or nose-up and nose-down settings, for each of the various phases of a flight. A problem with the airspeed system should not result in a crash under most circumstances, according to safety experts.

Safety experts said investigators will look at why Lion Air didn’t ground the plane.

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