UPDATE 4-Boeing alerts pilots after Indonesia crash, FAA plans directive

* Erroneous readings from system could cause dive - source

* Sensor changed day before doomed flight - investigators (Adds FAA statement, planning to issue safety directive)

ZHUHAI, China/WASHINGTON, Nov 7 (Reuters) - Boeing Co said on Wednesday it had issued a safety bulletin reminding pilots how to handle erroneous data from a key sensor in the wake of last week's Indonesian jetliner crash.

The U.S. planemaker said investigators probing the Lion Air crash off the coast of Indonesia, in which all 189 on board were killed, had found that one of the "angle of attack" sensors on the brand-new Boeing 737 MAX jet had provided erroneous data.

Experts say the angle of attack is a crucial parameter that helps the aircraft's computers understand whether its nose is too high relative to the current of air - a phenomenon that can throw the plane into an aerodynamic stall and make it fall.

Some modern aircraft have systems designed to correct the posture of the aircraft automatically to keep flying safely.

There are also procedures for pilots to follow in the event of missing data from damaged sensors on the fuselage, but it remains unclear how much time the crew of flight JT610 had to respond at the relatively low altitude of around 5,000 feet.

An angle of attack sensor had been changed by mechanics on the ground in Bali the day before the crash, Indonesia's National Transportation Safety Committee (KNKT) said.

The captain and first officer flying from Bali to Jakarta the night before the crash had indicators displaying differences in angle of 20 degrees, KNKT said, but that flight landed safely despite the issues in the air.

Boeing said in a statement received at China's largest air show in Zhuhai that its note to airlines underscored "existing flight crew procedures" designed to address circumstances where information coming into the cockpit from the sensors was wrong.

The Boeing 737 MAX has three such blade-shaped sensors. Erroneous readings can in some circumstances cause the 737 MAX to point the nose down sharply to keep air under the wings and avoid a stall, according to a person briefed on the matter.

FAA DIRECTIVE

Boeing's bulletin related only to the 737 MAX, the latest version of the world's most-sold family of aircraft, which has been in service for just over a year.

The U.S. Federal Aviation Administration said it would issue an airworthiness directive to make Boeing's advice mandatory.

"The FAA continues to work closely with Boeing, and as a part of the investigative team on the Indonesia Lion Air accident, will take further appropriate actions depending on the results of the investigation," it said in a statement.

Boeing has delivered 219 737 MAX jets to customers globally, with 4,564 orders for jets yet to be delivered.

The Boeing 737 MAX is a more fuel-efficient version of the manufacturer's best-selling single-aisle 737 series.

The Lion Air crash was the first involving the new version, which airlines introduced into service last year.

Indonesian authorities have downloaded information from the flight data recorder that showed a cockpit indicator on the Lion Air jet was damaged for its last four flights.

A search for the cockpit voice recorder, the second so-called "black box", remains underway.

KNKT said it would attempt to reconstruct the jet's last flight using Boeing simulators in Seattle. The angle of attack sensor replaced in Bali would be analysed at its place of manufacture in Chicago, the accident investigator said. (Reporting by Tim Hepher and David Shepardson; additional reporting by Cindy Silviana in Jakarta, Jamie Freed in Singapore and Allison Lampert in Montreal; editing by Himani Sarkar and Alexandra Hudson)