Following disclosure that a faulty altimeter played a critical role in the Turkish Airlines B-737-800 (TC-JGE) crash at Amsterdam’s Schiphol Airport on Feb. 25, Boeing is advising 737 pilots to carefully monitor cockpit instruments during critical phases of flight.

The crash killed nine of the 135 people aboard (128 passenger and seven crew) and injuring more than 50 others. Both of the pilots flying the GE/Snecma CFM-56-powered jetliner and a third junior pilot observing, were among the dead. Turkish Airlines Flight 1951 from Istanbul was on final approach to land on Runway 18R when the twinjet slammed into a muddy field and broke into three pieces.

The aircraft maker said the warning results from the preliminary report from the Dutch Safety Board regarding the fatal accident.

Dutch air safety investigators said the readout from the B-737’s flight data recorder shows that one of two altimeters, a device that measures altitude, was faulty, leading the automatic pilot to drastically reduce power, causing a stall on final approach to the Dutch airport.

"Boeing reminds all operators to make sure flight crews pay close attention to all primary flight controls during critical stages of a flight," says a Boeing spokesman.