NEW DELHI: The spectre of " sleeping pilots " is back to haunt Indian flyers. A Jet Airways aircraft winging its way from Mumbai to Brussels last Friday suddenly dropped 5,000 feet in the Ankara airspace over Turkey. The commander of the Boeing 777 aircraft was taking 'controlled rest', which means a nap as per rules. The aviation regulator is now probing if the co-pilot too had dozed off.The co-pilot, who was supposed to hold fort in the cockpit, claims she was busy on her tablet and did not notice that the aircraft had lost altitude. It took a call from an alarmed Ankara ATC, asking why 9W-228 had moved away from its assigned flight level of 34,000 feet, for the situation to be rectified.European ATCs in particular have become very particular about watching aircraft sticking to their flight path in this extra-sensitive region. Other aircraft had been assigned the flight level at which Jet's B-777-300 had descended to.The Directorate General of Civil Aviation (DGCA) got to know about this incident on Tuesday when its joint DG Lalit Gupta got an anonymous SMS about the "level burst", or uncontrolled descent.In its emergency communication, Ankara ATC asked the Jet aircraft to immediately ascend to 32,000 feet as this region's airspace has become very busy in recent days with airlines avoiding countries such as Iraq and Ukraine. The co-pilot immediately woke up the commander and he yoked the aircraft to 32,000 feet.On Tuesday evening, Jet confirmed the incident to Lalit Gupta, who is in charge of flight safety in DGCA. DGCA summoned both the commander and the co-pilot on Wednesday to initiate a probe into what has been termed as a "serious incident". The two have been grounded pending investigation."The commander was taking controlled rest, which means he was sleeping as per global airline norms where pilots take rest in the cockpit by turn on long flights. The co-pilot, who was supposed to ensure that the aircraft flew steadily on its assigned path, told DGCA investigators that she was on her electronic flight bag (EFB) — a tablet that has all aircraft documents loaded on to it. She did not realize that the aircraft had dropped below its assigned level," said a source who spoke to the two pilots.The regulator is now probing how the aircraft dropped 5,000 feet without the co-pilot realizing it. "Was some button pressed wrongly and what was the co-pilot doing — whether she was watching something on the tablet or she too had dozed off — needs to be probed," said a senior DGCA official.In a statement, Jet said it had initiated an internal inquiry into the matter. "The airline is also extending all cooperation to the DGCA by providing all necessary assistance for the inquiry. Safety is of paramount importance to Jet Airways, as is also the welfare of our guests and crew, and the airline will always take appropriate steps to ensure the same," it said.Following this case, DGCA has decided to audit the airline's training procedure for pilots. "The pilots should have informed the airline and they in turn should have apprised the accident investigation board about this case. The captain did not file a report. The two pilots kept flying after this case, which took place last Friday, till we grounded them on Tuesday. We got to know about it only from an anonymous complaint," said an official.