The Dutch government’s safety board wants to publicize the existence of false glideslope indications that could cause the aircraft, when coupled to the autopilot, to pitch up rather than down. The insights were gathered during an investigation into a pitch-up incident on a Boeing 737 in which the incident “digressed” until the aircraft’s stick shaker activated.
The board wants pilots to understand the dangerous information these false glideslope signals can send to an aircraft’s autopilot that might cause the system to operate in a manner opposite to what the cockpit crew expects.
The report, published last month, focused too on the result of the unexpected pitch up. “The resulting loss of airspeed may cause the aircraft to stall,” the report said. The Dutch authorities said that while false glideslope indications are not new, until recently it was believed that even false signals would head the aircraft in the proper direction, down and toward the runway, just at a higher rate of descent.
Further investigations revealed that four similar incidents have occurred in Europe, while 19 were found in a U.S. database, all at different airports and involving different operators.