The French aviation investigation agency, BEA, is recommending operational or design changes to autopilot systems after an upset incident of an Air France A340-300 on approach to Paris Charles de Gaulle Airport (CDG) early on the morning of March 12, 2012.

The incident occurred as the A340 approached CDG at 4:40 am in low visibility conditions after an overnight flight from Mali. Due to the visibility, the crew was using a Category 3 instrument approach system (ILS) approach, which allows pilots to use the autopilot to bring the aircraft down to as low as 50 ft. above the runway before taking manual control.

The autopilot uses lateral reference data from a localizer and vertical guidance from a glideslope, both of which are transmitted from ground stations near the runway.

Due, in part, to miscommunications between pilots and controllers, the A340 was too high late in the approach, resulting in its automation system selecting “sidelobe” of the glideslope signal for its guidance. While the autopilot normally commands the aircraft to converge on a 3 degree slope inbound to the runway, if the system becomes locked on the sidelobe, the autopilot signals become inverted. 

Pilots and controllers are aware of the potential problem, particularly when intercepting a glideslope from above.

According to the BEA, the A340 was 1,600 ft. above its proper glidepath at a point 2 nm from the runway 08 Right threshold when the pilot activated the “glide path capture mode” on the autopilot and the system began controlling to the sidelobe.

Rather than reducing the A340’s pitch to capture the glideslope, the autopilot commanded the nose upward.

“The pitch attitude increased from 1 deg. to 26 deg. in 12 seconds,” says the BEA. “The [pilot not flying] stated that he had called out the difference in the pitch attitude when the chevrons appeared.” The BEA explains that on the A340, when the pitch attitude reaches 30 deg., chevrons appear on the primary flight display showing the direction that the pilot should pitch the aircraft to recover from a nose-high attitude.

The BEA says the aircraft’s speed dropped from 163 kt. to 130 kt. during the pitch-up, with vertical speed changing from 1,600 ft./min. downward to 3,300 ft./min. upward. “When the pitch attitude reached 26 deg., the crew disconnected both autopilots and the [pilot flying] made a pitch down input almost down to the stop,” the BEA says.

The pilot later took manual control of the aircraft and performed a go-around and returned to the airport to land.

In the final incident report, published Sept. 14, the BEA attributed the incident to inadequate monitoring of the aircraft’s flight path by the controller and by the crew during the approach, as well as the crew’s decision not to abandon the approach earlier. Contributing factors include the autopilot’s capturing of the ILS signal from a sidelobe.

The agency also says fatigue may have contributed both for the pilots and the controller.

The BEA is recommending that the European Aviation Safety Agency (EASA) “ensure” that aircraft ILS modes are not engaged on the wrong signal. “Failing this, a system enabling the crew to be alerted be put in place,” the BEA says.

EASA also is called on to ensure that regulators define explicit operational limits for crews when intercepting glideslopes from above.

The BEA is asking French regulators to investigate tools that would give controllers a way to determine the vertical position of an aircraft in relation to glide path.