French investigators have disclosed that Airbus is introducing a modification for conventional A320-family aircraft to prevent excessive pitch in the event of a false-glideslope encounter.

The development is contained in the investigation authority BEA’s findings into the unexpected pitch-up by an Air France A318 after it intercepted a false 9° ILS glideslope during an approach to Toulon in December 2019.

Such an interception causes a reversal of the autopilot response compared with the true 3° glideslope, and BEA says the aircraft rapidly pitched to a nose-up attitude of 30°.

BEA says the implementation of pitch-up excursion protection in the avionics of the latest Airbus models – the A380, A350 and A320neo – avoids the excessive attitude.

It states that the protection will be rolled out across all version of flight computers fitted to the conventional A320 family from 2023.


Source: Airbus

BEA states that the modification for older A320-family variants will be available from 2023

Dutch investigators carried out an analysis of false-glideslope events after an incident in Eindhoven, in May 2013, involving a Ryanair Boeing 737-800 that intercepted a 9° glideslope.

This triggered a pitch-up upset and caused the airspeed to drop, despite the autothrottle’s commanding higher thrust, leading to stick-shaker activation – circumstances similar to those encountered by the A318 crew.

The Dutch Safety Board’s examination, published in June 2014, stated that Boeing was incorporating a software change into the flight-control computer which would limit the climb rate in glideslope mode.

“This change has been shown to eliminate the pitch-up when the [aircraft] captures the reversed-signal 9° glideslope,” it added.

Boeing also told the board that simulations had shown that other models – including the 747, 757, 767, 777 and 787 – used a different glideslope interception and anomaly-detection logic which prevented their capturing the false 9° signal.

Air France published a document for pilots last year, in the aftermath of the A318 incident, regarding the interception of glideslopes from above.

It states that aircraft above a 5° slope with respect to the threshold face a “significant risk” of capturing a side lobe from the ILS transmissions, and therefore encountering a false glideslope – pointing out that a simple height-to-distance check can help pilots assess the situation.