High descent profile exposed A340 to glideslope trap

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Pilots of an Air France Airbus A340-300 fell into a false glideslope trap as the jet descended far above the normal approach profile, leading to serious instability during the low-visibility landing.

The aircraft, arriving from the Malian capital Bamako, had been attempting a Category III approach to Paris Charles de Gaulle's runway 08R.

It had been initially cleared to 8,000ft while 30nm from the threshold, maintaining 250kt, but almost immediately overflew the top of descent for a 3° glideslope and began diverging from the normal approach profile.

While traffic constraints had initially forced the approach controller to keep the A340 above the glideslope, he subsequently forgot to clear the crew for further descent. At 17.5nm from the threshold, the aircraft was 1,275ft too high.

The controller apologised for his error, clearing the crew to descend to 3,000ft and intercept the glideslope from above.

French investigation authority BEA says the controller assumed the crew would mention any difficulty with performing the manoeuvre. But the crew, it says, similarly assumed that the controller would provide the necessary guidance.

"Each thought that the other would report to them if there was a problem," it states.

Instead of using the A340's vertical-speed mode, the crew selected the open-descent mode, which prioritised reduction of the airspeed over the acquisition of the necessary altitude. This slowed the descent and exacerbated the deviation from the glideslope.

When the flight was transferred to Paris tower, the controllers were not informed that the A340 was above the glideslope. Further instructions to reduce speed continued to worsen the deviation and, 4nm from the threshold, the aircraft was still at 3,700ft - around 2,100ft too high.

Such was the height discrepancy that the aircraft became vulnerable to a false glideslope. The phenomenon is caused by mirroring of the ILS antenna transmissions, and creates the illusion of a second glideslope located above - and therefore steeper than - the true one.

When the A340's glideslope capture engaged, at 2,850ft, the aircraft rapidly pitched up, reaching 12° nose-up in 12s. As the attitude increased, the airspeed fell from 163kt to just 130kt. The crew disengaged the autopilot, and the captain pushed his sidestick fully forward to raise the airspeed and avert the risk of a stall.

BEA says the aircraft passed the runway threshold at 3,700ft. The captain realised that the approach was unstable and executed a go-around at 2,000ft. The inquiry attributes the serious incident - which took place on 13 March 2012 - to inadequate monitoring of the A340's trajectory by both crew and controller, and the pilots' using an "inappropriate" technique to capture the glideslope from above.