French investigators are reiterating that carriers should establish specific operating limits for crews attempting to capture a glideslope from above, after Air France suffered two serious destabilisation incidents within two weeks.
Investigating authority BEA has found that an Airbus A319 crew had been preparing for a descent to runway 29 at Tunis but was then offered the straight-in approach to runway 19.
This change shortened the approach by 20nm but also meant the aircraft was far above the 3° glideslope.
The captain, flying, selected the open-descent mode which prioritises airspeed over acquisition of the required altitude.
While 33nm from the threshold the aircraft was still above flight level 200, putting it some 10,000ft above the glideslope, travelling at 276kt.
It was cleared for the instrument approach to runway 19 and descended at 5,000ft/min, chasing the glideslope. But the aircraft slowed and reduced its descent rate, failing to close the gap by the time the captain disengaged the autopilot - with the aircraft still at 10,000ft just 14nm out.
BEA says the crew executed a procedure for capturing the glideslope from above - which involved switching to vertical-speed mode - but the subsequent descent rate of 4,400ft/min far exceeded a restriction of 2,500ft/min.
This rapid descent continued and, after air traffic control had given landing clearance, the first officer advised that the aircraft was "a bit high". The aircraft passed through the glideslope 2.8nm from the threshold, still flying at 220kt and descending at 2,200ft/min.
It continued to descend to 398ft above ground - triggering sink-rate warnings and a "pull up" order - before the crew applied go-around thrust. The aircraft later landed safely after a visual approach to the same runway.
The incident, on 24 March 2012, occurred just 11 days after an Air France A340's low-visibility approach to Paris Charles de Gaulle was destabilised by a false glideslope capture from above.
BEA says the A319 crew embarked on an unstable approach to Tunis having failed to communicate adequately and plan properly following the change of runway.
Although the captain realised the approach was "compromised", says BEA, he was encouraged to pursue the glideslope as a result of the good weather. Although the first officer twice informed the captain that the aircraft was too high, he said he "did not want to encroach" on the captain's decisions.