French investigators have found a controller at Nice missed a prolonged safe-altitude alarm and only intervened to halt a Bombardier CRJ200’s low approach after a separate system warning.
The Armavia aircraft, arriving from Yerevan at night, descended to just 550ft while still some 7nm (13km) east of the airport.
While a minimum safe altitude warning sounded the controller only responded to an area proximity warning which began 11s later.
French investigation authority BEA says the crew originally agreed to a runway 04L approach. But after the pilots missed a radial intercept, the controller directed them instead to a runway 22R approach.
The CRJ approached from the east at 3,000ft and, around the final approach fix, began to descend to 1,500ft – the minimum descent altitude.
But the BEA says the aircraft continued to lose height, passing through this minimum altitude while 10nm from the airport, and then deviated to the right of the approach path.
The safe-altitude warning triggered but was not acknowledged by the Nice controller, who responded only after the subsequent area proximity warning.
As the CRJ descended at 1,700ft/min the controller urgently told the pilots that they were “mistaking the bay”, indicating that the crew had misidentified a bay east of Cap Ferrat as the one in which the airport lay.
The aircraft turned to the left and began to climb, and the altitude warning – which had sounded for 41s – stopped.
In its analysis of the incident the BEA says the controller “focused his attention on managing the aircraft’s trajectory in the horizontal plane, to the detriment of management in the vertical”.
It adds that the CRJ’s crew had been “saturated” by the change in runway and the complexity of the approach procedure. The flight was the captain’s third to Nice, and the first officer’s second.
Although the inquiry credits the controller’s reaction to the area proximity warning – which signals an airspace deviation – it says the failure to notice the safe-altitude alarm delayed the intervention.
BEA says the safe-altitude alarm was prone to nuisance alerts and audio was inhibited to an extent. The Armavia incident occurred on 30 December 2011 and, over the following January, only two of the 57 alerts generated were considered reasonable.
Analysis of Montpellier and Marseille airport operations revealed “significantly lower” nuisance alarm rates than Nice, the inquiry adds. BEA investigators have recommended that steps be taken to cut the number of unnecessary alerts from safe-altitude warning systems.