Tailwind 737's low approach traced to fix confusion

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French investigators believe the pilots of a Turkish-operated Boeing 737-400 misidentified a crucial waypoint on approach to Lyon, resulting in a premature descent which generated minimum safe altitude alerts.

The Tailwind Airlines aircraft had been cleared to descend to 3,000ft and carry out a localiser approach to runway 36R, as the glideslope was unavailable.

But while the procedure requires a descent from 6.9nm (12.8km) out, the aircraft began to lose altitude at about 10nm.

At the time the cloud base was around 1,200ft, close to the minimum descent altitude, and a preceding flight's crew had noted this in an exclamation to air traffic control.

The 737 stayed below the normal glideslope and eventually triggered a minimum altitude warning, prompting the controller to order a go-around.

French investigation authority BEA says the crew was unaware of the reason for the go-around instruction. The aircraft had descended to 250ft above ground level while still 1.4nm from the runway.

None of the 105 occupants was injured during the 7 September 2010 incident. The aircraft (TC-TLE) had been operating on behalf of Tunisair.

Inquiries determined that the pilots had erroneously identified the final approach point. France's aeronautical charts indicated the approach should begin at 10nm after clearance to 4,000ft but also featured a 6.9nm fix for approaches from 3,000ft.

The BEA acknowledges that the crew was not fully informed as to which approach should be performed, but questions the pilots' control of their descent.

It points out that, nine days later, another crew on the same approach detected their own similar error by routinely cross-checking their distance and altitude. The pilots corrected their descent at 2,400ft without triggering any alarms.

BEA says the two approach fixes on the same chart contributed to the Tailwind incident and has recommended that charts featuring multiple fixes be removed.