Tailwind B734 at Lyon on Sep 7th 2010, descended below glide on LOC approach

Last Update: April 22, 2013 / 15:07:12 GMT/Zulu time

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Incident Facts

Date of incident
Sep 7, 2010

Classification
Report

Flight number
TU-750

Destination
Lyon, France

Aircraft Registration
TC-TLE

Aircraft Type
Boeing 737-400

ICAO Type Designator
B734

A Tailwind Airlines Boeing 737-400 on behalf of Tunis Air, registration TC-TLE performing flight TU-750 from Tunis (Tunisia) to Lyon (France), was on an approach to Lyon's runway 36R, there were few cloud at 500 feet AGL near the minimum descent altitude for the localizer/DME procedure to runway 36R, the glideslope transmitter was inoperative due to maintenance works. ATIS had informed crews of visibility of 10km, light rain, few clouds at 1800 feet, scattered cumulus cloud at 3100 feet, the glideslope transmitter 36R was out of service, runway 36R was active for landings. The aircraft was vectored to intercept the localizer at 3000 feet, the preceding approach reported they broke out of cloud at 1250 feet stating "we really drilled minima" and recommending to return the glideslope to service or change to runway 36L. Tower relayed the information to TU-750 stating the preceding aircraft broke out of cloud at 1200 feet, no reply. 15 seconds later tower again relayed the information, at that time a radar warning activated at the tower indicating the aircraft had descended below minimum safe altitude, tower responded by instructing TU-750 to go-around, the crew acknowledged to go around. The aircraft climbed out to safety, positioned for another approach to runway 36L this time and landed safely.

According to radar data the aircraft had descend to 250 feet AGL about 1.4nm before the runway threshold (editorial note: on a 3 degree glide required height would be 450 feet).

The BEA released their final report in French (English version released on Apr 22nd 2013) concluding the probable causes of the serious incident were:

- an incorrect identification of the final approach fix on the approach chart
- inadequate monitoring of the final approach vertical profile by the crew

Contributing factors were:

- the publishing of two final approach fixes as widespread on approach charts requiring to identify which one of the two to be used
- lack of information to the crew to identify the exact procedure to be used

The BEA complained, that following the minimum altitude alert issued at the control tower ATC prepared a safety report, which was handed to the BEA the following morning only, the crew did not notify the BEA at all. This prevented the BEA from downloading cockpit voice and flight data recorders.

In addition the BEA reported that according to documentation forwarded it was initially believed the supervising authority of the aircraft would be Tunisia, an according report was filed with Tunisia's CAA on Sep 8th, it turned out the Turkish CAA was responsible authority for the aircraft on Sep 9th and a report was filed with Turkey on Sep 9th, the NTSB representing the state of manufacture of the aircraft was notified as well.

The crew consisted of a captain (64, ATPL, 22,310 hours total, 16,100 hours on type) and a first officer (31, CPL, 1,150 hours total, 700 hours on type).

Approach control had initially cleared the aircraft to 4000 feet while on vectors for final approach, then cleared the crew to intercept the localizer at heading 320, the crew did not read back the heading. After the crew reported passing through the localizer, approach issued an updated vector of heading 020, cleared the aircraft to descend to 3000 feet and cleared the aircraft for the Localizer/DME approach to runway 36R, all instructions were read back correctly. The aircraft was subsequently handed off to tower.

The BEA annotated, that all aircraft were cleared to 3000 feet to begin their localizer approach during the period of time, the BEA processed the ATC tapes of that day. One of the most common approach charts depicted a localizer approach from 4000 feet showing an option for ATC clearance to 3000 feet and thus creating two final approach fixes.

The BEA reported that according to Boeing and operator flight crew operating manual the aircraft should be configured for landing before reaching the final approach fix (FAF), at crossing the FAF the MCP vertical speed should be adjusted to the previously computed rate of descent, the pilot monitoring then is responsible to monitor crossing points and intermediate altitudes. Upon reaching the minimum descent altitude (MDA) the pilot flying should call out whether the threshold of the runway is in sight permitting the approach to be continued below MDA or not requiring a go-around to be commenced.

The BEA analysed that the crew was never told the full title of the approach to be used, Localizer DME Y or Z approach to runway 36R with the Y/Z identification omitted, it was logical however that procedure Y would be used (intercept at 4000 feet). Due to lack of recordings available the BEA could not determine whether the crew had considered the possibility of procedure Z (intercept at 3000 feet) being used.

The aircraft was level at 3000 feet about 2:20 minutes prior to starting the final approach, the crew did not perceive the FAF at 10 DME (as provided for an intercept at 4000 feet) unusual. The aircraft thus began the descent at that FAF.

Multiple segments indicating minimum altitudes at fixed DME distances were provided on the vertical profile, the aircraft had to remain above such minimum safe altitudes for each of the segments at all times. Besides the use of EGPWS this was the crew's only safe guard against terrain obstacles. The BEA analysed, that this monitoring of segments, crossing points and intermediate altitudes has clearly not been applied.

The BEA further analysed that it appeared the crew did not understand why they had been instructed to go around and stated that the instruction to go-around did not match standard phraseology.

4 safety recommendations were released as result of the investigation, amongst them to discontinue the publication of two FAFs on the same approach chart and require controllers to fully identify the procedure to be used.
Incident Facts

Date of incident
Sep 7, 2010

Classification
Report

Flight number
TU-750

Destination
Lyon, France

Aircraft Registration
TC-TLE

Aircraft Type
Boeing 737-400

ICAO Type Designator
B734

This article is published under license from Avherald.com. © of text by Avherald.com.
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