Tunisair B736 at Geneva on Nov 24th 2012, runway excursion on landing

Last Update: April 26, 2015 / 20:06:09 GMT/Zulu time

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Photo of Tunisair TS-IOL, Boeing 737-600 — Tunisair TS-IOL, Boeing 737-600 (Photo credit: Tab59 / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Nov 24, 2012

Classification
Incident

Cause
Runway excursion

Airline
Tunisair

Flight number
TU-700

Departure
Tunis, Tunisia

Destination
Geneva, Switzerland

Aircraft Registration
TS-IOL

Aircraft Type
Boeing 737-600

ICAO Type Designator
B736

A Tunisair Boeing 737-600, registration TS-IOL performing flight TU-700 from Tunis (Tunisia) to Geneva (Switzerland) with 124 passengers and 6 crew, landed on Geneva's runway 23 in reduced visibility when the aircraft contacted lights at the left runway edge at 09:50L (08:50Z). The aircraft vacated the runway and taxied to the apron, no injuries occurred.

Switzerland's BFU opened an investigation into the incident reporting the aircraft damaged two lights at the left runway edge during roll out in reduced visibility.

The incident aircraft was able to perform the return flight on schedule.

In April 2015 Switzerland's SUST (former Swiss BFU) released their final report in French concluding the probable cause of the serious incident was:

The serious incident has been caused by a brief runway excursion shortly after touchdown when spatial disorientation had resulted in loss of control.

Inappropriate use of the automated flight control systems contributed to the serious incident.

SUST reported that the first officer (30, CPL, total hours not reported, 1,522 hours on type) was pilot flying, the captain (45, ATPL, 8,565 hours total, 1,200 hours on type) was pilot monitoring. When the aircraft was descending towards Geneva in dawn, mist was forming reducing horizontal visibility to 800 meters, vertical visibility to 200 meters and darkening the sky, Geneva Airport had thus invoked low visibility procedures with runway 23 being the active runway.

The aircraft ws guided for an ILS approach to runway 23 with the instruction to maintain 150KIAS or more until 4nm. Approach control communicated low visibility procedures were in effect, RVR was 500 meters. While the aircraft remained on approach frequency, approach told a number of other aircraft RVRs of 400 and 450 meters. The aircraft was cleared for the ILS approach, the crew decided to perform the intercept of the localizer and glideslope on automation engaging two autopilots.

Upon contacting tower tower advised RVR in the touch down zone was 550 meters, in the middle section of runway 375 meters and cleared the aircraft to land on runway 23, the aircraft was 5.4nm before touchdown at that point.

When the aircraft reached decision height the captain announced runway in sight and the first officer disengaged the autopilot and increased the attitude. The aircraft crossed the runway threshold at 92 feet and passed over the aiming point at 47 feet AGL maintaining that height. About 10 seconds after crossing the threshold the aircraft turns left prompting the commander to announce "Control Left" (editorial note: probably with the meaning "I have control"), the commander pushed the aircraft down, the aircraft descended rapidly and touched down 1290 meters past the runway threshold near the left edge of the runway. A few seconds later, about 1400 meters down the runway, the aircraft crossed the left edge of the runway, contacts and takes out a runway edge light while running 120 meters over soft ground before returning onto the runway. The aircraft rolled out without further incident and taxied to the apron.

In the meantime the control tower received an alert of total failure of runway edge lights, a power outage was ruled out as cause. A maintenance engineer showed up an hour later and identified three runway edge lights had been damaged.

A passenger told ground staff that the aircraft had landed out of the runway and was rolling over grass, this information was relayed to tower.

When the crew reported ready for departure for the return flight TS-701, tower queried the crew about the passenger statement, the commander reported they had a hard landing at the left side of the runway however they did not go over grass. The engineer's finding then prompted tower to recognize a relationship between the runway light alert, the aircraft landing, the damaged lights and the passenger report, informed the supervisor who in turn requested another runway inspection. The runway inspection reported that there was no debris remaining on the runway, there had been two runway edge lights damaged and there were tracks of wheels on soft ground exiting and entering the runway.

SUST reported the flight data recorder showed the autopilots remaining engaged until CAT I decision height, then the autopilots were disengaged and the aircraft began to roll right and left reaching bank angles of 5-10 degrees. The pitch changes from 3.2 to 7 units nose up and the aircraft levelled off at 47 feet AGL crossing the aiming point at that height. The aircraft continued along the runway center line for another 10 seconds at 129 knots over ground, then the pitch changes to 4 degrees and the aircraft rapidly descends onto the runway touching down at a vertical acceleration of +2.5G before going off the left edge of the runway.

The captain informed maintenance about the hard landing, no grass was foun on the tyres, a hard landing inspection was not required if the crew determined that landing did not occur with all wheels simultaneously or the nose gear first. The maintenance engineer required the captain to sign the technical log with the annotation that a hard landing inspection was not required. The aircraft performed return flight TS-701, in Tunis a tear on the inside of the outboard left main tyre was discovered and the wheel was replaced.

After learning the following morning, that the aircraft had gone off the runway and tyre damage had been found in Tunis, the maintenance engineer reported the event to Tunis to suggest the hard landing inspections to be carried out.

SUST analysed that there were no technical defects with the aircraft or airport that contributed to the sequence of events. The visibility was sufficient for the ILS approach.

The flight was uneventful and stabilized until the autopilots were disengaged. As the aircraft was pitched up and started to maintain 47 AGL, it went past the touch down zone and entered the middle zone of RVR of 375 meters insufficient for a CAT I landing. The continuation of the landing therefore was inappropriate, a go-around should have been initiated.

SUST analysed that the crew performed a CAT I ILS approach in weather conditions that were marginal for CAT I, with two autopilots coupled for the approach instead of one autopilot coupled only as required for CAT I. While the coupling of two autopilots permits for an automated go-around, the coupling of two autopilots changes how autothrust and autopilot interact to balance the aircraft below decision height, with two autopilots engaged the automatic flight system is designed only to be disengaged after touch down.

With two autopilot's coupled approaching decision height the aircraft's trim is set to an equilibrium requiring the thrust to be reduced or nose down inputs for continuing the descent. As the autopilot was disengaged at decision height but autothrust was kept active, the aircraft was trimmed in a way to take the nose up, the autothrust however did not reduce thrust until below 27 feet AGL and there were no nose down inputs. The first officer should have reduced thrust to continue the descent.

The use of two autopilots for a CAT I approach thus is being considered inappropriate. Disengaging the autopilot at decision height destabilized the aircraft, the autothrust was not disengaged and thus kept the speed set as target until the aircraft descended below 27 feet AGL, the combination of the aircraft's equilibrium pitching the nose up and the autothrust system maintaining speed thus diverted the aircraft from its intended trajectory and caused temporary loss of control.

Metars:
LSGG 240950Z VRB01KT 1200 R05/0600U R23/0700V2000U BCFG BKN002 07/07 Q1020 BECMG 1500 BR
LSGG 240920Z VRB02KT- 0250 R05/0375V0500N R23/0400V0550D FG VV001 06/06 Q1020 BECMG 1200 BCFG BKN003
LSGG 240850Z VRB03KT 0300 R05/0325N R23/0900VP2000D FG VV002 07/07 Q1020 NOSIG
LSGG 240820Z 24002KT 0700 R05/0900VP2000D R23/0800VP2000U FG VV002 06/06 Q1021 NOSIG
LSGG 240750Z VRB02KT 0800 R05/P2000D R23/P2000N FG VV002 05/05 Q1021 NOSIG
LSGG 240720Z VRB01KT 2500 PRFG BR SCT002 BKN140 05/04 Q1020 TEMPO 1500 BCFG
LSGG 240650Z VRB02KT 3000 PRFG BR BKN003 05/04 Q1021 TEMPO 1500 BCFG
LSGG 240620Z 29001KT 3500 BR SCT004 BKN140 05/04 Q1020 BECMG 3000
LSGG 240550Z VRB01KT 4000 BR FEW005 BKN140 04/03 Q1021 BECMG 1500 BCFG