Austrian DH8D at Innsbruck on Apr 4th 2019, tail strike on landing

Last Update: February 5, 2022 / 18:07:09 GMT/Zulu time

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Incident Facts

Date of incident
Apr 4, 2019

Classification
Accident

Flight number
OS-903

Aircraft Registration
OE-LGM

ICAO Type Designator
DH8D

Airport ICAO Code
LOWI

An Austrian Airlines de Havilland Dash 8-400, registration OE-LGM performing flight OS-903 from Vienna to Innsbruck (Austria), landed on Innsbruck's runway 08 but struck its tail onto the runway. The aircraft rolled out without further incident and taxied to the gate.

The return flight was cancelled.

The airline confirmed the tail strike commenting the passengers did not become aware of it. Trails were visible on the tail section of the aircraft. At the time of landing there was a strong foehn storm which probably contributed to the occurrence. The aircraft is being examined, the damage is to be assessed. The return flight OS-904 needed to be cancelled, the passengers were rebooked onto other flights.

The occurrence aircraft remained on the ground for two months and returned to service on Jun 4th 2019 with flight OS-1492 from Innsbruch to Vienna.

On Jun 27th 2019 Germany's BFU reported the occurrence was rated an accident, the aircraft sustained substantial damage when it suffered a tail strike on landing. The BFU is assisting the investigation representing the country of first officer according to ICAO Annex 13.

The Austrian SUB released their final report concluding the probable cause of the accident was:

Sudden decrease in wind speed just before touchdown (Low-level negative wind shear)

The SUB analysed:

At the time of the occurrence, good visibility and moderate cloud cover prevailed at Innsbruck Airport. The southern foehn situation that was just building up indicated possible wind shear and turbulence. Warnings of wind shear for the airport were only active until 07:00. Therefore, warnings were not active at the time of landing or in the hour before. In the MET REPORT, the wind shear warning was removed at 07:50. At the same time, turbulence warnings were downgraded from MOD/SEV (medium to severe) to MOD (medium). The crew was aware of the information about "moderate turbulence".

After the situation had been discussed, there was no change of pilot flying. The approach was described as turbulent. Nevertheless, based on the available information the wind shear that was flown through about three seconds before landing could not be anticipated in this form. This wind shear or downdraught immediately before landing can be regarded as the cause of the accident, and as crucial to the further chain of events, including the contact of the tail of the aircraft with the runway.

Handling of the weather situation in Innsbruck

Due to the specific location of Innsbruck Airport, the special meteorological characteristics in Innsbruck and their handling were discussed in the course of the accident investigation with the aviation meteorological service located at Innsbruck Airport. Up to this accident, the practice in a classic foehn situation was that wind shear warnings (Bulletin, METAR, MET REPORT) were only issued until the foehn breakthrough, as long as the southerly foehn wind in the approach and aerodrome area meets the westerly wind from the Upper Inn Valley. If the south or south-east wind also prevailed in the direction of the Upper Inn Valley, the wind shear warning was omitted and only the turbulence warning for the approach remained.

Although this practice is correct from a meteorological point of view, it was possible that pilots could sometimes seem to interpret that information differently. It is conceivable and not uncommon for wind shear to be perceived as more dangerous than turbulence.

One of the reasons for this is that wind shear tends to be perceived as a danger in combination with the inherent risk during take-offs and landings, whereas turbulence is usually more often connected to cruising flight. This unconscious prioritisation of wind shear to be more dangerous than turbulence, ultimately leads to the fallacy that the cancellation of a wind shear warning is considered as a relaxation of the wind situation.

This also applies if the turbulence warning persists. The correct procedure of the Innsbruck aviation meteorological service could therefore possibly inadvertently convey that the situation is relaxing or less risky as soon as the foehn has established itself, since no further wind shear warning is issued. However, this is a fallacy, as approaches are by no means easier, because the foehn wind from the south or south-east in the aerodrome area is much more gusty and turbulent than the westerly wind. Based on these considerations, the Innsbruck aviation meteorological service concluded that the wind shear warnings should be maintained even after the foehn breakthrough.

This is justified, among other things, by the fact that after the foehn breakthrough there would indeed no longer be two air masses meeting at a shear plane. Even so, wind shears in the form of turbulences and rotors occur in the same way, if not more intensely.

History of flight

Visibility was good during the approach and at the time of the accident at 08:56. The crew was informed comprehensively about the weather situation both before and during the flight, and they obtained weather information themselves. A warning regarding wind shear that was issued earlier that day was no longer valid at the time of the accident. The crew had known at least since the pre-flight briefing that the approach would begin at a time close to a southern foehn situation that was currently forming.

During the pre-flight briefing, it was discussed that the experienced captain, who was familiar with such weather situations, would perform the approach and landing if the conditions deteriorated. Based on the available weather information, the flight crew did not make use of this possibility.

The flight preparation and also the in-flight weather evaluations were carried out in accordance with the applicable standards and regulations. The approach was performed by the co-pilot in accordance with the published foehn approach procedure. The criteria for a stabilized approach were briefly exceeded on the final approach, but these were recognized immediately and corrected in a targeted manner. The approach is therefore to be designated as a stabilized approach in accordance with OM-A 8.4.5.15.

The exceedance of the nose-up pitch limit while touching down was minimal (7.65° when touching down vs. 7.6° mechanical limit with full compression of the struts, see section 1.6.7). It is possible that the operationally necessary crown of the runway contributed to the accident.

In the very last phase of the approach, there was a sudden decrease in wind speed. The increase in engine power in response was correct, but it came too late. The decrease in wind speed did not occur until about 3 seconds before touchdown, which in itself gives the pilot very little time to react. If more power had been set, this would have further increased the nose-up pitching moment. Taking into account the reaction time of the pilot and the inertia of the aircraft, if at all, contact of the tail with the runway in the given circumstances could only have been prevented if a higher vertical acceleration (G-load) was accepted during landing.

Wind shear and turbulence are typical of foehn conditions. Fluctuations in wind speed, as in the present case, can occur not only in combination with foehn conditions but generally at any time. Whether an immediately initiated go-around manoeuvre would have prevented the tail of the aircraft from contacting the runway cannot be clearly deduced from the parameters available.

Metars:
LOWI 041020Z VRB09G27KT 9999 FEW060 SCT150 17/02 Q0994 NOSIG=
LOWI 040950Z VRB09G29KT 9999 FEW060 SCT150 17/02 Q0994 NOSIG=
LOWI 040920Z 04014G24KT 360V070 9999 FEW060 SCT080 16/02 Q0993 NOSIG=
LOWI 040850Z 06013KT 010V130 9999 FEW060 SCT080 16/02 Q0993 NOSIG=
LOWI 040820Z 08013G24KT 040V130 9999 FEW060 SCT080 16/02 Q0993 NOSIG=
LOWI 040750Z 08012G23KT 340V140 9999 FEW060 SCT080 15/02 Q0994 NOSIG=
LOWI 040720Z 07015KT 030V120 9999 FEW060 SCT080 15/01 Q0993 WS ALL RWY NOSIG=
LOWI 040650Z 04018KT 020V090 9999 FEW050 SCT080 14/01 Q0993 WS ALL RWY NOSIG=
LOWI 040620Z 05016G28KT 010V110 9999 FEW050 SCT080 13/M00 Q0994 WS ALL RWY NOSIG=
LOWI 040550Z 04011KT 350V120 9999 FEW050 SCT080 13/01 Q0994 WS ALL RWY NOSIG=
Incident Facts

Date of incident
Apr 4, 2019

Classification
Accident

Flight number
OS-903

Aircraft Registration
OE-LGM

ICAO Type Designator
DH8D

Airport ICAO Code
LOWI

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