Arann AT72 at Shannon on Jul 17th 2011, nose gear collapse

Last Update: May 15, 2013 / 15:27:40 GMT/Zulu time

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Incident Facts

Date of incident
Jul 17, 2011

Classification
Accident

Aircraft Type
ATR ATR-72-200

ICAO Type Designator
AT72

The Irish AAIU released their final report concluding the probable cause of the accident was:

Excessive approach speed and inadequate control of aircraft pitch during a crosswind landing in very blustery conditions.

Contributory Factors

- Confusing wording in the FCOM that led the crew to compute an excessive wind factor in the determination of Vapp.

- Incorrect power handling technique while landing.

- Inexperience of the pilot in command.

- Inadequate information provided to flight crew regarding crosswind landing techniques.

The captain (29, ATPL, 2,882 hours total, 2,444 hours on type) was pilot flying for the sector, the first officer (37, CPL, 1,678 hours total, 1,351 hours on type) was pilot monitoring.

The AAIU analysed that both approaches were flown in blustery and turbulent conditions with the flight data recorder showing deviations of IAS by 10 knots and roll angles of up to 10 degrees. The aircraft was 5 tonnes below maximum landing weight, center of gravity was within limits and trim settings were satisfactory. The aircraft had been downgraded to CAT1 approaches only, this however was not a contributing factor. No technical anomaly occurred prior to the accident.

The AAIU continued analysis that "following examination of the DFDR, airport security videos and from the Flight Crew interviews the Investigation is satisfied that the nose gear was intact when the second landing was attempted. The nose gear then failed at ground impact as evidenced by the CVR recording an unsafe gear warning and the DFDR recording blue hydraulic failure as the nose gear folded to the rear and hydraulic lines fractured. Metallurgical testing of the nose gear stub axle indicated that its failure occurred due to a single event overload, the 2.32 g contact the nose gear made with the runway, as a result of which plastic deformation of the attachment eye and fracture of the drag brace link occurred, thus leading to the nose gear folding rearwards."

The AAIU analysed that the speed deviations were short term, typically 1 to 2 seconds, concluding the speed deviations were the result of turbulence and not windshear (longer term speed deviations). The AAIU stated: "The Investigation notes from the DFDR readouts that the gains and losses of IAS though significant were of short term duration, typically over one or two seconds, and is satisfied that the longer term average airspeed changes were due to engine power changes and not wind shear."

The AAIU reported, that a hangar had been built 20 years ago followed by pilot complaints throughout all years of turbulence in the touch down zone of runway 24 believed to be induced by the hangar. The AAIU reported that a study in 1996 concluded that there was a wake effect some 370-490 meters downwind which includes the touch down zone of runway 24 with little effect along short final. A mathematical model also confirmed the touchdown zone of runway 24 was in the wake effect of the hangar. The AAIU thus stated: "The Investigation therefore concludes that, depending on the prevailing atmospheric conditions at the time (wind velocity, turbulence, turbidity and instability), the wake effect can locally augment turbulence at the TDZ of RWY 24. The continued reports of turbulence recorded by pilots over the years lend validity to the assumption that the wake effect of the Hangar has a local effect at the TDZ of RWY 24 in strong winds between west and northwest and therefore the turbulence caution is considered warranted." The AAIU continued: "The Investigation notes that when the Hangar was originally built, RWY 13/31 was still in use and could be used by aircraft such as the ATR 72 with winds from the northwest or southeast. This is no longer the case as the runway is now de-classified and therefore approaches are conducted in wind conditions where previously the option was available to land on a TDZ unaffected by the Hangar wake effect. This places additional demands on pilots who attempt to land at their destination airport in difficult crosswind conditions and, due to the non-availability of the crosswind runway, it is inevitable that some flights divert due to crosswinds and turbulence."

The AAIU analysed that the wind from the right required the aircraft to approach the runway with a 15 degrees crab angle which reduced to 11 degrees during the flare with the aircraft entering a side slip maneouver to align with the runway, right wing low. This maneouvre assists in keeping the runway on the runway centerline in steady cross winds however in gusty conditions the ATR-72 has a tendency to weather cock. The AAIU stated: "aircraft geometry together with aileron and rudder authority limits dictate the capability of an aircraft to maintain a steady side slip close to the ground and no guidance is provided by the Manufacturer."

According to regulations, demonstrated cross wind capability and standard operating procedures by the operator the aircraft was within permitted cross wind limits for the approach, the correct cross wind speeds were applied by the crew. However, the procedures did not require to account for the wind gusts. The AAIU noted, that the operator since reduced the cross wind limits at Shannon by 5 knots.

The AAIU analysed that the Vapp was erroneously computed at 114 knots by the crew, the correct calculation would have resulted in 107 knots with Vref at 99 knots. The AAIU annotated that the wording of the relevant procedures was not sufficiently clear and could mislead crews to include full headwind velocity (not component) rather than adding 1/3 of the headwind component.

Irrespective of the computed Vapp the aircraft flared reaching 139-140 knots, about 20-25 knots above computed/correct Vapp.

The AAIU analysed: "In such gusty crosswind conditions the key focus of the pilot must remain on handling the aircraft and keeping it aligned on the runway centreline after the flare. In turbulence, such as that which was experienced, the ability of a pilot to monitor airspeed is significantly reduced and assistance should be given by a co-pilot calling airspeed deviation from Vapp. The Investigation notes in the CVR recording the absence of SOP speed callouts being requested or given during both approaches, which would have assisted the PF in controlling airspeed in such conditions. It therefore recommends that the Operator reviews this issue during its training."

The AAIU analysed that during the first approach the aircraft floated long and needed to be pushed down onto the runway resulting in the nose gear making ground contact first resulting in a vertical acceleration of 1.3G followed by a sharp pitch up, subsequently the main gear "rocked" onto the runway resulting in 1.7G vertical acceleration. The AAIU stated: "As the landing gear subsequently retracted normally the Investigation is of the opinion that no significant damage was caused to the nose gear during the first landing."

The AAIU analysed that following the first approach the crew should have discussed the conditions of the first approach and prevailing weather conditions which would have resulted in a diversion. Instead the crew immediately requested another approach, chose a different touchdown position this time in the standard touch down zone due to the long float in the first approach. During flare engine power was added, "which resulted in increased airspeed, the aircraft bouncing and not descending onto the runway. Ultimately the control column was pushed forward and the aircraft reached a significant nose down attitude, impacting the runway at 2.3g. This caused the nose gear to fracture and the hydraulics to fail."

With respect to human factors the AAIU analysed: "The Investigation believes that the severity of the prevailing conditions was not recognised by either of the Flight Crew, either before or after the go-around. Whilst the objective was to land at EINN, this was not evaluated against the prevailing weather conditions and their relative experience levels. Although the CVR recorded discussions regarding turbulence from the Hangar and crosswinds, the Flight Crew appeared to not unreasonably consider that the turbulence was due to the Hangar and that this turbulence could be avoided by landing in a different position on the runway. However, the CVR recorded that the stress levels of the PF had increased after the first approach."

The AAIU analysed that the operator had experienced two other landing events with their ATR-72 which resulted in nose gear damage. In both events the aircraft had bounced during landing in gusting crosswind. The AAIU thus concluded: "These bounces being the result of unsuccessful crosswind landing attempts, the Investigation is therefore of the opinion that the Operator should review the crosswind landing training it provides to its pilots, particularly in gusty conditions, and issues a Safety Recommendation in this regard."

Four safety recommendations were released as result of the final report.
Incident Facts

Date of incident
Jul 17, 2011

Classification
Accident

Aircraft Type
ATR ATR-72-200

ICAO Type Designator
AT72

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