Description

On 21 January 2002, an Airbus A321-100 being operated by All Nippon Airways on a scheduled passenger flight from Nagoya to Hakodate encountered sudden negative windshear just prior to planned touchdown and the pitch up which followed resulted in the aft fuselage being damaged prior to the initiation of a climb away to position for a further approach which led to a normal landing. Three of the cabin crew sustained minor injuries but the remaining 90 occupants were uninjured.

Investigation

An Investigation was carried out by the Japanese Aircraft and Railway Accidents Investigation Commission. It was established that the First Officer had been PF for the approach and that the Instrument Landing System (ILS) flown had been normal and made into a strong but somewhat gusty headwind with fairly constant direction and unexceptional speed fluctuation between 20 and 40 knots until below 100 feet agl. The Autopilot had been disconnected at 500 feet agl. It was noted that although both predictive and reactive wind shear detection was active on the aircraft, only a reactive alert had been generated by the wind shear encountered.

Examination of DFDR data showed that when the aircraft was below 100 feet agl, there was a lull in the wind speed and CAS dropped by around 36 knots during a period of seven seconds to reach its minimum value prior to fuselage ground impact of around 117 knots with a rate of descent of around 1220 ft/min. Following the reactive wind shear alert which occurred as a consequence, the Captain, although still PNF, operated his side stick simultaneously with the First Officer without first operating the take-over button on his side stick or announcing that he was taking control. As a result of these combined control inputs, aircraft pitch attitude briefly reached 15.5° at a time when α Floor Protection, which is designed so that an pitch-up initiates the advance of the thrust levers to takeoff position automatically was likely to be inoperative because the flight crew response to the wind shear alert occurred below 100 feet agl. With the rate of descent still approximately 1220 fpm, the aircraft rear fuselage struck the ground with a 1.9g impact and a pitch angle of 11.2 degrees. Ground contact of both main landing gears followed and then, after a short period of simultaneous and conflicting pitch inputs on the two side sticks, a go around climb was established and the Captain formally took control for an uneventful circuit back to final approach from which a normal landing was accomplished.

It was considered by the Investigation that although the flight crew could not have anticipated the wind shear encounter before onset, the presence of strong winds with significant speed variations just before the accident should have acted as a general warning of the risk.

The Investigation noted that Operations Manual guidance on thrust setting whenever wind shear encounter was ‘a possibility’ was not fully followed by the First Officer and that as a consequence the energy management of the aircraft had not been optimum immediately prior to the wind shear encounter.

It also reviewed the Operator procedure for use of ‘SELECTED SPEED’ mode on approach rather than the Airbus-recommended ‘MANAGED SPEED’ Mode. It was considered that, from the point of view of preventing recurrence of this type of accident, the advantages of the latter over the former included:

• The automatic on-board computer calculation of final approach speed.
• That even if airspeed increases, there should not be an excessive reduction of thrust to maintain a fixed ground speed during approach and landing.

It was found from flight simulator tests assuming similar conditions to those which prevailed that wind shear recovery from the condition actually encountered would have been possible and that use of ‘MANAGED SPEED” mode would have been effective.

The minor injuries sustained by three of the cabin crew were considered to have been related to the much reduced thickness of the seat base padding of cabin crew jump seats as compared to passenger seats.

The overall conclusion was that although recovery actions were taken when the aircraft encountered severe wind shear at approximately 100 feet agl, a sufficient rate of climb was not attained in time to prevent the lower surface of the aft fuselage striking the runway, resulting in damage to rear frames and the aft pressure bulkhead.

It was considered that the following factors had contributed to the accident outcome:

• A delay in engine thrust response to the thrust levers being advanced to recover from the wind shear, combined with a reduced approach speed, meant that an increase in speed could not be obtained immediately (so that a) sufficient rate of ascent could not be attained in time to prevent the tail strike.
• A reduced approach speed resulted from the fact that the First officer, acting as PF, had selected a slightly low value for the final approach speed, and that he reduced thrust in response to an increase in headwind.
• The reduction of engine thrust in response to the increase in headwind occurred because the pilot was operating the thrust levers manually in “SELECTED SPEED” mode and was attempting to maintain a constant CAS, and because the flight crew had not adequately anticipated an increasing then decreasing wind speed.
• The selection of a slightly low value for final approach speed occurred because appropriate consideration was not given to wind speed and because there were concerns over touch down attitude related to high approach speeds.
• The lack of sufficient consideration of wind speed by the First Officer resulted from the use of a simple calculation formula that did not separately consider the additional airspeed margins to take account of wind speed changes and the head wind component, but that treated these together.
• The concerns over aircraft touch down attitude at high approach speeds arose because the measures described in the operator’s AOM for dealing with a nose-down attitude assumed only the case of full flap landings and the First Officer considered that it would be difficult to land an aircraft with this relatively nose-down attitude.

The Final Report of the Investigation was published on 26 September 2003 and may be seen in full at SKYbrary bookshelf.

No Safety Recommendations were made.

Further Reading

• Low Level Wind Shear
• Situational Awareness (OGHFA BN)
• Wind Shear Encounter During Go-Around (OGHFA SE)
• Go-around from Low Airspeed/Low Thrust