Description

On 25 November 2004, a V2500 powered Airbus A320 being operated by MyTravel Airways UK on a passenger flight from Narvik to Norwich UK lost directional control during the early stages of an attempted night take off from surface-contaminated runway 35 in normal visibility and the aircraft departed the left side of the runway. The aircraft stopped short distance off the paved surface and the occupants were subsequently evacuated from the rear of the aircraft using normal aircraft steps before being taken back to the terminal by bus. Damage to the aircraft was minor and confined to the NLG.

Investigation

An Investigation was carried out by the Accident Investigation Board Norway (AIBN) with DFDR and CVR data available. It was found that the First Officer, who had only recently qualified on the aircraft type, had been designated as PF despite recognition of the potentially challenging departure conditions by the aircraft commander

It was noted that before and after the accident, the wind had been light and variable along the length of the runway averaging about 6 knots with good visibility in light Snow and an air temperature around minus 4-5°C.

The flight crew were aware prior to departure that the runway surface was covered with dry snow overlying sanded ice giving and during taxi out were informed by ATC that the braking coefficients for runway 35 were 32/32/30 and therefore satisfied the minimum permitted braking action of “MEDIUM” under the MyTravel Airways SOPs.

It was found that there had been no abnormal aircraft handling issues during the taxi out but as the First Officer had tried to turn the aircraft round on the runway, there had been no response to nose wheel steering input and so he had stopped the aircraft to allow the aircraft commander to take over. When he was not able to turn the aircraft either, the aircraft held position for some 6 minutes whilst extra sanding of the surface took place which resolved the problem. With the aircraft eventually aligned for take off, DFDR data showed that the thrust levers had been advanced, with no pause for stabilisation, to a Throttle Lever Angle (TLA) of 33.75° and without following the available engine fan ice shedding procedure. The achieved EPRs of the two engines had been different with 1.34 on the right but only 1.11 on the left. Left engine N2 had also been lower than that on the right engine.

Both thrust levers had been retarded to idle at a ground speed of 10 knots as the aircraft began to deviate from the runway centreline and this deviation increased as ground speed rose to a peak of 22 knots before deceleration began. Braking was then initiated as the aircraft left the runway. The aircraft had come to a stop in snow and soil at an angle of approximately 40° to the runway centre line and with the nose and tail of the aircraft respectively 35 metres and 12 metres from the edge of the 45 metre wide runway.

It was established that at the time of the accident, the runway used had been covered with approximately 6 mm of loose dry snow overlying sanded ice. The Investigation noted that UK CAA did not permit operations on runways contaminated with frozen deposits and considered that UK pilots would not routinely experience such conditions. It also noted that Operator Procedures for contaminated runway take offs had been based on the Airbus premise of a “fluid contaminated runway” which was judged to be a “very uncertain (concept) based on insufficient scientific documentation”. It was considered that the investigated accident and another recent Airbus runway excursion incident in Norway served to “indicate that the Airbus procedures are difficult to relate to Norwegian winter operations'.”

The report also noted that at the time of the investigation, AIBN had already begun a special investigation into “Winter Operations and Friction Measurements” as a result of a series of 24 events over a period of 8 years involving slippery runways and the measurement and reporting of runway surface coefficients of friction. This ongoing wider investigation was noted to have already led to the issue of 4 immediate general safety recommendations to the Norwegian CAA.

In respect of the asymmetric thrust which had resulted from the setting of a similar thrust lever position for both engines, it was concluded that: During the 6 min stop while waiting for extra sanding in front of the aircraft, ice was probably forming on the No. 1 engine Low Pressure Compressor (LPC) blades, the LPC guide vanes and the Fan Exit Guide Vanes (FEGV). Relatively more fan icing was considered to have accumulated on the left engine fan blades during this time because the right engine was shielded from the prevailing wind by the fuselage whereas the left engine was not and was also running slightly above idle after being so selected in an attempt to assist the turn on to the runway. It was therefore believed that the ice in the No. 1 engine led to restricted air flow within the compressor causing asymmetric thrust.

The Investigation also considered that circumstances of the investigated event had confirmed that:

• AIP Norway information governing winter maintenance of runways is outdated

• the Norwegian practice of sand gritting on loose slush, wet or dry snow on top of ice or compact snow does not provide the expected aircraft braking coefficient” and

• the practice of measuring runway friction coefficient on wet conditions provides erroneous coefficients of friction

The Investigation made Significant Findings as follows:

• Due to icing in the No. 1 engine resulting in asymmetric thrust, the aircraft veered

to the left during engine acceleration.

• Due to an extremely slippery runway, with a CF much lower than the measured

and reported friction number of around 30 (MEDIUM), the actual runway friction was too low for nose wheel steering and for braking.

• Airbus Industrie’s concept of basing aircraft take off and landing performance on

“Fluid Contamination” and “Equivalent to Wet Runway” is misleading and not substantiated by scientific research. AIBN investigations and Norwegian experience show that “fluid contaminations” very often result in POOR braking action, contrary to the present belief of some organisations.

• The Norwegian practice of measuring friction on compact snow or ice covered by

loose dry snow, wet snow or slush may be outside the approved acceptable conditions for the measuring devices.

The Investigation resulted in the issue of four Safety Recommendations as follows:

• that MyTravel Airways UK review their OM Part A related to these types of operations. [SL 2007/25T]

• that MyTravel Airways UK review their training requirements for operations on contaminated runways in Norway. [SL 2007/26T]

• that Avinor (the Norwegian State ANSP and Airport Operator) review the extent to which the available friction measuring devices should be used in the various surface conditions which may prevail [SL 2007/27T]

• that Airbus Industrie review their concept of “Fluid contamination being Equivalent to Wet Runway” for (operations) on contaminated runways. [SL 2007/28T]

The Final Report of the Investigation was published on 7 August 2007

Related Articles

RE

• Runway Excursion;
• Runway Surface Friction;
• Runway De-icing;
• Accident and Serious Incident Reports: RE.

GND

• Aircraft Ground De/Anti-Icing.
• Engine Core and Fan De/Anti-icing