What happened
Over a ten-year period, the Accident Investigation Board Norway (AIBN) analyzed 30 different incidents and accidents involving operations on contaminated or slippery runways. These occurrences, which included nine serious accidents or serious aviation incidents, frequently resulted in aircraft departing the runway because the surface conditions were more slippery than anticipated. The investigation examined a decade of data to understand why aircraft braking coefficients often failed to align with reported runway friction coefficients.
The investigation
The AIBN conducted a thematic study of the broader framework surrounding winter operations. The board reviewed a large volume of documentation, research, and test data from both national and international sources. The investigation focused on the interplay between meteorological conditions, runway maintenance, regulatory standards, and operational procedures. Specifically, the board looked at how the transition from subjective pilot assessments to the use of cockpit performance computers (CPC) has impacted safety margins during winter operations.
Findings
- The investigation found that the aircraft braking coefficient (ABC) often does not match measured or estimated runway friction coefficients (FC).
- There is significant uncertainty regarding the reliability of various friction measuring devices, particularly when dealing with moisture, low dew point spreads, or layered contaminants.
- Current national regulations for winter operations in Norway are less stringent than those for summer operations, failing to achieve an equivalent level of safety.
- The use of cockpit performance computers has led to a more numerical, less subjective approach to flight planning, which may overlook critical environmental safety indicators.
- Many incidents occurred when slippery runway conditions were combined with crosswinds, often exceeding the conservative limits used in other jurisdictions.
Safety action
The AIBN issued seven safety recommendations aimed at reducing uncertainty and improving safety margins. Key recommendations include:
- Requesting that ICAO, FAA, EASA, and CAA Norway review and validate the permitted measurement ranges for approved friction-measuring equipment.
- Revising the SNOWTAM table to reduce the degree of friction uncertainty.
- Evaluating whether the use of reverse thrust should be included in landing distance calculations for contaminated runways.
- Reviewing airline crosswind limits in relation to friction values.
- Updating the ICAO Airport Services Manual to better reflect modern winter operations, including the use of de-icing chemicals and new measurement technologies.