Approach and Landing
The flight crew planned an autoland to runway 4R. Winds were from 340 degrees at 17 knots, gusting to 23 knots. The runway had been plowed and was covered with thin wet snow. On final approach, the control tower advised winds from 340 degrees at 15 knots and reported braking action as poor.
Runway Excursion
About one second after nose wheel touchdown, the airplane deviated left. The autopilot system corrected the deviation. Approximately 11 seconds after touchdown, the airplane deviated left again and did not correct. The pilot disconnected the autopilot to regain directional control but was unable to restore runway heading. He kept the airplane straight as it departed the left side of the runway.
Friction Measurements
Mu readings of 0.28, 0.28, and 0.26 given to the flight crew and broadcast on the ATIS were five hours old. A more recent Mu reading of 0.21, 0.23, and 0.24 was three hours old but had not been placed on the ATIS. The control tower did not have a tracking procedure for adding revised Mu readings to the ATIS, nor was one required. Airport operations personnel reported transmitting the revised readings to the control tower.
Weather and Runway Conditions
Heavy snow had been falling earlier. Light snow was reported at the 0506 and all subsequent weather observations. The runway was last plowed and treated three hours earlier. An FAA inspector reported the runway condition as loose snow over patches of hard packed snow and ice.
Aircraft Performance Guidance
The airplane Flight Crew Operating Manual stated that automatic rollout performance was approved on dry and wet runways but had not been demonstrated on snow-covered or icy runways.