Aviation Safety Letter 3/2003

Inaccurate CRFI Contributes to Runway Excursion

Artist's impression of runway excursion.
Artist's impression of runway excursion.

On March 27, 2002, a Fokker F-28 was on a night flight from Toronto, Ontario, to Saint John, New Brunswick, with 4 crew members and 51 passengers on board. The aircraft landed on the centerline of Runway 05 in Saint John at 00:30 local time. After the nose wheel touched down, the aircraft started to drift uncontrollably to the left and the left main wheels went off the side of the runway for approximately 900 ft before regaining the runway surface. The left main gear track was 15 ft from the runway edge at its furthest point. Aircraft damage was limited to minor cuts in the tires of the right main gear and the nose wheel. There were no injuries to the passengers or crew. This synopsis is based on TSB Final Report A02A0038.

The crew had been provided with a Canadian Runway Friction Index (CRFI) measurement for Runway 05, taken at 00:12, of 0.52 - well above the recommended minimum CRFI. A value of 0.52 is equated with good friction characteristics, approximately equivalent to a wet runway covered with 0.02 in. of water. With this information in hand, even though Runway 05 was reported to be 100 percent snow-covered with up to ¼ in., the crew declined an offer for a centerline sweep. Conditions were deteriorating rapidly however, and the non-landing Runway 14/32 was measured at 00:22 with a CRFI reading of 0.23, with an equal amount of contamination. The significance of the discrepancy in the CRFIs was not recognized by ground personnel and consequently, there was no re-assessment of the validity of the Runway 05 CRFI measurement.

The value of the CRFI for the non-landing runway was not passed on to the crew. It is not known if the provision of this information would have altered their decision to land on Runway 05, or to reconsider the offer for snow clearing. The CRFI for Runway 05 had been measured 20 min prior to the landing and was reported to the crew 10 min before touchdown. Given this relatively short time, the crew would not expect a significant change to the friction characteristics and consequently relied on the Runway Surface Condition (RSC)/CRFI report to establish the suitability of Runway 05 for landing.

As the temperature was slightly above freezing, melting under the snow cover on Runway 05 was likely either undetected at the time of the CRFI run, or it happened mostly after the measurement was taken. In either case, the CRFI reading of 0.52 was considered valid when the measurement was taken, but was not an accurate indication of the runway's friction characteristics at the time of landing.

The TSB concluded that the poor friction characteristics of the runway, due to slush contamination, did not allow the crew to correct the aircraft's ground track after touchdown and the aircraft slid off the side of the runway.

Safety action taken - In May 2002, the TSB forwarded a safety advisory to Transport Canada (TC) regarding the adequacy of RSC/CRFI reporting and crews' knowledge of the limitations of these reports. The advisory suggested that TC consider a means of advising aircrews and other members of the aviation community of the limitations of RSC and CRFI reports, particularly when airport ambient temperatures are near freezing and precipitation or visible moisture is present. In addition to this article on the Saint John occurrence, TC published the article "Just a bit of slush." in ASL 1/2003, and a third article on how much performance is affected by slush is planned for ASL 4/2003.

The operator of the occurrence described in this article took steps to reduce the likelihood of further runway excursions in conditions where slush might be encountered, including the publication of a Flight Operations Bulletin advising flight crews of the potential for CRFI reports to become invalid soon after the reading was taken, particularly during changing weather conditions where temperatures are at or near the freezing level and surfaces are contaminated with snow, slush, ice or standing water, or where precipitation or visible moisture is present during the approach and landing. It also directed crews to consider delaying a landing and consider the validity of CRFI reports only after the runway has been swept, giving due consideration to depth of contaminates between the time of the CRFI measurement and the landing.

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