Winter Departure With Contaminated Runway and Icing Risk
You are the pilot-in-command of a twin-engine turboprop, MTOW 9,500 kg, departing a northern Canadian airport at 07:20 local time in February. Temperature is -8°C, dew point -11°C, wind 340° at 12 knots gusting 22, runway 31 in use, 1,800 m long, dry snow 2 cm over compacted snow, braking action reported as medium. The aircraft has a clean airframe but the wings carry a thin layer of frost from overnight radiation cooling, and freezing drizzle was reported 40 km west moving toward the field. Anti-ice and de-ice systems are serviceable. Your accelerate-stop distance on the contaminated surface is 1,650 m and your accelerate-go distance is 1,720 m. The company minimum for medium braking action is 1,700 m. Your takeoff weight is 9,100 kg.
1. Is a departure legal and safe under these conditions? Walk through the performance and contamination decision.
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First, compare the runway available (1,800 m) against both accelerate-stop (1,650 m) and accelerate-go (1,720 m). Both fit within 1,800 m, so the field length is not the limiting factor. However, the company minimum for medium braking action is 1,700 m, and the accelerate-go distance of 1,720 m exceeds that minimum by 20 m - a marginal violation that must be resolved before departure.
Second, the 2 cm of dry snow over compacted snow is a contaminated runway condition. The reported medium braking action is the controlling value; do not substitute dry-runway figures. Because the accelerate-go distance exceeds the company medium-braking minimum, you must either reduce weight (offload fuel or payload), wait for improved braking action, or select a different runway with better reported conditions.
Third, the frost on the wings is disqualifying. FAR/CAR guidance and manufacturer data require a clean airframe - frost, snow, or ice on critical surfaces degrades lift and increases drag. Even a thin layer must be removed with de-icing fluid before takeoff. The presence of freezing drizzle 40 km west means holdover time must be computed and the takeoff completed within it, or the aircraft re-de-iced.
The safe decision is to de-ice, reduce weight to bring accelerate-go under 1,700 m, and confirm braking action is still medium before committing.
2. Freezing drizzle arrives and the field is now reporting light freezing drizzle with visibility 1.5 sm. What changes?
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Light freezing drizzle changes the contamination picture significantly. The runway surface will accumulate a thin, clear ice film, which typically degrades braking action from medium toward poor. A poor braking action report would require a longer accelerate-stop and accelerate-go distance than the 1,700 m medium minimum - likely exceeding the 1,800 m available.
Additionally, freezing drizzle in the holdover-time tables for the de-icing fluid in use carries a short holdover, often in the range of 15 to 30 minutes depending on fluid type and precipitation intensity. You must start the holdover clock at the beginning of the final application and be airborne before it expires. If the takeoff is delayed, the aircraft must return for re-de-icing.
The decision logic: re-check braking action, re-compute performance against the new report, verify holdover time against the actual start-of-takeoff-roll time, and if any margin is lost, do not depart. Waiting for the band to pass or diverting to a field with better conditions is the conservative choice.