Freezing Rain Below the Warm Layer: Known Icing Is Not Enough
Single pilot, single-engine pressurised turboprop, flight into known icing approved, on a night IFR flight from Oslo to a regional aerodrome. Cruising FL190, OAT -24 C. The forecast for the destination gives surface temperature -1 C, calm wind, visibility 3,000 m in snow, and FZRA from 2,000 ft up to 8,000 ft, with a warm layer at FL090 to FL110 at +3 C. The destination has one RNAV (GNSS) approach to LNAV minima and no ILS. Fuel: 900 kg at top of climb, consumption 300 kg/h, 35 minutes to destination, alternate 25 minutes further at +4 C in rain, final reserve 30 minutes. Weather radar shows no returns. The aircraft has pneumatic de-icing boots and electrically heated props and windshield, no weeping-wing fluid system. The pilot has never flown into the destination aerodrome. ATC offers a descent at pilot's discretion.
1. Explain what the forecast freezing rain means for this flight, and whether the aircraft's known-icing approval covers it.
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FZRA implies a specific temperature profile: a warm layer aloft, with sub-freezing air beneath it. Snow falling through the warm layer melts, then becomes supercooled liquid water as it falls into the cold air below. The result is supercooled large droplets (SLD), which are nothing like the cloud droplets in the certification envelope.
Appendix C icing certification covers a defined range of droplet size, liquid water content (up to roughly 2.5 to 3 g/m3) and temperature. Freezing rain lies outside it: the droplets are much larger, and they strike and run back well beyond the boots, freezing as ridges behind the protected area (runback ice), and freezing on unprotected surfaces such as the upper wing aft of the boots.
So the answer is no. 'Flight into known icing approved' does not authorise flight into freezing rain. The boots can be bridged or bypassed, the ice accretion rate can exceed anything the certification tested, and the performance penalty can be severe and rapid. Additionally, if the aircraft is cold-soaked at -24 C, descending into the +3 C layer will not immediately melt accumulated ice.
Equally important: a pilot's discretion descent from FL190 would take the aircraft down through the warm layer and into the 2,000 to 8,000 ft band where the freezing rain is forecast. That clearance should not be accepted as offered.
2. Decide between continuing to destination, holding, and diverting, and support it with the fuel arithmetic.
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Fuel: 900 kg at top of climb, minus 35 min at 300 kg/h (175 kg), leaves about 725 kg at the destination. The alternate is 25 minutes (125 kg) further and the final reserve is 150 kg, so arrival at the destination with alternate plus reserve intact needs roughly 275 kg plus approach and missed-approach fuel. Fuel is therefore not the limiting factor, which is exactly why the decision must be made on icing rather than on fuel.
The destination approach descends through 8,000 ft to 2,000 ft inside the forecast freezing rain band, at night, with no ILS, at an aerodrome the pilot does not know. That is not a viable approach.
Holding is also poor: the freezing rain is associated with a synoptic warm-layer structure, not a shower that will clear in ten minutes; 20 minutes of holding costs 100 kg and does not change the profile.
Divert to the alternate now, remaining at FL190 well above the warm layer and the icing band: 60 minutes from the present position is 300 kg, landing with about 600 kg, which is over two hours of fuel. Request the diversion and the destination weather, decline the descent, and report the icing hazard to ATC.
3. If the destination were the only option, what else would you have to account for before and during the approach, and what would you tell ATC?
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Cold-temperature altimeter correction. With the aerodrome at -1 C, the LNAV minima must be corrected for the true altitude error, using the height above the altimeter setting source and the aerodrome's procedure design temperature if published. At these temperatures the correction is small but not zero, and the corrected altitude becomes the hard floor; the same applies to any step-down fixes.
Runway condition and landing performance. Snow and a -1 C surface mean contamination; obtain the braking action and the declared distances, and check that the landing distance required still fits the landing distance available with the correct contamination factor. Expect reduced braking and a longer roll-out.
Approach and monitoring. Check RAIM for the GNSS approach, brief the missed approach in detail (there is no ILS to fall back on), and expect airframe ice on the approach with a possible change in handling and a higher approach speed if ice is carried.
Tell ATC: 'Unable the descent at this time due to forecast freezing rain, request diversion to (alternate), request latest destination weather and any pilot reports of icing.' A pilot report of severe icing in the area is worth more to the next aircraft than anything else you can give.