Night Freight into a Frontal Squall Line
At 21:40 local a twin turboprop is at FL190 on a night freight sector, 70 NM from destination, a single-runway airport with one non-precision VOR/DME approach (MDA 640 ft, visibility 1 600 m) and no ground radar. The aircraft's weather radar shows a continuous line of echoes 25 NM wide lying across the approach area, tops displayed well above the aircraft's ceiling, and the crew have been using maximum gain. A preceding aircraft reports severe turbulence and continuous moderate icing in the line between FL180 and FL200. The destination reports surface temperature 0 °C, dewpoint −1 °C, and freezing rain. The aircraft can shed light icing only; its anti-ice systems are serviceable but bleed-air limited, so with engine and airframe anti-ice on the single-engine service ceiling is FL140. Fuel allows one approach at destination and a diversion to an alternate 55 minutes away, or a diversion now with about 40 minutes of holding at the alternate. The destination METAR ends with a temporary group of visibility 2 000 m.
1. What does the surface freezing rain tell you about the vertical temperature structure, and what is the significance of the radar display and the maximum-gain setting?
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Freezing rain is a temperature-profile message. Rain reaching the surface as liquid at 0 °C with a dewpoint of −1 °C means the drops fell through a layer warmer than 0 °C aloft (melting any snow), then through a shallow sub-freezing layer near the ground without freezing. The hazard is therefore not at the top of the cloud but in the cold layer between the surface and the top of the inversion - precisely the layer an aircraft flies through on descent. That layer contains supercooled large drops, the worst icing regime there is: it can exceed the protection of any anti-ice system and it is where the aircraft cannot shed ice.
The radar display. A continuous 25 NM line across the approach area with tops above the aircraft ceiling means the flight cannot overfly it. On maximum gain, the nearest echoes paint strongly, but strong returns also attenuate the beam: what looks like a weak gap behind the first line may be a second, more intense core that the radar cannot see. Never treat a low return behind a strong line as a gap. The magenta/intense display indicates gradients and hail potential, and the radar gives no information about turbulence or icing anyway - the PIREP does that.
The combination is a warning: the terminal area is a convective, icing, low-level-wind-shear environment at night, with one non-precision approach and no ground radar.
2. The captain proposes descending to 8 000 ft to get beneath the line and continue visually. Assess that plan.
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It is the most dangerous option available.
- Descending to 8 000 ft puts the aircraft into the sub-freezing layer where the freezing rain is forming, at the altitude where the supercooled drops are largest and most concentrated. The aircraft can shed light icing only; the anti-ice system is the barrier, and this is the regime that defeats it.
- With engine and airframe anti-ice on, the single-engine service ceiling is FL140. Below that in a convective line, an engine failure or a flame-out from icing leaves no performance margin over terrain at night.
- "Visually" is not available: 4 000 m visibility at night, a 1 800 m runway, a displaced threshold, terrain before the threshold and no approach lighting give no meaningful visual horizon; the crew would be flying an improvised approach in a thunderstorm at night.
- The line is 25 NM wide and sits across the approach area, so there is no short leg underneath it.
The professional point. The plan is being generated by the desire to complete the sector, not by the weather. The correct response to a convective line across the only approach at night, with severe turbulence, icing and freezing rain, is to stay out of it: divert while the fuel still allows 40 minutes of holding at the alternate, or hold clear of the line and re-assess with a hard fuel limit, accepting that the flight may end at the alternate.
The temporary group of 2 000 m visibility is not usable as a planning basis for a 1 600 m non-precision approach at night.
3. State your decision and the reasoning that ranks the threats.
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Decision: divert now to the alternate.
Threat ranking. The controlling threat is not fuel or time - both are adequate for the alternate - it is that the only approach at destination passes through a convective line containing severe turbulence, moderate-to-severe icing and freezing rain, at night, with no vertical guidance, no ground radar and no approach lighting. That combination removes the barriers which normally make a non-precision approach acceptable: a visual segment, a stable approach path, and a prompt go-around option.
Why not one approach? The fuel plan allows one approach and a diversion, but that assumes the approach is flown to a missed approach and the aircraft is still in a fit state to divert. After penetrating the line, the aircraft may be icing-affected, the crew degraded by turbulence and workload, and the alternate fuel partly consumed. The one-approach option is only rational if the line can be avoided, and it cannot.
Why divert now rather than hold? Holding clear of the line preserves the option to return if the line moves or breaks, but it burns the alternate holding fuel and increases crew fatigue on a night sector. With 40 minutes of holding available at the alternate, the diversion is robust.
After landing: report the icing and turbulence to ATS for onward PIREP, note the radar attenuation in the debrief, and record the decision and its basis - the value of the case is the explicit ranking of icing and convection above schedule.