Night Freight into a Warm Nose: Icing with No FIKI and No Oxygen
A single-engine turboprop Caravan, unpressurized and not FIKI-certified, is flying a night freight sector from Karachi to Quetta in mid-February at 12,000 ft. The route sounding shows: surface +8 °C; 850 hPa (about 5,000 ft) +4 °C; a frontal inversion near 9,000 ft where temperature rises from −1 °C to +3 °C; then 700 hPa (10,000 ft) −3 °C and 600 hPa (14,000 ft) −11 °C. Layered stratocumulus lies between 6,000 and 11,000 ft with tops at 11,000 ft, and the freezing level is 9,500 ft. A ten-minute-old PIREP reports trace rime at 8,000 ft. The pilot is at 12,000 ft, OAT −6 °C, in and out of cloud tops, with pitot heat and propeller de-ice on. The nearest suitable alternate is 40 NM back toward Karachi; the minimum safe altitude for the mountainous segment ahead is 11,400 ft, and there is no oxygen on board.
1. From the sounding, identify the icing type, likely severity, and the band of greatest risk.
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This is a textbook warm-nose profile. The 0 °C isotherm is at 9,500 ft and the frontal inversion places +3 °C air directly over −1 °C air. Snow falling from the cloud above the inversion melts in the warm layer, then re-enters sub-freezing air below without a freezing nucleus - the classic supercooled large droplet (SLD) mechanism. The band from roughly 9,500 ft to the top of the warm layer, say 10,500–11,000 ft, is where freezing rain/drizzle and large-drop icing are most likely, and because the cloud there is extensive and layered the exposure can be prolonged. Expect moderate, locally severe, clear or glaze icing, with runback and ice potentially spreading beyond protected surfaces.
Above the inversion the cloud is stratiform, with temperatures from −3 °C at 10,000 ft to −11 °C at 14,000 ft; that regime gives light to moderate rime, which de-ice equipment may partly handle - but the aircraft is not FIKI-certified, so any sustained accumulation is unacceptable. The 8,000 ft trace rime PIREP sits below the freezing level: it confirms ice is present but badly understates the hazard above 9,500 ft. The PIREP should not be used to justify continuing.
2. Compare climb, descent and turn-back as escape options, and state the decision.
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Climb: tops are 11,000 ft, so 12,000–12,500 ft is above cloud, and with OAT −6 to −8 °C and clear air there is no icing. But the practical ceiling is 12,500 ft - above that for more than 30 minutes, oxygen is required and none is carried, so 14,000 ft is not a legal option. The aircraft is already at 12,000 ft with only 600 ft above an 11,400 ft MSA; rising terrain plus a possible downdraft in ice is a poor place to be.
Descend: below 9,500 ft the air is above freezing and ice would shed, but the MSA ahead is 11,400 ft, so descent is not possible on the planned route - it is viable only if the aircraft turns back.
Turn back: a 180° toward Karachi descends into warmer, drier air and removes both the SLD band and the terrain problem.
Decision: with no FIKI, no oxygen, 600 ft of terrain margin and an SLD signature ahead, turn back to Karachi (or divert to an airfield south of the freezing level), advise ATC, and file a PIREP stating type, intensity and level. Continuing over high terrain with no oxygen and possible ice-induced loss of climb is the classic accident chain.