IFR En-Route Descent into an Unforecast Icing Layer - Decision at the Top of Descent
You are flying a single-pilot IFR helicopter at FL120 on an airway toward Salalah. The aircraft is approved for flight in known icing conditions only with the engine anti-ice and rotor blade de-ice systems serviceable; the blade de-ice system is currently showing an intermittent fault on the caution panel. OAT at FL120 is −18 °C; the freezing level is forecast at FL140. The destination METAR is +28 °C, but the approach will require a descent through a layer between FL100 and FL060 where the forecast gives SCT Cu with tops at FL090 and a temperature of 0 °C at FL080. You have 1,900 kg fuel, giving 2 h 10 min endurance. The nearest suitable alternate is 35 minutes away with a cloud base of 1,200 ft. ATC offers you either a descent now or a hold at FL120 for 20 minutes.
1. Assess the icing risk on the descent and decide whether to accept the descent now or hold.
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Icing assessment. The critical band is FL100–FL060, where the forecast temperature reaches 0 °C at FL080 and the cloud tops are at FL090. That means your descent from FL100 to FL060 will pass through a supercooled water region in Cu - the most efficient icing producer. The blade de-ice system is faulty (intermittent caution), so you are not fully equipped for known icing.
The regulatory/operational point. If the aircraft is not fully serviceable for known icing, you must avoid known icing conditions. A descent through a Cu layer with tops at FL090 and 0 °C at FL080 is known (forecast) icing. Therefore the descent as offered is not acceptable with the de-ice fault.
Hold vs descend. Holding at FL120 keeps you above the icing layer (OAT −18 °C, clear of cloud) and gives 20 minutes for the fault to clear or for you to replan. That is the safer choice provided the hold does not eat into your fuel reserve unacceptably. 20 minutes of hold leaves ~1 h 50 min - still comfortably above the 35-minute diversion plus reserves.
Decision. Hold at FL120. Use the 20 minutes to attempt to reset/confirm the de-ice fault, obtain an updated forecast, and consider an alternate routing that keeps you above FL100 until closer to destination, or a diversion to the 1,200 ft cloud-base alternate if the fault persists.
2. If the fault does not clear and you must descend, what is the least-risk profile and what ATC request do you make?
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Least-risk profile:
- Descend as late and as fast as possible through the icing band - a high-rate descent reduces time in the supercooled layer. Request a 'high rate' or 'expedited' descent.
- Stay out of cloud where possible: request a descent in VMC or on top until the last moment, then a continuous descent through the layer rather than level flight in it.
- Avoid level flight in the 0 to −10 °C band - that is where accretion is fastest. If you must level, do it above FL100 or below FL060.
- Keep anti-ice on and monitor engine parameters; be ready for blade imbalance/vibration.
ATC request: 'Request expedited descent through FL100 to FL060, and if able, a heading that keeps us clear of cloud until top of descent; we are not fully equipped for icing.' Also request the latest destination/alternate weather and a PIREP if any traffic has been through.
If conditions worsen (heavy accretion, vibration, performance loss): declare and divert to the 1,200 ft alternate, descending only through the thinnest part of the layer. The decision to divert is made before the aircraft is degraded, not after.