Icing on the Descent: Boots, Bleed Air, and the Speed Decision
You are flying a turboprop at FL180 in IMC. OAT is −8 °C. The aircraft has pneumatic de-icing boots on the wing and tail leading edges, electrically heated props and windshield, and an engine bleed system that supplies both the boots and the cabin pressurisation. You enter a layer of supercooled stratiform cloud and pick up moderate rime ice; the boots are cycling normally and the ice is shedding in patches. The first officer proposes increasing speed to Vmo to "blow the ice off" and selecting maximum bleed for the boots. Engine indications are normal, but you notice the cabin altitude creeping up slightly and the bleed pressure annunciator flickering amber.
1. Evaluate the first officer's proposal to accelerate toward Vmo.
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Do not accelerate to Vmo. Several reasons:
- Ice on the wing increases drag and reduces lift; the aircraft will need a higher angle of attack for the same speed, and the stall speed rises. Accelerating to Vmo does not remove ice - the boots do that - and it puts you near a speed limit you cannot easily recover from if you need to slow for turbulence or an approach.
- High speed with residual ice on the tail is the classic setup for tail-plane stall on descent/flap extension. The tail is the more dangerous surface because it is often out of the crew's direct view.
- The correct response to airframe icing is to maintain a safe speed above the icing/stall speed (the AFM's icing speed or a comfortable margin above the clean stall), keep the boots cycling, and exit the icing layer - change altitude or route.
So: keep a sensible speed, do not chase Vmo, and prioritise leaving the cloud.
2. Interpret the rising cabin altitude and flickering bleed pressure annunciator, and decide what to do.
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The bleed system is supplying both the boots and pressurisation. A flickering amber bleed pressure annunciator with a slowly rising cabin altitude means the bleed is marginal or failing - it is not delivering enough pressure to hold cabin altitude and drive the boots simultaneously.
Implications:
- If you select maximum bleed to the boots, you will starve pressurisation further and the cabin altitude will climb faster.
- If you select maximum pressurisation, the boots may under-perform and ice will accumulate.
Action:
- Reduce bleed demand from the boots - cycle them on a normal schedule, not maximum, and accept some residual ice while you get out of the layer.
- Protect pressurisation - you need cabin altitude under control more than you need perfect boot performance at this moment. Monitor cabin altitude and rate; if it continues to climb, be ready to don masks and descend.
- Leave the icing layer - request a lower or higher level where the cloud tops out or the temperature is outside the icing range.
- Treat the flickering annunciator as a system fault and plan for a possible loss of pressurisation: brief the emergency descent, masks on.
3. What is the correct sequence if the cabin altitude continues to rise?
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Sequence:
- Masks on, establish communications - crew oxygen first, then check passenger oxygen has deployed automatically (or manually).
- Declare - MAYDAY or PAN PAN to ATC, state the problem and your intention to descend.
- Emergency descent - idle power, speedbrake as required, descend to a level where you can breathe (typically 10,000 ft or the highest terrain-safe level), respecting structural and icing considerations. Do not descend into terrain - check MSA and terrain clearance first.
- Manage the bleed - isolate the faulty bleed if the AFM procedure calls for it; this may restore pressurisation or confirm the fault.
- Divert - plan to land at the nearest suitable airport; a pressurisation fault is a land-as-soon-as-practical event, not a continue-to-destination event.
Key point: the icing problem and the pressurisation problem are linked through the bleed. Solve the pressurisation first - you can survive some residual ice, you cannot survive prolonged hypoxia.