Residual Ice and Increasing Nose-Up Trim
During cruise at FL190 in a turboprop, OAT is -6°C and you are in visible moisture. The airframe ice-protection system uses pneumatic boots on the wing and tail leading edges, electrically heated propellers and heated pitot/static. The ice detector has been active for 25 minutes. Boots are cycling normally: 6 seconds inflate, 10 seconds deflate. Airspeed has fallen from 185 to 168 kt IAS at constant power, torque has risen 9%, and the aircraft needs increasing nose-up trim. The boots are shedding ice from the outer wing but a rough ridge remains on the inner wing and on the tailplane leading edge. There is no ice on the windshield. The autopilot is engaged in altitude hold. The nearest airport is 30 minutes away; a descent to FL140 would put you in air at -2°C, still in cloud. The landing flaps are available.
1. Assess the icing situation. What does the residual ice and trim change suggest, and what is the immediate risk?
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Residual ice on the inner wing and tailplane despite normal boot cycling means the system is not removing all accretion. The rise in torque and fall in IAS at constant power show increased drag; the increasing nose-up trim is a warning that ice is changing the tailplane flow. Tailplane icing can cause a tail stall, often triggered by flap extension or power changes, and the autopilot may mask the trim change. The immediate risk is an abrupt pitch-down or loss of control, not just reduced performance. The aircraft is already in a degraded state and should not remain in this icing environment while troubleshooting.
2. State your actions, including use of autopilot, flaps, power and altitude.
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Disengage the autopilot so you can feel trim and pitch changes, and avoid flap extension. Reduce angle of attack: lower the nose, keep speed up, and make power changes slowly. Do not make large nose-up trim inputs. Leave the boots cycling; if the system has a manual maximum cycle, use it. A descent toward warmer air only helps if the temperature actually rises above freezing; FL140 at -2°C is still icing, so that level alone does not solve the problem. Divert to the 30-minute airport, plan a reduced-flap or no-flap landing, and expect a higher approach speed. If pitch control becomes abnormal, treat it as a tail-stall event and reduce power and angle of attack.