The Unexpected Icing Encounter
You are flying a non-FIKI certified aircraft at FL180 in instrument meteorological conditions. The outside air temperature is -5°C, and you encounter supercooled large droplets. You notice ice accumulating on the leading edges of the wings and on the propeller, and the aircraft begins to experience a slight buffet. The pitot heat is on, but the static ports are not heated. Your airspeed indicator begins to read lower than expected, and the altimeter shows a gradual climb. You are 40 nautical miles from your destination, which has a 2000 ft ceiling and 5 km visibility, and you have 30 minutes of fuel remaining above reserve.
1. What are the hazards of this icing encounter, and what immediate actions should you take?
Read the worked answer
The hazards include reduced lift and increased drag from airframe icing, propeller icing reducing thrust, and pitot-static system icing causing erroneous airspeed and altitude indications. The buffet indicates that the aircraft is approaching the stall, and the ice is disrupting the airflow over the wings. The static port icing causes the altimeter to show a climb and the airspeed indicator to read lower, which can mislead you. The immediate actions are to turn on all available anti-ice and de-ice equipment, including propeller heat and pitot heat, and to avoid further icing by changing altitude or routing. You should not rely on the indicated airspeed and altitude; instead, use pitch and power settings, and if possible, use GPS altitude. You should declare an emergency and request a lower altitude or diversion. The priority is to exit the icing conditions and land as soon as possible.
2. How should you handle the erroneous pitot-static indications?
Read the worked answer
The erroneous indications are caused by ice blocking the static ports. The altimeter shows a climb because the static pressure is trapped or reduced, and the airspeed indicator reads lower because the pitot pressure is correct but the static pressure is wrong. You should recognize that the indications are unreliable and use alternative sources. If the aircraft has a GPS, use GPS altitude and groundspeed. Use pitch and power settings to maintain a safe flight path. If the aircraft has a standby instruments or an alternate static source, select it. You should also consider that the static port may clear if you descend to a warmer altitude. Do not make abrupt control inputs based on the erroneous indications. Inform ATC of the situation and request priority handling. The key is to maintain aircraft control and avoid stall or upset.