Pitot heat failure and an over-reading ASI in the climb
You are the pilot flying a Beech King Air 200 from NZAA to NZWN at FL180. Passing FL150 in IMC, OAT -20°C, the left pitot heat fails. The captain's airspeed indicator, on the left side, reads 220 KIAS and is slowly increasing although torque and power settings are unchanged and the first officer's ASI reads a steady 180 KIAS. The standby ASI reads 182 KIAS. Both altimeters agree at 15,000 ft and the VSI shows +500 ft/min. The autopilot is engaged in altitude hold. ATC has cleared you to FL180. There is no turbulence and no icing reported above FL160. The aircraft has a three-axis autopilot that uses the captain's air data for altitude and speed inputs. You need to decide what is valid, what to do next, and how to manage the clearance.
1. What is the most likely defect, and why does the captain's ASI read high while the altimeters and VSI remain normal?
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The most likely defect is a blocked left pitot tube with the static port still open. In the climb, the trapped pitot pressure stays at the value when the blockage occurred while static pressure falls. The ASI measures pitot minus static, so the pressure difference rises and the ASI over-reads. A static blockage would freeze the altimeter and drive the VSI to zero; both altimeters agree at 15,000 ft and the VSI is responding, so static is not blocked. The first officer's and standby ASIs agreeing near 180 KIAS confirm that the captain's indicator is the odd one out. The failed pitot heat is the likely cause. Check the heat switch and circuit breaker, but do not use the captain's ASI for speed control.
2. State the immediate actions and the instrument cross-check you would use.
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Disconnect the autopilot because it may use the captain's air data; fly attitude and power. Select pitot heat on and check the circuit breaker; if it pops, leave it out. Use the first officer's ASI and the standby ASI as the primary speed reference. Cross-check attitude, torque/N1, first officer's ASI, standby ASI, altimeter, VSI, and outside references if visual. Advise ATC of the unreliable ASI and request a level-off at FL160 or lower. Avoid a further climb until you have a reliable speed indication. If the ASI cannot be restored, plan a diversion to an aerodrome with visual conditions or a long runway. Do not chase the erroneous ASI with pitch; use power and attitude.
3. Can you accept the clearance to FL180? Explain.
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Not on the captain's ASI. The clearance itself is not invalid because the altimeters are reliable, but accepting it means continuing a climb with a failed primary speed indicator and failed pitot heat. The safer decision is to level at FL160, tell ATC 'unable FL180 due unreliable airspeed', and resolve the defect. If the first officer's ASI and standby agree and the autopilot can be fed from a valid source, a careful climb may be possible, but the over-reading ASI creates overspeed risk and the autopilot may use bad data. The conservative answer is to stop the climb and return or divert.