Static pressure blockage in climb
During climb in IMC at FL180, the pilot notices the altimeter has stopped winding, the VSI reads zero, and the airspeed indicator is reading 15 kt higher than the flight director command speed, while the standby altimeter (separate static source) continues to climb normally. The aircraft is in a 3° pitch-up attitude, autopilot in ALT hold. Pitot heat is on.
1. Identify the failure and explain the three indications.
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The main static source is blocked. With static trapped at the pressure of the blockage altitude:
- Altimeter: reads a constant altitude (stops winding) - it is frozen at the altitude where the blockage occurred.
- VSI: reads zero because static pressure is not changing.
- ASI: the pitot pressure continues to fall as the aircraft climbs, but static is trapped at a lower-altitude (higher) pressure, so the indicated airspeed reads higher than actual. The standby altimeter, on a separate static source, is correct. This is the classic static-blockage signature: altimeter frozen, VSI zero, ASI over-reading in climb (under-reading in descent).
2. What immediate actions should be taken, and what is the danger if the crew selects the alternate static source?
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Actions: select the alternate static source, cross-check with the standby instruments, disengage the autopilot (ALT hold is now unreliable because it uses the blocked static), and level off or continue the climb using the standby altimeter and pitch/power. Advise ATC of the unreliable pressure instruments. Danger of the alternate static source: it vents to the cabin, whose pressure is normally slightly lower than the true static pressure, so the altimeter will read slightly high, the ASI slightly high, and the VSI will show a brief climb indication when selected. Treat the readings as approximate and apply the AFM correction.