Blocked Static Source in the Climb
You are pilot flying a twin turboprop on a scheduled night IFR service from Christchurch to Auckland, in smooth air, climbing through FL150 for FL190 with the autopilot in IAS mode. Passing 14,800 ft the captain's airspeed begins to fall; a minute later it reads 165 kt while the first officer's display is steady at 212 kt, and torque, attitude and cabin conditions are unchanged. The captain's altimeter now reads 14,020 ft and the gap to the FO's altimeter grows as you climb; the captain's VSI sits at exactly zero. The standby altimeter and standby airspeed indicator, which use their own separate static source, agree with the FO. Pressurisation is normal at cabin altitude 7,400 ft, pitot heat has been on since 10,000 ft, and there are no ice warnings or electrical faults. ATC has just cleared you to maintain FL190 and asks you to confirm your level.
1. Identify the failed system and explain how the three indications on the captain's PFD prove it. State what the indications would have been had the pitot tube been the blocked source.
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The captain's static source is blocked at about 14,000 ft - most likely ice or an obstruction at the port. The three indications are a matched set:
- Altimeter frozen at 14,020 ft - it senses a trapped static pressure that no longer changes with height.
- VSI exactly zero - no rate of change of static pressure.
- Airspeed under-reading and falling - pitot pressure is correct and rising in the climb, but the trapped static is higher than ambient, so pitot minus static is smaller than the true dynamic pressure. The error grows with height.
The discriminator is the pitot case: a blocked pitot leaves the static working, so the ASI would over-read and increase in the climb, while the altimeter and VSI would be normal. A leaking static would give an altimeter that over-reads in the climb and a VSI showing a climb. A frozen altimeter, zero VSI and falling ASI can only be a blocked static. On descent the same fault reverses: altimeter over-reads and ASI over-reads.
2. The captain proposes opening the alternate static source at FL190 (cabin altitude 7,400 ft). Describe the resulting indications and the hazards of doing so.
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On this aircraft the alternate static valve vents the captain's instruments into the cabin. At FL190 the cabin pressure (cabin altitude 7,400 ft, roughly 770 hPa) is far higher than the true outside static pressure (roughly 466 hPa), so at the instant of selection the instruments see a large false increase in static pressure:
- The altimeter would jump from 14,020 ft down to about 7,400 ft - an apparent 6,600 ft descent.
- The ASI would fall towards zero. Pitot total pressure at FL190 is only about 505 hPa, so with 770 hPa applied as 'static' the differential is negative and the pointer sits at or below zero - expect a stall warning or stick shaker.
- The VSI would show a transient as internal lines equalise.
The autopilot, altitude alert, EGPWS and transponder may all follow the false altitude. Because the FO's and standby systems are serviceable, leave the valve closed, cover or annotate the captain's instruments and continue on the good systems. The alternate static valve helps only in an unpressurised aircraft, where it causes a small over-read rather than a catastrophic one.
3. You are cleared to FL190. State your decision and the threats you must manage for the rest of the flight.
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Continue, using the FO's PFD as the primary flight display and the standby instruments as the independent cross-check; the captain flies on those, with the failed side covered and annotated. Manage these threats:
- Altitude source. Establish which static source feeds the altitude encoder. If it is the captain's side, ATC is receiving a frozen Mode C (about 14,000 ft) while you climb to FL190 - a loss-of-separation risk. Tell ATC Mode C is unreliable and revert to voice level reports.
- Automation. Check that autopilot altitude hold, the altitude alert and any cross-comparison logic are using the FO's air data; many types will disconnect or degrade when air data disagree.
- Cabin. Some pressurisation controllers use static pressure - confirm the outflow valve is responding normally.
- Warning systems. If EGPWS or TCAS takes barometric altitude from the failed side, their alerts are unreliable; verify the source.
- Plan the descent and approach on the good system, record the defect, and do not attempt to 'clear' the port in flight.