Blocked Static Port on Climb-Out
A CPL candidate departs Brunei (elevation 22 ft) in a light twin on a hot afternoon. During the climb, the pilot notices the altimeter is lagging behind the expected profile, the VSI is indicating a steady climb that does not match the aircraft's actual attitude, and the airspeed is reading higher than the pitch attitude and power setting would suggest. The pitot heat was on and the pitot tube is clear. At 4000 ft the airspeed reads 145 kt when 115 kt is expected. The static system drain was last checked six months ago. The pilot has a handheld GPS showing groundspeed 128 kt and a headwind component of 8 kt. The aircraft is IFR-equipped and the pilot is in IMC.
1. Diagnose the most likely instrument system fault and explain why each of the three indications behaves as observed.
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The pattern points to a blocked static port (or a partially obstructed static line). Reasoning:
- Airspeed over-reading: The ASI depends on the difference between pitot (total) pressure and static pressure. If static pressure is trapped in the line at a lower value than ambient (e.g. the port plugged while the aircraft was still near the ground), that difference is larger than it should be, so the ASI over-reads.
- Altimeter lagging: The altimeter capsule sees the same trapped static pressure. It will indicate an altitude close to the altitude at which the port became blocked, or drift slowly only through leak paths, so it lags the actual climb.
- VSI steady climb indication: With static pressure trapped at a lower value, the capsule remains deflected and the instrument shows a climb that does not reduce to zero as it would in a normal climb. A completely blocked static line commonly causes the VSI to freeze at the rate existing at the moment of blockage.
Pitot heat and a clear pitot tube rule out pitot icing. The GPS groundspeed plus known headwind (128 + 8 = 136 kt TAS roughly) confirms the ASI's 145 kt is wrong, supporting a static-side fault rather than a pitot fault. An alternate static source should be selected and the aircraft flown on partial-panel or GPS-derived data.
2. State the immediate pilot actions and the instrument cross-check you would use to maintain safe flight in IMC.
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Immediate actions:
- Select alternate static source if fitted; expect an immediate jump in ASI, altimeter and VSI readings and re-interpret them.
- Disconnect the autopilot if it is using the static-driven instruments, and hand-fly.
- Advise ATC of the failure and request a lower level or a route that reduces reliance on altitude separation.
- Use GPS altitude and groundspeed as the primary cross-check, remembering that GPS altitude is not pressure altitude and must be corrected for local pressure.
Cross-check:
- Attitude indicator and turn coordinator for bank.
- Altimeter cross-checked against GPS altitude and known terrain/MEA.
- Airspeed cross-checked against power setting, pitch attitude and GPS groundspeed with wind.
- VSI treated as unreliable.
If the alternate static source is not available, fly a known pitch-and-power profile and treat the ASI, altimeter and VSI as advisory only. Declare an urgency and request vectors to VMC or a suitable alternate.