Static Port Blocked After a Rain Shower
After a rain shower, you rig a club glider and notice that the static port on the rear fuselage is partly covered by a piece of tape left from a recent repair. The pitot tube appears clear. You have a mechanical airspeed indicator, a sensitive altimeter set to QNH 1013, a variometer plumbed to the total-energy probe, and a turn coordinator. During the pre-flight, the altimeter reads 120 ft higher than the surveyed field elevation. On the launch, the airspeed indicator reads 55 kt during the climb while the tow plane is climbing at 70 kt; the variometer shows a steady climb. After release at 800 m AGL, you accelerate to 60 kt and the airspeed indicator reads 48 kt. The altimeter has stopped changing during the descent.
1. Diagnose the instrument indications and explain the physical reason for each discrepancy.
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The symptoms are consistent with a blocked static source, not a blocked pitot.
- The altimeter is stuck because it measures pressure from the static port; if that port is blocked, the capsule sees a trapped pressure and cannot respond to altitude change. The 120 ft pre-flight error is a warning sign of a static system error, though it could also be a QNH setting error or a mechanical offset.
- The airspeed indicator reads low in the climb and low in the descent. A blocked static source traps pressure inside the case. In a climb, the trapped static pressure is higher than the actual outside pressure, so the case pressure is too high relative to the pitot pressure, and the indicated airspeed reads low. In a descent, the opposite would occur and the indicator would read high. The observed low readings in both phases suggest the static line is leaking or partly blocked rather than fully sealed, so the case pressure drifts toward cabin/fuselage pressure.
- The variometer is plumbed to a total-energy probe, which uses a compensated source; it can continue to show a climb even when the static port is compromised, which is why a vario reading is not a reliable cross-check for static system health.
The key diagnostic is the combination: an altimeter that fails to respond plus an airspeed indicator that disagrees with the tow plane's known speed. Either alone could be a sensor fault; together they point to the static system.
2. State the operational decision and the cross-checks you would use to continue or abandon the flight.
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The decision is to treat the aircraft as having an unreliable primary altitude and airspeed indication and to abandon the flight at the earliest safe opportunity. The reasoning:
- Airspeed is the primary control reference for approach and stall avoidance. An indicator that reads 12 kt low at 60 kt means your actual speed is higher than indicated, so a normal approach speed would be flown too fast - but the error is not constant, so you cannot simply apply a fixed correction.
- The altimeter is unusable for height keeping, which removes your ability to judge circuit height and terrain clearance.
- Cross-checks available: the tow plane's known climb speed, GPS ground speed corrected for wind, attitude and pitch references, and the variometer for climb/descent trend. None of these substitutes for a calibrated airspeed indicator on approach.
Practical action: release or continue to a safe landing area, fly a wide, long approach using attitude and GPS ground speed as the primary references, and land with a generous margin. After landing, have the static system inspected and the tape removed, and record the defect. The judgement tested is recognising that a partial static blockage produces non-linear errors, so a fixed airspeed correction is not a valid workaround.