Static source blockage in the climb: three instruments that disagree
A twin turboprop is climbing in IMC on an IFR clearance from FL120 to FL200. It has been in cloud for 12 minutes at -12°C with airframe anti-ice on. Pitot heat is on, but the static-source heater switch was not selected at the after-start check. Passing an indicated 17,600 ft the crew notices that the captain's altimeter has not moved for three minutes, the VSI reads exactly zero, and the ASI reads 158 kt although pitch attitude and torque have not changed since FL120. The standby altimeter, which is fed from a separate static source, reads 18,050 ft and climbing at 900 ft/min. ATC clears the aircraft to FL200 and asks for the present altitude. The first officer suggests the standby instrument is the faulty one because two of the three captain's instruments agree with each other.
1. Diagnose the failure and explain why each of the three captain's instruments reads what it does.
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The static source has been blocked by ice. The static heater was not selected, and the blockage happened at about 17,600 ft.
- Altimeter: it is a barometer connected to static pressure. With the static trapped at a constant pressure, it displays the altitude at which the blockage occurred and then freezes at 17,600 ft.
- VSI: it displays the rate of change of static pressure. Trapped static = no change = zero.
- ASI: it displays pitot total pressure minus static pressure. The pitot is heated and clear, so total pressure falls correctly as the aircraft climbs, but the trapped static stays at the higher pressure of 17,600 ft. The difference (Pt − Ps) is therefore too small and the ASI under-reads by an amount that grows with height gained.
The two "agreeing" instruments are not independent - both are driven by the same blocked line. The standby altimeter uses a separate source and is telling the truth: the aircraft is about 450 ft above the captain's altimeter and still climbing. The first officer's logic is exactly backwards.
2. State your immediate actions, including what you tell ATC and how you level off.
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- Disconnect the autopilot and fly pitch and power; the autopilot may be holding an altitude that does not exist.
- Level off and control altitude on the standby altimeter, not the captain's.
- Select the alternate static source if installed; expect a transient jump in ASI, altimeter and VSI as the system equalises, and re-check the indications.
- Stop the climb - do not accept FL200 on these instruments. Tell ATC: "unreliable airspeed and altitude indications, levelling at FL180 on standby, request holding/vectors and no speed control."
- Select static heat on and leave the icing environment if possible; descend to warmer air if the situation allows.
- Cross-check with GPS altitude and groundspeed (sanity check only), and with attitude, power and pitch.
- Consider a diversion to an aerodrome with good weather, and declare urgency (PAN PAN) if remaining in IMC with unreliable ASI.
Do not make any rapid or large attitude change until the situation is stabilised and the standby instruments confirm the result.
3. Why is the same failure more dangerous in a descent? State the hazard and the mitigation.
Read the worked answer
In a descent, ambient static pressure rises above the trapped value, so the trapped Ps becomes lower than the true static. The ASI then over-reads: the crew sees a high airspeed that does not exist, while the altimeter stays frozen at 17,600 ft and the VSI stays at zero. The aircraft is actually descending through 17,600 ft towards terrain while the panel says it is level there, and the crew may reduce power or pitch up because the ASI looks fast - driving the real speed towards the stall in a high-altitude, low-margin regime. Loss of altitude awareness plus a false high-speed indication is the classic combination for a controlled flight into terrain or a stall event.
Mitigation: at the first disagreement, treat the system as failed rather than "probably fine"; identify the standby/alternate static source; limit the descent rate; keep a positive pitch and power target; use GPS altitude and groundspeed as a cross-check; and tell ATC immediately so terrain separation is provided. If the alternate static source is selected, re-brief the approach because all static-driven indications change.