Frozen static source in an icing descent
You are descending in IMC from FL100 to 6,000 ft at 500 fpm in icing conditions, OAT −4 °C, pitot heat ON. Three things are wrong on the panel: the airspeed indicator, which reads 148 kt, is slowly increasing although you have reduced power and are descending; the altimeter has not moved from 9,200 ft for four minutes; and the VSI reads exactly zero although the attitude indicator and engine instruments confirm a steady descent. The turn coordinator is wings-level, heading is steady, and the suction gauge reads 4.8 inHg. The aircraft has a heated pitot tube and two static ports, one each side of the aft fuselage, with an alternate static source selectable in the cabin. The destination is 40 NM ahead with an ILS to runway 27; the field OAT is +8 °C and the MSA is 7,500 ft. You can hear ATC clearly but have not yet reported the discrepancy.
1. Diagnose which system is blocked and justify the diagnosis from the three indications.
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The static system is blocked - most probably one or both static ports iced over.
Reasoning:
- Altimeter frozen at 9,200 ft. The altimeter is a pure static-pressure instrument. If the static source is sealed, the capsule sees a constant pressure no matter what the aircraft does, so the pointer stops. Four minutes of a 500 fpm descent should have produced a 2,000 ft change - that it produced none is decisive.
- VSI reads zero. The VSI is a rate-of-change device fed from the same static source. With trapped static pressure, there is no rate to display. This also rules out a purely mechanical instrument failure: three static instruments have failed together, which points to a shared source, not three coincidences.
- ASI over-reading and increasing in the descent. With the static trapped at the pressure of the altitude where it sealed (about 9,200 ft, i.e. lower pressure than the aircraft is now in), the pitot pressure rises as you descend while the trapped static stays fixed. The difference (Pt − Ps) grows, so the ASI indicates an increasing speed even though the actual speed is steady or falling. The pitot is evidently clear - pitot heat is on and the ASI is alive.
Gyroscopic instruments are healthy (suction 4.8 inHg, attitude and turn coordinator consistent), confirming this is a pitot-static problem, not a vacuum or electrical failure.
2. What would the indications have been if instead the pitot tube had been blocked with its drain hole open? How would you distinguish the two cases on the panel?
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Pitot blocked, drain open - the ASI becomes an altimeter.
With the impact hole sealed and the drain hole open, the pitot line is connected to static pressure. The ASI capsule then sees static pressure on both sides and reads approximately zero - but as the aircraft climbs or descends, the pressure inside the line changes through the drain, so the ASI responds to altitude change, not to speed.
Indications would be:
- In a descent: the ASI reading decreases steadily as altitude is lost, regardless of actual airspeed.
- In a climb: the ASI reading increases steadily, regardless of actual airspeed.
- Altimeter and VSI: normal and correct. They are fed from an undisturbed static source.
This is the exact mirror image of the static blockage:
| Static blocked | Pitot blocked, drain open | |
|---|---|---|
| ASI in descent | increases | decreases |
| Altimeter | frozen | correct |
| VSI | zero | correct |
In both cases the attitude indicator is the only reliable pitch reference and must be used to fly. The discriminator here is that the altimeter and VSI are dead, so the fault is on the static side; if they were healthy, it would be the pitot.
3. What do you do now, and how do you manage the alternate static source and the ILS approach?
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Immediate actions:
- Fly attitude and power - the attitude indicator, turn coordinator, heading and RPM are now the primary references. Ignore the ASI, altimeter and VSI entirely.
- Report to ATC without delay: state that you have lost static pressure, that your altitude and speed indications are unreliable, and request a new altitude readout from radar and a heading/altitude service all the way to the approach. Ask for a long final and no last-minute changes.
- Select the alternate static source and expect a step change: cabin pressure is lower than true static pressure, so the altimeter will jump higher (over-reading), the ASI will over-read, and the VSI will show a brief climb before settling to zero. Note the magnitude of the error on the ground/at a known altitude and apply a correction, typically a few hundred feet and a few knots.
- Attempt to clear the ports - descend out of icing if terrain and MSA permit, and check whether the alternate source is itself iced.
ILS to runway 27:
- Use the alternate static source and apply the known correction to the glide-slope check, but do not rely on the altimeter as the sole means of terrain clearance.
- Use DME/GNSS/radar-derived altitude as the cross-check for the outer marker and minimums.
- If the alternate source cannot be trusted or the approach requires accurate altitude, divert to an airport with VMC and land visually. Making an ILS with an unusable static system and a 7,500 ft MSA is not worth the risk.