IAS Rises in the Climb with Normal Altimeter and VSI
A light twin-jet is climbing from FL120 to FL240. The PF holds 250 KIAS and 1,800 ft/min; thrust and pitch are unchanged, and the FMS ground speed is steady at 410 kt. Passing FL168, the captain notices the IAS has risen to 278 kt and continues to increase about 2 kt per 1,000 ft, although nothing has changed. The altimeter and VSI agree with each other and with the planned profile, and the standby altimeter is within 40 ft. TAT is −21 °C and the pitot heat has not been switched on. There is no ice detector warning, but the aircraft has been in visible moisture since FL150. The crew has 90 minutes of fuel over destination and no nearby diversion. ATC has cleared them to continue climbing.
1. What is the most likely explanation for the IAS behaviour, and why do the altimeter and VSI evidence point to that cause rather than a static blockage?
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The IAS is behaving like an altimeter: it rises as the aircraft climbs even though pitch, thrust, and ground speed are unchanged. That is the signature of a blocked pitot tube with the drain open. The pitot line is sealed at the tip, so the pressure trapped in the line remains at the value corresponding to FL168. As the aircraft climbs, static pressure falls; because the ASI measures pitot minus static, the differential grows and the indicated speed increases. If the drain were also blocked, the trapped volume would be fixed and the IAS would freeze at the blockage value. If the static system were blocked, the altimeter would freeze and the VSI would read zero, which is not the case. The normal altimeter and VSI therefore rule out a static blockage and point to the pitot side. The absence of an ice warning does not exclude ice, because the pitot heat was off in visible moisture at −21 °C.
2. What immediate actions should the crew take, and which instruments should they use to fly the aircraft if the indication does not recover?
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Select pitot heat ON immediately, because the most likely cause is ice in the pitot tube; if the indication returns to a sensible value, treat it as an icing event and record it. Cross-check the IAS against GPS ground speed, pitch attitude, power setting, and angle of attack. The altimeter and VSI are still valid, so the static system can be used. Do not select the alternate static source; it would introduce a deliberate static error and would not fix a pitot blockage. If the IAS remains unreliable, fly by attitude, power, and AoA, and use GPS ground speed for speed control. Notify ATC that the airspeed indicator is unreliable and request a suitable routing. Consider descending to a warmer level only if terrain and fuel permit, but do not rely on the IAS clearing. The crew has 90 minutes of fuel over destination, so a diversion is not immediately required, but they should plan for a possible diversion if other systems degrade.
3. If the aircraft later descends at 250 KIAS with the same blockage, what would the ASI indicate and why is that dangerous?
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In a descent, static pressure rises while the trapped pitot pressure remains constant. The pressure difference across the ASI capsule therefore falls, and the ASI under-reads the true airspeed. The crew could believe they are slower than they are and increase power or lower the nose, risking an overspeed and structural damage. The error grows with the depth of the descent. The correct technique is to ignore the IAS, use GPS ground speed and AoA, and respect the aircraft's Vmo/Mmo by reference to attitude and power, because the ADC's Mach indication is also derived from pitot and static and is therefore unreliable. If the blockage clears, the ASI may jump; warn the crew. Do not attempt an approach with an unreliable ASI if a precision approach is needed; use a suitable alternate with a long runway and good weather.