Blocked Pitot Tube in the Climb
During a climb from 2,000 ft to 9,000 ft in a non-pressurised single, the pilot notices the airspeed indicator rising from 100 KIAS at 2,000 ft to 145 KIAS at 9,000 ft, although power setting, attitude and the absence of vibration suggest about 100 KIAS. On levelling off the ASI slowly falls and settles near 120 KIAS. GPS ground speed is 118 kt with an estimated 10 kt tailwind component. The VSI reads zero, the altimeter shows 9,000 ft and agrees with the area QNH. Pitot heat was selected on for the whole flight, and the aircraft has a heated pitot with drain holes and fuselage static ports. The pilot has an alternate static source available and must decide what has failed and how to proceed to destination in VMC.
1. Diagnose the instrument failure, using the three pressure instruments to support your conclusion.
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The ASI is displaying the difference between total pressure at the pitot and static pressure. If the pitot is blocked while the static port remains clear, total pressure is trapped at the value it had when the blockage occurred, while static pressure falls as the aircraft climbs. The difference therefore grows and the ASI reads increasing speed - in effect it has become an altimeter, reading higher as altitude increases. That is exactly the observed rise from 100 to 145 KIAS.
The altimeter still responds correctly to 9,000 ft and agrees with the QNH, and the VSI is steady in level flight. Both of those instruments use static pressure only, so static is clearly serviceable. That rules out a static blockage, which would freeze the altimeter, drive the VSI to zero and make the ASI over-read in the climb. The fault is in the pitot or its plumbing, possibly ice that the heater did not clear, or an insect or foreign body in the tube.
2. What would the ASI show in a descent, and why is that dangerous?
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In a descent, static pressure rises while the trapped total pressure stays fixed, so the indicated difference falls. The ASI would decrease as the aircraft descends, even though true airspeed was unchanged or increasing - potentially reading close to zero in a normal descent.
The danger is that a pilot who trusts the ASI would lower the nose to regain speed and then, on the approach, pitch up to raise the indicated speed. That could put the aircraft close to the stall at low altitude with a false low-speed indication, or produce an excessive speed on final if the pilot over-corrects. It also corrupts every V-speed decision: rotate, best climb, approach and flap limiting speeds cannot be referenced from a failed ASI. The situation is worst in IMC or at night, where attitude and power are the only remaining cues; here the flight is in VMC, which allows a safe visual approach.
3. State your actions and your plan for the approach and landing.
Read the worked answer
Cover or disregard the ASI, confirm pitot heat is on, and check the pitot drain and any secondary airspeed source. Advise ATC of the unreliable airspeed and request any assistance available, such as radar-derived groundspeed. Do not select the alternate static source: the static system is working, and selecting it would corrupt the altimeter and VSI as well.
Fly by attitude, power and GPS ground speed, corrected for wind. Use known power settings and pitch attitudes for climb, cruise and descent, and cross-check with the altimeter and VSI. Plan a long, stable, visual approach with ample runway, no significant crosswind if avoidable, and a configuration and speed flown by attitude and power. Expect to carry a little extra speed and accept a longer landing distance rather than a slow approach. After landing, write up the defect specifically as a suspected pitot blockage and ensure the system is checked before the next flight.