The Airspeed Indicator That Behaves Like an Altimeter
Departing Enugu in a PA-28-181 in moderate rain, you climb through 500 ft AGL at full power with pitot heat still off. At rotation the airspeed indicator read 65 KIAS. You set the attitude for 78 KIAS, but the needle keeps rising: 95, then 110, then 125 KIAS, while the tachometer holds 2,450 RPM, the altimeter winds up smoothly and the VSI shows +700 ft/min. You level at 3,000 ft and reduce to 2,300 RPM; the ASI holds near 118 KIAS even though power and attitude now correspond to about 100 KIAS. You begin a 500 ft/min descent back towards Enugu and the ASI falls steadily through 80 to 62 KIAS, while the altimeter unwinds smoothly and the VSI reads −500 ft/min. The engine is smooth throughout, the attitude is normal, and there is no autopilot. Enugu is 8 minutes away with a 3,000 m runway; cloud base is 1,500 ft and visibility 6 km in rain.
1. Diagnose the failure and explain how the three pressure instruments fit together.
Read the worked answer
The failing instrument is the airspeed indicator, and the pattern identifies a blocked pitot head with the static port still clear.
- When the pitot (ram) port and the drain hole both block - typically water or ice in the head - the pressure trapped in the pitot line is fixed at the value it had when the blockage formed.
- In the climb, outside static pressure falls but the trapped pitot pressure does not, so the differential the ASI sees grows and indicated airspeed rises even though nothing about the aircraft's speed has changed. That is your 65 → 125 KIAS at a steady 2,450 RPM.
- On levelling off the differential stops growing, so the needle holds near its high value rather than settling back to the true 100 KIAS.
- In the descent static pressure rises while the trapped pitot pressure cannot, so the differential falls and the ASI under-reads - your 62 KIAS at a normal descent attitude.
Crucially, the altimeter and VSI behave normally, which proves the static source is clear, and the steady tachometer proves the engine is fine. The ASI has effectively become a crude altimeter.
2. What do you do in the air, and how do you plan the approach and landing?
Read the worked answer
Actions:
- Pitot heat ON immediately. If the blockage is water or ice in the head, heat may clear it; watch for the needle returning to a sensible value.
- Stop chasing the ASI. Fly pitch and power: known attitude and RPM settings for climb, level and descent, cross-checked against the tachometer and GPS ground speed.
- Tell ATC/other traffic that the ASI is unreliable and that you are returning VFR.
- Do not enter cloud. Without a trustworthy ASI you cannot fly a safe instrument approach in IMC, and the cloud base at 1,500 ft gives you no room to experiment.
Approach and landing: fly a normal circuit by attitude and GPS ground speed; use a long runway (Enugu's 3,000 m helps), a normal sight picture and the tachometer for power. Expect the ASI to read nonsense on final - use attitude, height and groundspeed instead. If the ASI recovers, treat the reading with suspicion until it has been checked on the ground; if it does not, land and write it up before the next flight.
3. How would the indications have differed if the static port had been blocked, and why does that distinction matter?
Read the worked answer
A static blockage gives a diagnostic mirror image:
- The ASI would read low in the climb, not high, because the trapped static pressure is higher than the真实 outside pressure, reducing the differential; it would read high in a descent.
- The altimeter would freeze and the VSI would read zero, because both depend on the static source. That is the giveaway.
- Pitot heat does nothing for a static blockage; you would use the alternate static source if fitted, expecting a transient as cabin pressure equalises, and navigate on GPS altitude and groundspeed.
Why the distinction matters: a pitot blockage corrupts speed information but leaves you height and rate of climb; a static blockage takes away altitude, which is far more dangerous, and it cannot be cured from the cockpit. If both are blocked you have lost the whole pitot-static group, and the response is to fly by attitude and GPS and land as soon as practicable - which, in rain and a 1,500 ft cloud base, means turning back now rather than pressing on.