An airplane's pitot tube becomes completely blocked by insects during preflight, but the pitot drain hole remains open and the static ports are functioning normally. During a subsequent climb, how will the airspeed indicator most likely behave?
- AIt will act like an altimeter, increasing as the airplane climbs and decreasing as it descends
- BIt will read erratically, fluctuating rapidly with no consistent pattern
- CIt will freeze at the airspeed value present when the blockage occurred
- DIt will slowly decrease toward zero regardless of pitch changes
Show answer & explanationAnswer & explanation
Correct answer: A. It will act like an altimeter, increasing as the airplane climbs and decreasing as it descends
With the pitot tube blocked but the drain hole open, ram air pressure can no longer enter the pitot line, but static pressure can still leak into the pitot side through the drain hole. This causes the airspeed indicator to respond to changes in static (ambient) pressure like an altimeter — indicating an increase in airspeed during a climb and a decrease during a descent, opposite of normal behavior.
Why the other options are wrong
- B. Erratic fluctuation is not the expected consistent behavior from this specific type of blockage.
- C. Freezing at a fixed value would occur only if both the pitot tube and drain hole were blocked with the static system unaffected.
- D. A steady decrease to zero would occur if pitot pressure dropped to zero with no drain hole leak, not the case here.
Pitot Blockage with Open Drain Hole
A pitot-static system malfunction where a blocked pitot tube with a functioning drain hole causes the airspeed indicator to behave like an altimeter, rising in climbs and falling in descents.
- Pitot blockage alone (drain hole also blocked) freezes ASI at last correct reading
- Pitot blockage with open drain hole lets static pressure enter, causing altimeter-like ASI behavior
- Static port blockage affects altimeter, VSI, and ASI differently than pitot blockage
Memory trick: Open drain lets static sneak in and fake the climb.