FAA Flight Instructor Airplane (FIA)Aircraft Systems and InstrumentsMedium
A pilot's pitot tube becomes completely blocked by insects during preflight, but the drain hole remains open and the static port is unobstructed. What will happen to the airspeed indicator during a climb after takeoff?
- AThe airspeed indicator will behave like an altimeter, increasing in indicated airspeed during a climb
- BThe airspeed indicator will freeze at the last accurate reading before the blockage occurred
- CThe airspeed indicator will read zero regardless of actual airspeed
- DThe airspeed indicator will read accurately throughout the flight
Show answer & explanationAnswer & explanation
Correct answer: C. The airspeed indicator will read zero regardless of actual airspeed
With the pitot tube fully blocked but its drain hole open, ram air pressure cannot enter the system, and the trapped pressure equalizes to ambient pressure through the drain hole. Since the static port is unobstructed, both the pitot chamber and static chamber read the same ambient pressure, resulting in zero pressure differential and an airspeed indication of zero.
Why the other options are wrong
- A. The altimeter-like behavior occurs when both the pitot inlet and drain hole are blocked, trapping pressure.
- B. Freezing at the last reading occurs only if the drain hole is also blocked, trapping pressure.
- D. Accuracy requires an open pitot inlet, which is blocked.
Pitot Blockage with Open Drain Hole
When the pitot tube is blocked but its drain hole remains open, the airspeed indicator reads zero because pressure cannot build in the pitot line.
- Drain hole open + pitot blocked = ASI reads zero
- Drain hole blocked + pitot blocked = ASI acts like altimeter
- Static port blockage affects both ASI and altimeter differently
Memory trick: 'Open drain lets the pressure drain away—needle stays at zero'