Airspeed with the Static System Blocked
The airspeed indicator compares two pressures: ram pressure from the pitot line, inside its diaphragm, and static pressure from the static line, in its case. When the static line blocks, the case keeps the pressure it had at that moment.
Pitot line
Carries ram pressure, the pressure of the air stopped in the pitot tube. It rises with speed, and it falls as the airplane climbs into thinner air.
Case, static line blocked
Holds the static pressure from the altitude where the line blocked. It does not change when the airplane climbs or descends.
The static line blocks at 8,500 feet while the airplane cruises at 120 knots. It keeps flying at 120 knots. Predict what the airspeed indicator shows at each altitude, then select Check.
Each bar shows a pressure in inches of mercury (in Hg). The needle reads the difference between the pitot line and the case.
Climb
Outside pressure falls, so ram pressure falls with it, while the case holds the higher trapped pressure. The difference is too small, and the airspeed reads low.
Descent
Outside pressure rises, so ram pressure rises, while the case holds the lower trapped pressure. The difference is too large, and the airspeed reads high.
If the static system becomes blocked but the pitot tube remains clear, the ASI continues to operate; however, it is inaccurate. The airspeed indicates lower than the actual airspeed when the aircraft is operated above the altitude where the static ports became blocked because the trapped static pressure is higher than normal for that altitude. When operating at a lower altitude, a faster than actual airspeed is displayed due to the relatively low static pressure trapped in the system.
Source: FAA-H-8083-25B, Pilot’s Handbook of Aeronautical Knowledge, Chapter 8, Blocked Static System. A US government work in the public domain.Why It Teaches
Committing to a prediction, even a wrong one, improves memory for the answer that follows. A 2024 meta-analysis of prequestions found an effect of about g = 0.54 on the predicted items (St. Hilaire, Chan and Ahn). The prediction error is what does the work.
So the options are the three real possibilities, the reveal comes on the same page within seconds, and the student's own pick stays marked beside what happens. The reveal is corrective feedback on the student's model of the instrument, not a score.
It needs a basis for the guess. In the lesson, this page comes after the pages that teach the pitot line, the static line and what a blockage traps, which is why the two cards above restate them.
Principles
Craft note
How It Is Built
The numbers are not typed in. The page computes them with the lesson's own pressure model: the standard atmosphere gives the outside pressure at each altitude, and the airspeed indicator reads the pitot line minus its case. At 9,000 feet that gives about 76 knots, at 8,000 feet about 152 knots, and the bars fill from those pressures.
Predict, then reveal is a local part. The shared ask card says "Not quite" and asks to try again, which suits a check better than a prediction. The quoted handbook text is the real FAA wording, shown on its own paper.