12.1 Pitot-Static Instruments and Blockage Errors

ACS PA.I.G.K1h (pitot-static, vacuum/pressure, and associated flight instruments) and the knowledge under PA.VIII basic instrument maneuvers are one plumbing problem. The Private Pilot Airplane knowledge test does not ask you to design a Kollsman window. It does ask which instrument dies when ice covers which hole, whether a climb with a blocked pitot makes the airspeed needle rise or fall, and whether density altitude is a height above the ground.

Two holes, three instruments

PHAK Chapter 8 is blunt about the sources.

The airspeed indicator is a differential-pressure instrument. It compares pitot (total) pressure with static pressure. The difference is dynamic pressure, the q in the lift equation. The altimeter is a static-only instrument: sealed aneroid wafers expand as static pressure falls (climb) and contract as it rises (descent). The Kollsman window adds a local altimeter setting so those wafers can be referenced to a sea-level pressure rather than to a raw vacuum. The vertical speed indicator is also static-only: a diaphragm sees the rate of change of static pressure through a calibrated leak. Instantaneous VSIs add accelerometers; trainers usually have the simple leak design.

If you remember nothing else: pitot blockage cannot freeze the altimeter or the VSI, because those two never see ram air. Static blockage can lie to all three.

The blockage table PAR actually writes

Ice, tape left over a static port after painting, a bug in the pitot, or a forgotten pitot cover are the same exam. Work each failure from the plumbing, not from a memorized cartoon.

Pitot ram inlet blocked, drain hole OPEN. Remaining pressure in the pitot line vents overboard through the drain. Pitot pressure falls to essentially static. Dynamic pressure goes to zero. The ASI drops to zero. Altimeter and VSI are untouched. This is the “ASI to zero” row.

Pitot ram inlet AND drain hole both blocked. Pressure in the pitot line is trapped at the value that existed when the ice sealed both holes. The ASI now compares a constant total pressure with a changing static pressure, so it acts like an altimeter.

• Level flight: ASI is frozen at the indicated speed when the blockage occurred.
• Climb: outside static pressure falls, the trapped-minus-static difference grows, and the ASI overreads (shows faster than you really are).
• Descent: static pressure rises, the difference shrinks, and the ASI underreads.

Altimeter and VSI still work. The trap is treating every pitot failure as “needle to zero.” Zero is only the open-drain case.

Static port(s) blocked, pitot open. Static pressure in all three instruments is trapped at the blockage altitude.

• Altimeter freezes at the altitude where the port iced or was taped.
• VSI goes to zero and stays there; the calibrated leak no longer sees a changing pressure.
• ASI is wrong in the opposite sense from the both-holes-blocked pitot case. In a climb, trapped static is higher than the real outside static, so the indicated dynamic pressure is too small and the ASI underreads. In a descent, trapped static is lower than real static and the ASI overreads.

Pitot and static both blocked. All three instruments freeze at the values they held when the last hole closed. You are now flying a clock and a compass.

That table is the chapter. If a stem says “both the pitot tube and the drain hole ice over, then you climb,” the needle rises. If it says “only the ram inlet ices and the drain is still open,” the needle dies. If it says “static port ices and you descend,” the ASI overreads and the altimeter does not move.

Alternate static — PHAK’s direction, not a vibe

Most trainers have an alternate static source under the panel that opens the static lines to cabin air. In an unpressurized airplane the cabin is usually at a slightly lower pressure than the true outside static (venturi effect of the slipstream, leaks, the heater). PHAK’s direction is therefore fixed:

• Altimeter indicates a slightly higher altitude than true.
• ASI indicates a slightly higher airspeed than true.
• VSI shows a momentary climb, then settles to a usable rate.

Do not invert that for a 172. A pressurized cabin is a different machine; PAR is asking about the unpressurized trainer. After you pull the alternate-static knob, expect the three needles to jump, then fly the new slightly-high picture. Opening a window or breaking the VSI glass is the old emergency static trick some orals still mention; the certificated alternate-static valve is what the AFM tells you to use.

Six altitudes, five airspeeds

The test recycles vocabulary until you mix “true” with “absolute.”

Pressure altitude is a setting, not a weather product. Density altitude is a performance number, not a height AGL. Absolute is the only one that cares about the dirt under you. True is MSL. If the stem says “height above the surface,” the word they want is absolute.

Airspeed is the same kind of ladder:

IAS is what you fly. TAS rises with altitude at a given IAS because density falls. Groundspeed is a navigation number, not a pitot-static gauge. The ASI never “knows” the wind.

Pitot heat is the blockage you can prevent

The pitot mast has an electric heating element. It is anti-ice for the ram inlet and, on many masts, the drain. Turn it on in visible moisture when the air is near freezing, and anytime the AFM/POH or icing checklist says so. It is not a 14 CFR 91.205 day-VFR required instrument. A failed pitot-heat breaker plus a winter climb through a cloud is how you get the “both holes blocked, ASI acts like an altimeter” row in real life. Glass or steam, the plumbing is the same: the air-data computer still drinks the same two holes.

Scenario: Priya’s taped static port

Priya launches after a fresh paint job. Nobody pulled the tape off the static port. Pitot is clear. At 3,000 feet indicated she starts a climb to 5,500. The altimeter stops. The VSI dies at zero. The ASI, which had been an honest 80 knots, winds down as she climbs — she is not slowing; trapped static is too high and the ASI is underreading. She does not chase the decaying needle with pitch. She levels, pulls alternate static, watches the altimeter and ASI jump slightly high, and flies the PHAK picture home. If the ice had been on the pitot ram and the drain instead, the altimeter would still have climbed and the ASI would have risen as she climbed. Same airplane. Different hole.