Slater Aviation · Flight instruments

Pitot-static lab

Fly a climb or a descent and watch the airspeed indicator, altimeter, and vertical speed indicator. Then block the pitot tube or the static port, see how each instrument lies, and fix it with pitot heat or the alternate static source.

Slater Aviation · Deeper Knowledge. Better Pilots.

Instrument panel

Click or tap an instrument to flip just that one.
Airspeed indicator
  1. 1Diaphragm. Pitot pressure fills it. Static pressure in the case pushes on the outside.
  2. 2Pitot inlet. Line from the pitot tube into the diaphragm.
  3. 3Static inlet. Line from the static port into the sealed case.
  4. 4Rocking shaft and sector gear. Turn the diaphragm's movement into rotation.
  5. 5Pinion and hairspring. Drive the pointer. The spring takes out play.
Altimeter
  1. 1Aneroid wafers. Sealed, with the air pumped out. They expand as case pressure drops.
  2. 2Static inlet. The only connection: static pressure fills the case.
  3. 3Rocking shaft and sector gear. Pass the wafers' expansion to the gear train.
  4. 4Gear train. Drives the 100-ft, 1,000-ft, and 10,000-ft hands.
  5. 5Barometric setting knob. Sets the Kollsman window by shifting the mechanism.
Vertical speed indicator
  1. 1Diaphragm. Static pressure fills it right away.
  2. 2Direct static inlet. Feeds the diaphragm with no delay.
  3. 3Calibrated leak. A tiny orifice that lets case pressure catch up slowly.
  4. 4Rocking shaft and sector gear. Pass the diaphragm's movement on.
  5. 5Pinion and hairspring. Drive the pointer.

What the airplane is doing vs. what the instruments say

ActualIndicatedError
Airspeed (kt)
Altitude (ft)
Vertical speed (fpm)
Outside static
Total (static + ram)
Cabin
Pitot line
Static line
VSI case
System

All normal.

Flight

Right to climb, left to descend. The airplane stays between 500 and 14,500 ft.

Malfunction

Pitot tube ice closes the ram air inlet only. "Pitot blocked" closes the inlet and the drain hole.

Remedies

Try a scenario

Pitot blockage

How the ASI fails depends on whether the drain hole is still open.

  • Inlet only (ice): pressure bleeds out the drain hole and the ASI drops to zero.
  • Inlet and drain: pitot pressure is trapped and the ASI acts like an altimeter. It reads high in a climb and low in a descent.
  • The altimeter and VSI are unaffected.

Pitot heat melts ice. It won't clear an insect nest, so check the tube on preflight.

Static blockage

  • The altimeter freezes at the altitude where the port blocked.
  • The VSI goes to zero and stays there.
  • The ASI reads low in a climb and high in a descent. Reading high while descending can lead you toward a stall or an overspeed.

Fix it by opening the alternate static source. As a last resort, breaking the VSI glass vents the system to the cabin, but the VSI then reads in reverse.

Alternate static source

In most unpressurized airplanes, cabin pressure is a little lower than outside static pressure because air flows fast around the fuselage. On alternate static:

  • The altimeter reads a little high.
  • The ASI reads a little high.
  • The VSI shows a brief climb, then settles.

The POH has a correction table for your airplane.

Inside the instruments

  • ASI: a diaphragm measures pitot minus static pressure.
  • Altimeter: sealed aneroid wafers measure static pressure alone.
  • VSI: a diaphragm compares static pressure now with case pressure that lags through a calibrated leak. The lag is 6 to 9 seconds.

None of these use a Bourdon tube. Bourdon tubes measure high pressures, such as oil pressure.

A simplified teaching model on a standard day with the altimeter set to 29.92 inHg. Cabin pressure on alternate static is set about 10% of ram pressure below outside static pressure. Numbers are illustrative and not for flight planning. Follow your POH. Reference: Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 8.