Practice test
Unit 5 · Flight Instruments · Lesson 1 of 3

The pitot-static instruments

10 min readLesson 21 of 593D lab

By the end of this lesson you'll be able to:
  • Explain which instruments use pitot and static pressure
  • Read the airspeed indicator markings
  • Describe the types of altitude and how to set the altimeter

Three instruments run on air pressure: the airspeed indicator, the altimeter and the vertical speed indicator (VSI). The pitot tube faces forward and senses ram (impact) air pressure. The static port, usually on the side of the fuselage, senses still-air (static) pressure.

InstrumentUses
Airspeed indicatorPitot and static
AltimeterStatic only
Vertical speed indicatorStatic only

Airspeed indicator markings

  • White arc: flap operating range, from VS0 (stall, landing configuration) to VFE (max flap extended).
  • Green arc: normal operating range, from VS1 (stall, clean or a specified configuration) to VNO (max structural cruising speed).
  • Yellow arc: caution range, from VNO to VNE. Smooth air only.
  • Red line: VNE, the never-exceed speed.

VA (maneuvering speed) is not marked on the indicator; find it in the POH or on a placard.

The altimeter and altimeter setting

The altimeter is a barometer that shows altitude. Because air pressure changes with the weather, you set the current local altimeter setting in the Kollsman window. Below 18,000 feet MSL, use the setting of a station within 100 NM of your route (14 CFR 91.121); at and above 18,000 feet, everyone uses 29.92.

Flying from high pressure to low pressure without resetting makes the altimeter read higher than your true altitude: "high to low, look out below." Flying from warm to cold air does the same.

Five kinds of altitude

  • Indicated: what the altimeter shows with the current setting.
  • True: height above mean sea level (MSL). Charts show obstacles and airports in MSL.
  • Absolute: height above the ground (AGL).
  • Pressure: what the altimeter shows set to 29.92. Used for performance charts and flight levels.
  • Density: pressure altitude corrected for temperature. It's how the airplane "feels" the air (Unit 11).

When it goes wrong

  • Blocked pitot tube (ram opening and drain hole both blocked): the airspeed indicator acts like an altimeter, showing higher airspeed as you climb and lower as you descend.
  • Blocked static port: the altimeter freezes, the VSI reads zero, and the airspeed indicator becomes inaccurate (reads low in a climb, high in a descent).
  • Alternate static source (inside the cabin, where pressure is slightly lower): the altimeter and airspeed read slightly high, and the VSI briefly shows a climb.
Try it in 3D: The six-pack, liveFly with sliders, watch every instrument respond, then fail the vacuum, pitot or static system.

Key takeaways

  • Airspeed uses pitot and static; altimeter and VSI use static only.
  • Green arc ends at VNO; red line is VNE; VA isn't marked.
  • High to low, look out below: you're lower than indicated.

What the examiner may ask

Practice answering out loud before you open each answer.

What happens to the instruments if the pitot tube and its drain hole are blocked?
The airspeed indicator acts like an altimeter: it shows an increase in a climb and a decrease in a descent.
How do you use the alternate static source, and what errors does it cause?
Open it per the POH. Because cabin pressure is slightly lower, the altimeter and airspeed read slightly high and the VSI briefly shows a climb.
What are the color markings on your airspeed indicator?
Know your airplane's VS0, VS1, VFE, VNO and VNE, and where VA is published.

Common mistakes

  • Forgetting to set the altimeter before takeoff or when flying into a new area.
  • Leaving the pitot cover on.
  • Thinking VA is marked on the airspeed indicator.

Read it in the FAA sources

PHAK · Chapter 8Flight InstrumentsOpens the PHAK at this chapter ↗

Lesson check

Get every question right to complete the lesson. Take your time; wrong answers explain why.

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