Pitot-Static Instruments

How the three split the pressure

Definition

Three of your flight instruments run on air pressure. The pitot tube senses ram (impact) pressure; the static port senses the ambient (still-air) pressure. The airspeed indicator uses both — ram minus static is the dynamic pressure that becomes your airspeed. The altimeter and the vertical speed indicator use static pressure only.

Explanation

You already met this plumbing in Module 3 (the pitot-static system and pitot heat) and saw the airspeed arcs as limitations in the Regulations module. Here the focus is what each instrument does with the pressure: one instrument compares ram to static (airspeed), and two read static alone (altitude, and its rate of change).

Figure 8-1. Pitot-static system and instruments. — Figure 8-1 · FAA-H-8083-25C, Pilot's Handbook of Aeronautical Knowledge (p. 204)

Why it matters

Knowing which instrument reads which pressure is the key to understanding both the normal readings and what a blockage does — the failures make sense once you know the plumbing.

A DPE might ask

Which instruments are on the pitot-static system?
Which pressures does each one use?

PHAK ch. 8 — Pitot-static flight instruments

The airspeed indicator + your V-speeds

Definition

The airspeed indicator measures dynamic pressure (ram minus static) and shows your indicated airspeed. But 'airspeed' has layers: indicated is what you read; calibrated corrects for instrument and position error; true airspeed corrects for altitude and temperature (it rises roughly 2% per 1,000 feet); and groundspeed is true airspeed adjusted for wind. Fill in your airplane's V-speeds below — tap the info icon for any you've forgotten.

Explanation

Several of the V-speeds are the color-arc endpoints you met in Regulations: Vso and Vs1 are the bottoms of the white and green arcs, Vfe the top of the white, Vno the top of the green, Vne the red line. Others — Vx, Vy, Va, Vg, Vr — aren't marked on the dial but are just as important to know cold. Your numbers live in Section 2 of your POH.

Figure 8-8. Single engine airspeed indicator (ASI). — Figure 8-8 · FAA-H-8083-25C, Pilot's Handbook of Aeronautical Knowledge (p. 211)

Why it matters

Your V-speeds are the numbers every takeoff, climb, approach, and emergency is flown by. Knowing your own airplane's speeds — not a generic trainer's — is basic airmanship a DPE will expect on sight.

A DPE might ask

What's the difference between indicated, calibrated, true, and ground speed?
What are your airplane's Vx, Vy, Va, and best-glide speeds?

PHAK ch. 8 — Airspeed indicator & airspeed types

The altimeter

Definition

The altimeter is an aneroid barometer: sealed capsules expand and contract as static pressure changes, and that movement drives the pointers. You tell it today's sea-level pressure through the altimeter setting in the Kollsman window (29.92 inches is the standard datum). On a two-pointer trainer altimeter, the long thin pointer reads hundreds of feet and the short fat pointer reads thousands.

Explanation

Know your altitudes: indicated (what you read on the current setting), pressure (what you read set to 29.92), density (pressure altitude corrected for temperature — the performance number), true (actual height above sea level), and absolute (height above the ground). And remember the trap: 'from high to low, look out below' — flying into lower pressure or colder air without resetting leaves your true altitude below what's indicated.

Figure 8-2. Altimeter. — Figure 8-2 · FAA-H-8083-25C, Pilot's Handbook of Aeronautical Knowledge (p. 205)

Why it matters

The altimeter keeps you above terrain and at your assigned altitude. Setting it correctly and knowing the high-to-low trap is a real terrain-clearance issue, not trivia.

Common error

A classic mistake is flying a long way — especially into worsening weather or colder air — without updating the altimeter setting. As you fly from high pressure to low (or warm to cold) without resetting, your true altitude drops below what the altimeter shows. Update your setting regularly.

You fly toward an area of lower pressure without resetting the altimeter. Your true altitude is:

A DPE might ask

How does the altimeter work, and what does the Kollsman window set?
Define indicated, pressure, density, and true altitude.

PHAK ch. 8 — Altimeter & altitude types

The vertical speed indicator

Definition

The VSI measures the rate at which static pressure is changing and displays it as feet per minute of climb or descent. Its dial reads zero at the 9 o'clock position, with climb above and descent below.

Explanation

A basic VSI has a built-in lag of a few seconds — it needs a moment to catch up to a pitch change. It shows a trend immediately (the needle starts moving the right direction) and the accurate rate once things stabilize. An instantaneous VSI (IVSI) uses accelerometer pumps to reduce that lag.

Figure 8-5. Vertical speed indicator (VSI). — Figure 8-5 · FAA-H-8083-25C, Pilot's Handbook of Aeronautical Knowledge (p. 209)

Why it matters

The VSI is how you fly a precise climb or descent rate. Knowing it lags keeps you from chasing the needle instead of setting a pitch and power and letting it settle.

A DPE might ask

What does the VSI measure?
Why does a basic VSI lag, and what's an IVSI?

PHAK ch. 8 — Vertical speed indicator

When the system blocks

Definition

Because these instruments run on pressure, a blocked port gives a wrong reading — and the wrong reading follows a predictable pattern. A blocked pitot tube (with its drain hole open) makes the airspeed fall toward zero. If the drain blocks too — usually ice — the trapped pressure makes the airspeed indicator behave like an altimeter: reading high in a climb and low in a descent. A blocked static port freezes the altimeter, zeroes the VSI, and makes the airspeed read low in a climb and high in a descent.

Explanation

The fix for a static blockage is the alternate static source, which most airplanes have — it draws static pressure from inside the cabin. If you have no alternate source in an unpressurized airplane, breaking the glass of the VSI opens the system to cabin static (which reads slightly high because cabin pressure is a bit lower than outside). Know what your airplane has before you need it.

Blockage	What happens
Pitot blocked, drain open	The airspeed indicator drops toward zero — no ram pressure reaches it.
Pitot & drain blocked (ice)	The airspeed acts like an altimeter — reads high in a climb, low in a descent.
Static blocked	The altimeter freezes at the blockage altitude, the VSI reads zero, and the airspeed reads low in a climb / high in a descent.
The fix Do this	Select the alternate static source. Unpressurized with no alternate, breaking the VSI glass gives cabin static (reads slightly high).

Why it matters

A blocked pitot or static has caused fatal accidents when pilots trusted a lying instrument. Recognizing the pattern and reaching for alternate static is exactly the airmanship this module exists to build.

Common error

The dangerous mistake is trusting an airspeed or altimeter that's behaving oddly instead of recognizing a blockage and cross-checking. If your static ports block, your altimeter and VSI go dead and your airspeed reads backward — select alternate static and fly pitch and power.

In flight the altimeter is stuck, the VSI reads zero, and the airspeed reads oddly. You should:

A DPE might ask

What happens to each instrument if the static system blocks?
What does a blocked pitot do, and how do you recover?

PHAK ch. 8 — Blockage of the pitot-static system

Wrap

Definition

Three instruments, one shared source of pressure. The airspeed indicator compares ram to static; the altimeter and VSI read static alone. Know your airspeed types (indicated → calibrated → true → ground) and your altitude types (indicated / pressure / density / true / absolute), keep your altimeter set, and reach for alternate static if the static system blocks.

Explanation

Everything here comes back to the plumbing: what each instrument senses tells you both its normal reading and its failure mode. Fill in your V-speeds, keep the altimeter current, and know your alternate static source — those are the habits that make the pitot-static system a tool instead of a trap.

Instrument	Senses / remember
Airspeed indicator	Ram minus static (dynamic pressure). IAS → CAS → TAS (≈2%/1,000 ft) → GS.
Altimeter	Static pressure. Set the Kollsman; know indicated / pressure / density / true / absolute.
VSI	Rate of static-pressure change (fpm), with a little lag.
When it blocks	Alternate static for a static blockage; a blocked-and-drained pitot reads like an altimeter.

Why it matters

This is the foundation the rest of Module 4 builds on — the gyroscopic instruments, the compass, and the instrument scan all assume you can read and trust these three first.

A DPE might ask

Summarize what each pitot-static instrument senses.
What are the airspeed and altitude types?

PHAK ch. 8 — Flight instruments summary