Flight Instruments
PHAK Ch 8 • Pitot-Static, Gyroscopic, Six-Pack Scan
Memory Aids
A TOMATO FLAMES, ANDS & UNOS, Pitot-Static, Transponder Codes
- AAirspeed
- TTachometer
- OOil pressure
- MManifold pressure (if applicable)
- AAltimeter
- TTemperature gauge
- OOil temp
- FFuel gauge
- LLanding gear indicator
- AAnti-collision lights
- MMagnetic compass
- EELT
- SSeat belts
- ANAccelerate → North — on E/W headings
- DSDecelerate → South — on E/W headings
- UNUndershoot North — turning through N/S
- OSOvershoot South — turning through N/S
Pitot = Speed, Static = Everything
Pitot tube feeds ONLY the ASI. Static port feeds ALL THREE: ASI + Altimeter + VSI. Block the pitot → ASI fails. Block the static → all three are wrong.
Transponder Codes
7500 = hijack (taken alive), 7600 = comm failure (can't talk), 7700 = emergency (going to heaven), 1200 = VFR
Key Takeaways
- The six-pack splits into two systems: pitot-static (ASI, Altimeter, VSI) and gyroscopic (AI, HI, TC).
- Pitot-static uses air-pressure differences; gyros use rigidity in space and precession.
- The Turn Coordinator is electric — your backup bank reference if the vacuum system fails.
- Realign the HI with the magnetic compass every 15 minutes (it precesses ~3°/15 min).
- ASI color arcs: White = flaps (Vso–Vfe), Green = normal (Vs1–Vno), Yellow = smooth air only (Vno–Vne), Red line = Vne; Va is not marked.
Attitude Indicator
Gyroscopic · Vacuum/Electric
Shows: Pitch & Bank
Key limit: ±60° pitch, ±100° bank
CFI tip: Primary pitch + bank reference. If it tumbles after unusual attitudes, cage and re-erect before trusting it.
Altimeter
Pitot-Static · Static pressure
Shows: Altitude MSL
Key limit: 1" Hg ≈ 1,000 ft error
CFI tip: Set Kollsman window to current altimeter setting. Above FL180, set 29.92. "High to low, look out below."
Airspeed Indicator
Pitot-Static · Pitot + Static
Shows: Indicated Airspeed
Key limit: Vne (red line)
CFI tip: Know your color arcs: White=flaps, Green=normal, Yellow=smooth air only, Red line=never exceed.
Turn Coordinator
Gyroscopic · Electric
Shows: Rate of Turn + Coordination
Key limit: Standard rate = 3°/sec
CFI tip: Your electric BACKUP if vacuum fails. "Step on the ball" — ball left, press left rudder.
Heading Indicator
Gyroscopic · Vacuum
Shows: Magnetic heading
Key limit: Precesses ~3°/15 min
CFI tip: Realign with magnetic compass every 15 minutes in straight-and-level, unaccelerated flight.
Vertical Speed Indicator
Pitot-Static · Static pressure
Shows: Rate of climb/descent (fpm)
Key limit: 6-9 sec lag
CFI tip: Use as a TREND instrument — don't chase it; lag is normal. An IVSI reduces lag with accelerometers.
Two Systems, Six Instruments
Pitot-Static System, Gyroscopic System
Pitot-Static System
Measures speed, altitude, and vertical rate from air-pressure differences.
- ASI — pitot (ram) vs static
- Altimeter — static only (aneroid wafers)
- VSI — static through calibrated leak
Gyroscopic System
Spinning gyros sense attitude and direction via rigidity in space and precession.
- AI — vacuum (or electric) · pitch & bank
- HI — vacuum · heading (precesses ~3°/15 min)
- TC — electric · rate + coordination
Per FAA-H-8083-25C (PHAK), Chapter 8 — Flight Instruments
Airspeed Indicator — Color Arcs
White, Green, Yellow Arcs & Red Line
| Arc | Range | Meaning | Detail |
|---|---|---|---|
| White Arc | Vso → Vfe | Flap operating range | Stall speed (landing config) to max flap speed |
| Green Arc | Vs1 → Vno | Normal operating range | Stall speed (clean) to max structural cruising |
| Yellow Arc | Vno → Vne | Caution range | Smooth air only — risk of structural damage in turbulence |
| Red Line | Vne | Never exceed | Structural failure possible above this speed |
Note: Va (maneuvering speed) is NOT marked on the ASI — it changes with weight.
Altimeter — How It Reads
Aneroid Wafers, Three Pointers, the Kollsman Window
Reading the pointers
Read it like a clock: the long pointer = 100 ft, the medium = 1,000 ft, and the short = 10,000 ft. A crosshatch flag shows when below 10,000 ft MSL.
Set the Kollsman window
Aneroid wafers expand and contract with static pressure. Set the current altimeter setting in the barometric (Kollsman) window — and switch to 29.92" Hg at or above 18,000 ft MSL (Class A).
Turn Coordinator & Coordination
Rate of Turn, the Inclinometer (Ball)
Rate of Turn
The miniature airplane shows turn rate. Align its wing with the index for a standard-rate turn — 3°/sec, a full 360° in 2 minutes (the "2 MIN" mark). It's electric, so it survives a vacuum failure.
The Ball (Inclinometer)
Ball centered = coordinated. Slipping = ball to the inside (too much bank / too little rudder); skidding = ball to the outside. Fix it by "stepping on the ball" — add rudder toward the ball.
Key Takeaways
- The attitude indicator is your primary reference; the scan radiates out to supporting instruments and returns to the AI.
- Different phases of flight emphasize different instruments — altimeter for level, airspeed for climbs/descents, bank for turns.
- Three common scan errors: fixation (staring too long), omission (skipping an instrument), and emphasis (over-relying on one).
- Use a continuous radial pattern covering all six instruments; if one disagrees with all others, suspect that instrument.
Radial Scan Technique
Straight & Level, Climbs, Turns, Descents
The attitude indicator is your primary reference; your scan radiates out to the supporting instruments and returns to the AI. Each phase of flight emphasizes different instruments.
Straight & Level
AI → Altimeter → AI → Heading → AI → Airspeed → AI
Emphasis: Altimeter for altitude hold
Climbs
AI → Airspeed → AI → Altimeter → AI → VSI → AI
Emphasis: Airspeed for Vy/Vx
Turns
AI → TC → AI → Heading → AI → Altimeter → AI
Emphasis: Bank angle + altitude hold
Descents
AI → Altimeter → AI → Airspeed → AI → VSI → AI
Emphasis: Airspeed to avoid Vne
Three Common Scan Errors
Fixation, Omission, Emphasis
FAA-H-8083-15B (IFH) Ch 6 — these are what the DPE watches for
Fixation
Staring at one instrument too long and missing deviations on others. Common with the AI or altimeter in turbulence.
Fix: Keep moving — the scan should be continuous, not locked.
Omission
Skipping an instrument in your scan, often the VSI or turn coordinator.
Fix: Include ALL six instruments. Use the radial pattern — always return to the AI.
Emphasis
Over-relying on one instrument instead of cross-checking, so you chase a single needle.
Fix: Trust the ensemble. If one instrument disagrees with all the others, suspect that one — not the rest.
Key Takeaways
- Vacuum failure affects the AI and HI — they drift gradually as the gyros spin down; fall back to the Turn Coordinator (electric) and magnetic compass.
- Blocked pitot + open drain → ASI reads ZERO; blocked pitot + blocked drain → ASI acts like an altimeter (reads high climbing, low descending).
- A blocked static port affects ALL THREE pitot-static instruments — altimeter freezes, VSI reads zero, ASI inaccurate.
- The alternate static source (cabin air) makes the altimeter and ASI read slightly HIGH due to the venturi effect.
- Gyros rely on rigidity in space (AI stability) and precession (HI drift, TC sensing).
Vacuum System Failure
Recognition & Response
Affects the Attitude Indicator and Heading Indicator — both slowly become unreliable as the gyros spin down.
Recognition:
- Vacuum gauge shows low/zero suction (normal is 4.5-5.5 in. Hg)
- AI and HI begin to drift and show conflicting info
- Gradual — not instant. The gyros coast for a while
Response:
- Primary bank → Turn Coordinator (electric, independent)
- Primary heading → Magnetic Compass (with ANDS/UNOS awareness)
- Inform ATC, maintain VMC, plan nearest landing
Pitot-Static System Failures
Blocked Pitot, Blocked Static, Alternate Static Source
Ram air blocked but pressure bleeds out the drain hole, so the ASI drops to ZERO. Only the ASI is affected — altimeter and VSI still work normally (static only).
Pitot tube becomes a sealed chamber, so the ASI acts like an altimeter — reads higher as you climb (static drops, trapped pitot stays), lower as you descend. DANGEROUS: may show safe speed during a stall at altitude!
Affects ALL THREE pitot-static instruments: altimeter freezes at the last altitude, VSI shows zero, ASI grows inaccurate (reads HIGH above the blockage altitude, LOW below). Use the alternate static source if available.
Draws cabin air (slightly lower pressure from the venturi effect). Altimeter reads slightly HIGH, ASI reads slightly HIGH, VSI shows a momentary climb then normalizes. Emergency: break the VSI glass to vent cabin air into the static system.
Gyroscopic Principles
Rigidity in Space, Precession
Rigidity in Space
A spinning gyro resists changes to its axis. The aircraft moves around the gyro — that's how the AI shows pitch and bank against the real horizon.
Precession
A force on a spinning gyro acts 90° in the direction of rotation. This drives HI drift (~3°/15 min) and lets the turn coordinator sense yaw from its canted gyro.
Key Takeaways
- The magnetic compass needs no power source but has errors you must compensate for.
- ANDS (Accelerate North, Decelerate South) errors occur only on east/west headings.
- UNOS (Undershoot North, Overshoot South) turning errors occur when rolling out through N/S; lead/lag ≈ your latitude.
- Variation is true-vs-magnetic north; deviation is aircraft interference — True ± Var = Magnetic ± Dev = Compass.
- Magnetic dip drives the UNOS turning errors; the ANDS acceleration errors come from the compass card's CG hanging below its pivot (pendulous mounting). "High to low, look out below" for both pressure and temperature.
The Magnetic Compass
Lubber Line, Float Card, Magnetic Dip
How it reads
The only direction instrument that needs no power — a magnetized float card hangs in fluid and aligns with Earth's magnetic field. Read your heading where the card meets the lubber line (the vertical reference line). Numbers drop a zero: 3 = 30°, 33 = 330°.
Why it lies
Earth's field also dips downward toward the poles. That magnetic dip causes the turning errors (UNOS); the acceleration errors (ANDS) come from the card's center of gravity hanging below its pivot. Either way, the compass is only fully trustworthy in straight-and-level, unaccelerated flight.
Magnetic Compass Errors
ANDS, UNOS, Variation & Deviation, Magnetic Dip
Accelerate → compass shows turn toward North
Decelerate → compass shows turn toward South
Only on east and west headings. On north/south headings, the compass stays accurate during speed changes.
Undershoot North — compass leads, roll out early
Overshoot South — compass lags, roll out late
Turning to east or west, the compass is roughly correct (minimal turning error).
Variation: the gap between true north and magnetic north, shown by isogonic lines on sectional charts. "East is least, west is best" when converting true→magnetic.
Deviation: error from aircraft electrical/metal interference. Listed on the cockpit compass correction card and changes with heading.
True ± Variation = Magnetic ± Deviation = Compass heading
Magnetic field lines angle downward toward the poles, pulling the compass card. This dip is the root cause of the UNOS turning errors. (The ANDS acceleration errors come from the card's center of gravity being mounted below the pivot, not from dip directly.)
Dip is zero at the magnetic equator (errors minimal) and grows toward higher latitudes. The compass is most accurate in straight-and-level, unaccelerated flight.
Altimeter Settings & Errors
Pressure Changes, Temperature Effects, Standard vs. Local Settings
Pressure Changes
"High to low, look out below" — fly from high pressure to low without resetting and the altimeter reads too high (you're lower than you think). 1" Hg ≈ 1,000 ft difference.
Temperature Effects
Cold air is denser → altimeter reads too high. "From hot to cold, or high to low, look out below." In colder-than-standard temps, true altitude is LOWER than indicated.
Standard vs. Local Settings
Below 18,000': use the local altimeter setting (current barometric pressure). Above 18,000' (FL180): set 29.92" Hg so all aircraft share one reference for vertical separation.
DPE Scenarios
The examiner will test how you recognize and respond to instrument failures. Practice these out loud.
First, recognize which instruments are now unreliable: the Attitude Indicator and Heading Indicator are both vacuum-driven and will slowly tumble. My primary bank reference becomes the Turn Coordinator (electric — still working). For heading, I cross-check the magnetic compass, mindful of ANDS and UNOS errors in turns. I'd inform ATC, continue VFR on outside visual references, and land at the nearest suitable airport. I'd also note the loss in my logbook and have the vacuum pump inspected before the next flight.
Follow-up: If this happened in IMC instead of VMC, how would your priorities change?
Classic blocked pitot tube with the drain hole also blocked. Trapped ram air stays constant while static pressure drops as I climb, so the ASI reads increasingly high — acting like an altimeter. I'd: (1) Turn on pitot heat if available, (2) Hold attitude and power settings for a safe airspeed, (3) Cross-check GPS groundspeed as a rough reference, (4) Fly a known pitch and power for the performance I want. The altimeter and VSI still work (static only).
Follow-up: What if the ASI dropped to zero instead? What would that tell you about the blockage?
Turning from 090 to 360 — left is shorter. Rolling out on a northerly heading, UNOS applies: Undershoot North. The compass leads (shows the heading before I reach it), so I roll out EARLY. At 360/North the undershoot error is maximum. I'd start the left turn, watch the compass, and roll out about 30° early (around when it shows 030) to let it settle on 360. Use a standard-rate turn for predictable timing, then fine-tune after wings level.
Follow-up: Would the rollout lead/lag be different if you were turning to a southerly heading?
A small bank (up to about 5°) on the ground is normal — the AI needs a few minutes to spin up and erect fully. During taxi, I'd check it turns in the correct direction and levels back reasonably. If the error persists after 5 minutes of running, or is large, that's a no-go. Per §91.205, the AI is required for VFR day if installed (the standard six-pack in most aircraft), and it must be functional.
Follow-up: What's the full list of required instruments for VFR day flight?
Rule of thumb: 1" Hg ≈ 1,000 ft. The difference is 30.12 − 29.72 = 0.40" Hg, so about 400 feet. The actual pressure is LOWER than what I had set, so my altimeter was reading too HIGH — I'm actually about 400 feet LOWER than indicated. "High to low, look out below." Resetting to 29.72 drops the altimeter about 400 feet to show true altitude. This is why current altimeter settings matter, especially near terrain.
Follow-up: At what altitude do you stop using local altimeter settings and switch to 29.92?
Training aid only — verify all data against your POH and current FAA publications.