Flight Instruments — Reading Every Gauge, Understanding Every Error
Flight instruments are your cockpit window into the aircraft's state — speed, altitude, climb rate, attitude, heading, and coordination. Every instrument has a specific source of information, specific errors, and specific limitations. Knowing what each one tells you, and critically what it doesn't, separates a pilot who reads instruments from one who genuinely understands them. That difference matters most when something fails.
- Identify all colored arcs and lines on the airspeed indicator and state the V-speed each represents
- Explain the four types of airspeed and when each is used
- Define density altitude and compute its effect on aircraft performance using concrete examples
- Describe what happens to the altimeter when flying high-to-low pressure without updating
- Explain how the attitude indicator and heading indicator work and their specific failure modes
- Describe ANDS and UNOS compass errors and explain how to compensate for each
- Define standard rate turn and explain the bank angle formula for any airspeed
Lesson 1 — The Airspeed Indicator
The airspeed indicator (ASI) measures the difference between pitot pressure — ram air entering the pitot tube from the relative wind — and static pressure (ambient atmospheric pressure from the static port). This pressure difference, called dynamic pressure, is proportional to airspeed. The ASI converts this pressure difference into a speed reading displayed in knots. Learn more about weather for pilots →
The four types of airspeed
Indicated Airspeed (IAS) is what the instrument displays — uncorrected for instrument error, position error, or atmospheric density. All published V-speeds in the POH are given as IAS. When your instructor says "fly 70 knots on final," they mean IAS.
Calibrated Airspeed (CAS) is IAS corrected for instrument and position error — the errors caused by the location of the pitot tube and static port on the airframe. In the normal operating speed range these corrections are small (typically 1–5 kts), but they can be significant near stall or at very high airspeeds. CAS tables are published in the POH.
True Airspeed (TAS) is CAS corrected for non-standard pressure and temperature — the actual speed of the aircraft through the air mass. TAS increases approximately 2% per 1,000 ft above sea level. At 10,000 ft, TAS is roughly 20% higher than IAS for the same flight conditions. TAS is used for E6B navigation calculations, filing IFR flight plans, and computing groundspeed with wind.
Groundspeed (GS) is TAS corrected for wind — the speed across the ground. Tailwind increases GS above TAS; headwind decreases it. GS is computed on the E6B and displayed by GPS, but is not shown on the ASI.
ASI colored arcs and V-speed markings
| Color/Mark | Speed range | Meaning |
|---|---|---|
| White arc | Vs0 to Vfe | Flap operating range. Bottom = Vs0 (power-off stall, landing config). Top = Vfe (max flap extended speed — structural limit). |
| Green arc | Vs1 to Vno | Normal operating range, flaps retracted. Bottom = Vs1 (clean stall). Top = Vno (max structural cruising speed). |
| Yellow arc | Vno to Vne | Caution range — smooth air only. Enter only if you are certain the air is completely smooth and undisturbed. |
| Red line | Vne | Never-exceed speed. Structural flutter or failure can occur above this speed. Exceed it never, under any circumstances. |
V-speed study tip for the oral exam: Your DPE will ask the V-speeds for your specific training aircraft. They are in the POH's Operating Limitations section (also sometimes called Section 2). Memorize them from your aircraft's POH — not generic values from a textbook. The numbers vary by aircraft serial number, weight, and configuration. See 14 CFR Part 23 for airworthiness standards that define what each speed designation means structurally.
Lesson 2 — The Altimeter and Density Altitude
The altimeter measures atmospheric (static) pressure and converts it to an altitude reading using the standard atmosphere relationship: pressure decreases approximately 1 inHg per 1,000 ft. Setting the current local altimeter setting in the Kollsman window tells the instrument what sea-level pressure is today, allowing it to display altitude above mean sea level (MSL) accurately.
Types of altitude — a critical distinction
Indicated altitude: What the altimeter displays with the current altimeter setting entered in the Kollsman window. This is what ATC references and what determines your altitude relative to other aircraft.
Pressure altitude: What the altimeter reads with 29.92 inHg set in the Kollsman window — altitude above the standard datum plane. Used for performance chart calculations and all flight at/above 18,000 ft MSL (Class A airspace), where all aircraft set 29.92 and report altitudes as Flight Levels.
Density altitude: Pressure altitude corrected for non-standard temperature. The altitude the aircraft's performance systems actually experience. This is the number that drives every performance chart calculation.
True altitude: Actual altitude above mean sea level, accounting for the difference between actual atmospheric pressure and standard. Needed for terrain clearance calculations; close to indicated altitude near sea level in standard conditions.
The pressure altimeter setting rule — §91.121
Under 14 CFR §91.121, the altimeter must be set to the current reported altimeter setting of the nearest station along the route when at or below 18,000 ft MSL. When no station is available within 100 nm, use the elevation of the departure or destination airport. Above 18,000 ft: set 29.92 inHg.
"High to low, look out below": Flying from high pressure to low pressure without updating the altimeter causes it to over-read — the instrument shows you higher than you actually are. A pilot maintaining "4,500 ft indicated" who has flown into a low pressure area without updating may be 300–400 ft lower than they believe. Near terrain or in mountainous areas this error is fatal. Always update the altimeter setting when receiving new ATIS, when ATC provides it, and when crossing into new area weather.
Density altitude — the performance killer
Density altitude is pressure altitude corrected for non-standard temperature. On a hot summer day at a high-elevation western airport, the combination of high field elevation and high temperature can produce density altitudes 3,000–5,000 ft above field elevation. Every POH performance chart — takeoff roll, climb rate, landing distance — is computed at standard conditions. At high density altitude, actual performance degrades dramatically from those charts.
Density altitude worked example:
Airport elevation: 4,500 ft MSL. Altimeter setting: 29.72 inHg. OAT: 95°F (35°C).
Step 1 — Pressure altitude: 4,500 + (29.92 − 29.72) × 1,000 = 4,500 + 200 = 4,700 ft PA
Step 2 — ISA temperature at 4,700 ft: 15°C − (4,700 × 2/1,000) = 15 − 9.4 = 5.6°C standard
Step 3 — ISA deviation: 35°C − 5.6°C = +29.4°C above standard
Step 4 — DA: 4,700 + (29.4 × 120) ≈ 4,700 + 3,528 ≈ 8,228 ft density altitude
The aircraft "feels" like it's at 8,228 ft while sitting on a 4,500 ft runway. Consult performance charts at 8,200 ft density altitude — not 4,500 ft.
Lesson 3 — Vertical Speed Indicator
The vertical speed indicator (VSI) measures the rate of change of static pressure and converts it to a climb or descent rate in feet per minute. Because it measures rate of change rather than absolute pressure, the VSI has a characteristic lag of approximately 6–9 seconds — it shows sustained trends, not instantaneous readings. An instantaneous VSI (IVSI) uses accelerometers to reduce this lag and is found in some aircraft.
In normal flight, use the VSI as a trend indicator rather than an absolute reference. During cruise, a VSI pegged at zero confirms level flight. During approach, target approximately 400–700 fpm descent for a standard 3° glidepath — the exact value depends on groundspeed (descent rate = GS × 5.2 for 3° approaches; at 90 kts GS, that's 90 × 5.2 ≈ 468 fpm).
Lesson 4 — Attitude Indicator and Heading Indicator
The attitude indicator (AI, also called artificial horizon) and heading indicator (HI, also called directional gyro) are gyroscopic instruments that use the property of gyroscopic rigidity — a spinning mass resists changes to its axis of rotation — to maintain a fixed reference as the aircraft maneuvers.
Attitude indicator — how to read it
The AI has two moving elements relative to the fixed miniature airplane: the artificial horizon line (where earth meets sky) and the bank angle indicator at the top. The miniature airplane is fixed to the instrument case — it represents your aircraft. The horizon card moves with the gyroscope. Bank angle is read from the degree markings at the top (typically 10, 20, 30, 45, 60°). Pitch is read from the horizon bar position — above center means nose high, below means nose low. The brown color conventionally represents earth; blue represents sky.
Heading indicator — use and limitations
The HI maintains heading reference through gyroscopic rigidity — it does not seek magnetic north. This means two things: it must be set to the magnetic compass before flight (and after any prolonged ground operation), and it drifts due to gyroscopic precession and Earth's rotation. Typical drift: 3–5° per 15 minutes. Per AIM guidance, realign the HI with the magnetic compass every 15 minutes in straight-and-level, unaccelerated flight. The HI is far more useful for turning (compass errors make the compass unreliable in turns) but requires the magnetic compass for initial and periodic calibration.
Vacuum failure — the insidious instrument failure: AI and HI gyros spool down over 3–5 minutes after vacuum loss, appearing functional initially before slowly diverging from reality. In IMC this is a deadly trap. Always monitor the suction gauge (green arc: 4.5–5.5 inHg). During vacuum failure: turn coordinator (electric), magnetic compass, ASI, altimeter, and VSI remain reliable. These five instruments are all you need for partial panel flight. See FAA Instrument Flying Handbook Ch. 6 for partial panel procedures.
Lesson 5 — The Magnetic Compass and Its Errors
The magnetic compass is the original navigation instrument and the ultimate backup — no electrical, vacuum, or other aircraft power required. A magnetized needle floats in compass fluid, aligning with Earth's magnetic field. Despite its simplicity, it has two significant, predictable error categories that make it unreliable as a primary instrument during turns and during acceleration or deceleration on certain headings.
Why the compass has errors — magnetic dip
Earth's magnetic field lines are not horizontal at the surface — they dip into the Earth, increasingly so at higher latitudes. The compass card assembly is designed to pivot horizontally but is slightly weighted to counteract this dip. This counterweight creates the error source: when acceleration or bank tilts the compass assembly, the weighting causes the compass to respond as if to a false turn.
ANDS — Acceleration/Deceleration Errors
Accelerate North, Decelerate South — in the Northern Hemisphere, when on an easterly or westerly heading: accelerating causes the compass to indicate a turn toward North; decelerating causes it to indicate a turn toward South. There is no acceleration error when accelerating on northerly or southerly headings. These errors are temporary — the compass returns to the correct reading as acceleration ends.
UNOS — Turning Errors
Undershoot North, Overshoot South — when turning to a northerly heading, the compass lags; roll out earlier than the target heading. When turning to a southerly heading, the compass leads; roll out later. The amount to compensate equals approximately your latitude. At 35° N latitude: undershoot/overshoot by approximately 35°. Turning errors are minimal on east and west headings. Use the HI for all turns and verify with the magnetic compass only in straight-and-level unaccelerated flight.
Compass correction card: Each aircraft also has deviation — a small error caused by the aircraft's own magnetic field (from the engine, wiring, avionics). This error is different on each heading and each aircraft. The compass correction card, mounted near the compass, shows the deviation correction for various headings. Use it when flying with only the compass for reference. Deviation is different from variation (covered in Module 6).
Lesson 6 — Turn Coordinator and the Inclinometer
The turn coordinator uses a gyroscope tilted approximately 30° from the vertical axis, making it sensitive to both roll rate and yaw rate simultaneously. The miniature aircraft on the instrument face indicates rate of turn — not bank angle. This is a common misconception: the turn coordinator does not show bank angle; it shows how fast the aircraft is turning.
Standard rate turn
When the miniature aircraft's wingtip aligns with the turn index (the L or R tick mark), the aircraft is at standard rate — exactly 3° per second. At 3°/sec, a complete 360° turn takes exactly 2 minutes. Standard rate is used for instrument procedures, ATC-assigned turns, and as the basis for timing turns.
Bank angle formula for standard rate: Bank angle ≈ (TAS ÷ 10) + 5. At 90 kts TAS: (90 ÷ 10) + 5 = 14°. At 120 kts TAS: (120 ÷ 10) + 5 = 17°. At 180 kts: 23°. Useful for setting up standard rate turns without relying on the turn coordinator alone.
The inclinometer (ball) — coordination reference
The inclinometer beneath the turn needle — a black ball in a curved fluid-filled tube — shows coordination. In coordinated flight, gravity (pulling down) and centrifugal force (pulling outward in a turn) combine to keep the ball centered. When the ball moves:
- Ball to the left (skid): Too much rudder in the direction of turn — reduce rudder pressure on the turn side, or add opposite rudder.
- Ball to the right (slip): Insufficient rudder in the direction of turn — add more rudder pressure on the turn side.
The mnemonic "step on the ball" — press the rudder pedal on the same side as the ball displacement — always corrects coordination regardless of which direction the ball has moved.
Lesson 7 — Required Instruments and Equipment
Under 14 CFR §91.205, specific instruments are required for different types of flight. The mnemonic ATOMATOFLAMES is commonly used for VFR day requirements, and GRABCARD for additional night VFR requirements.
| Mnemonic | Instrument/Equipment | FAR Reference |
|---|---|---|
| VFR DAY — ATOMATOFLAMES (§91.205(b)) | ||
| A | Airspeed indicator | §91.205(b)(1) |
| T | Tachometer (for each engine) | §91.205(b)(2) |
| O | Oil pressure gauge | §91.205(b)(3) |
| M | Manifold pressure gauge (if applicable) | §91.205(b)(4) |
| A | Altimeter | §91.205(b)(5) |
| T | Temperature gauge (liquid-cooled engines) | §91.205(b)(6) |
| O | Oil temperature gauge | §91.205(b)(7) |
| F | Fuel gauge (each tank) | §91.205(b)(8) |
| L | Landing gear position indicator (retractable) | §91.205(b)(9) |
| A | Anti-collision light system | §91.205(b)(11) |
| M | Magnetic direction indicator (compass) | §91.205(b)(12) |
| E | ELT | §91.207 |
| S | Safety belts / harnesses | §91.205(b)(10) |
| NIGHT ADD-ONS — GRABCARD (§91.205(c)) | ||
| G | Generator or alternator | §91.205(c)(1) |
| R | Radio (two-way communication and navigation) | §91.205(c)(2) |
| A | Airspeed indicator (already required) | — |
| B | Ball (inclinometer) | §91.205(c)(3) |
| C | Clock (with sweep seconds or digital display) | §91.205(c)(4) |
| A | Attitude indicator | §91.205(c)(5) |
| R | Rate-of-turn indicator (turn coordinator or turn-and-slip) | §91.205(c)(6) |
| D | Directional gyro (heading indicator) | §91.205(c)(7) |
- ASI arcs: White (Vs0–Vfe, flap range) · Green (Vs1–Vno, normal ops) · Yellow (caution, smooth air only) · Red line (Vne, structural limit).
- IAS: instrument reading. TAS: actual speed through air (~+2%/1,000 ft). GS: TAS ± wind. Use IAS for flying; TAS for navigation planning.
- Density altitude = pressure altitude corrected for non-standard temperature. Always compute performance from DA, not field elevation. Hot + high = very high DA.
- "High to low, look out below" — flying into lower pressure without updating the altimeter causes over-reading. Required by §91.121 to update altimeter setting continuously.
- AI and HI are vacuum-powered in conventional aircraft. Gyroscopic rigidity maintains attitude/heading reference. HI drifts — realign with compass every 15 minutes in straight-and-level flight.
- Vacuum failure: AI and HI spool down gradually — insidious in IMC. Turn coordinator (electric), compass, and pitot-static instruments remain. Monitor suction gauge continuously.
- Compass ANDS on east/west headings: Accelerate North, Decelerate South. Compass UNOS on N/S turns: Undershoot North, Overshoot South.
- Standard rate turn = 3°/sec = 360° in 2 minutes. Formula: bank angle ≈ (TAS ÷ 10) + 5.
- Ball = coordination. "Step on the ball." ATOMATOFLAMES = VFR day required. GRABCARD = additional night VFR required. Both per §91.205.