Flight instruments

In short

Flight instruments are the cockpit displays that convert air pressure, gyroscopic rigidity, magnetism and electrical signals into the pilot's picture of height, speed, attitude and heading. They matter because they are the only reliable reference when the horizon is lost in cloud or at night, where the human vestibular system fails within seconds. The subject is examined in B.Sc Aviation flight-theory papers, in DGCA ground papers (Air Navigation, Technical General) for CPL/ATPL, and in AME licence modules; it is tested in cadet-pilot screening (IndiGo, Air India, Akasa), air-traffic and airport-operations interviews (AAI, Adani Airports, GMR) and MRO technical rounds. The content splits cleanly into pressure instruments (pitot-static), gyroscopic instruments, compasses and modern electronic display systems. Every exam asks for instrument errors, their causes and the pilot's response.

Explained

The pitot-static system

• The system senses two pressures: total (pitot) pressure from a forward-facing tube and static pressure from flush ports on the fuselage side (two ports, one each side, to cancel sideslip error).
• Airspeed indicator (ASI): measures the difference (pitot − static) = dynamic pressure q. A capsule fed with pitot pressure, inside a case at static pressure, expands as q rises and drives the pointer. Only instrument fed by both pitot and static.
• Altimeter: a sealed aneroid capsule inside a case open to static pressure; as static pressure falls the capsule expands, moving the pointer through a gear train. A subscale (Kollsman window) sets the datum pressure in hPa or inches of mercury.
• Vertical speed indicator (VSI): a capsule fed by static pressure inside a case vented to static through a calibrated leak; the pressure difference across the capsule is proportional to rate of change of altitude. It therefore has a lag of 6-9 seconds; an IVSI adds accelerometer-driven dashpots to give an instantaneous indication.
• Alternate static source (cabin air) is used if the static ports ice up. Cabin pressure is slightly lower than outside static in an unpressurised aircraft, so the altimeter over-reads and the ASI over-reads slightly.
• Blockage behaviour (classic exam question):
  • Pitot blocked, drain open → ASI falls to zero.
  • Pitot blocked and drain blocked → ASI behaves like an altimeter: over-reads in a climb, under-reads in a descent.
  • Static blocked → ASI under-reads in a climb and over-reads in a descent; altimeter freezes; VSI reads zero.

Airspeed: IAS, CAS, EAS, TAS and Mach

• IAS (indicated airspeed): what the needle shows. The pilot flies IAS because stall, flap and gear limiting speeds all occur at a fixed dynamic pressure, hence a fixed IAS, at any altitude.
• CAS (calibrated airspeed): IAS corrected for instrument error and position (pressure) error — the latter largest at high angles of attack and low speed.
• EAS (equivalent airspeed): CAS corrected for compressibility, significant above roughly 200 kt or 10,000 ft; EAS is always ≤ CAS.
• TAS (true airspeed): EAS corrected for air density. Because density falls with altitude, TAS exceeds IAS by roughly 2% per 1,000 ft of altitude as a rule of thumb.
• Ground speed = TAS ± wind component; navigation and ETA use ground speed, never IAS.
• Mach number M = TAS / a, where the local speed of sound a = √(γRT) depends on temperature only. A Machmeter compares dynamic pressure to static pressure, so it needs no temperature input.
• ASI colour markings: white arc = flap operating range (VS0 to VFE); green arc = normal operating range (VS1 to VNO); yellow arc = caution, smooth air only; red radial line = VNE (never exceed). Blue line (VYSE) and red line (VMC) appear on multi-engine ASIs.

Altimetry and Indian altimeter-setting practice

• Pressure altitude: height above the 1013.25 hPa datum (subscale set to 1013.25) — used as flight level above the transition altitude.
• QNH: setting that makes the altimeter read elevation above mean sea level; QFE: makes it read height above the aerodrome (zero on the runway); QNE refers to the standard setting 1013.25 hPa.
• Below the transition altitude pilots use QNH and report altitude; above the transition level they use 1013.25 and report flight level. In India the transition altitude is published per aerodrome in the AIP (commonly 4,000 ft AMSL at many Indian airports, higher at high-elevation fields) — always read the chart rather than assume a single national value.
• Rule of thumb: 1 hPa ≈ 27-30 ft near sea level. Flying from high pressure to low pressure with an old setting makes the altimeter over-read — the aircraft is lower than indicated ("high to low, look out below").
• Temperature error: in air colder than ISA the altimeter over-reads, so true height is less than indicated — critical on approach into northern Indian airports in winter. Approximate correction: 4 ft per 1,000 ft per °C of ISA deviation.
• Altimeter errors: instrument (friction, hysteresis), position error, lag, barometric error (wrong subscale) and temperature error.
• A radio (radar) altimeter measures true height above the terrain by timing a reflected signal (4.2-4.4 GHz), used below about 2,500 ft for approach, autoland and GPWS; unlike the pressure altimeter it is unaffected by pressure or temperature.

Gyroscopic instruments

• A spinning rotor has two properties: rigidity in space (gyroscopic inertia — it holds its plane of spin) and precession (a force applied to the rim produces motion 90° ahead in the direction of rotation).
• Attitude indicator (artificial horizon): an earth-gyro with its spin axis vertical, giving two degrees of freedom; shows pitch and bank against a horizon bar. Errors appear in prolonged turns and in acceleration (an acceleration gives a false climb-and-right-turn indication in erection-corrected gyros).
• Heading indicator (directional gyro, DGI): a space-gyro with horizontal spin axis; it has no north-seeking ability, so it must be aligned with the magnetic compass every 10-15 minutes. It drifts from real wander (bearing friction) and apparent wander (earth rotation 15°/h × sin latitude, plus transport wander).
• Turn and slip indicator / turn coordinator: a rate gyro with one degree of freedom, canted about 30° in the turn coordinator so it senses both roll and yaw rate. The ball is an inclinometer showing balance, not bank: "step on the ball" to centre it. A standard rate turn is 3°/s, i.e. 360° in two minutes.
• Power sources: vacuum (engine-driven pump) for attitude and heading gyros in most piston trainers; electrical for the turn coordinator — deliberately split so one failure does not remove every gyro. Transport aircraft use ring laser gyros and MEMS sensors in an IRS/AHRS with no moving rotor.
• The six-pack layout: top row ASI, attitude indicator, altimeter; bottom row turn coordinator, heading indicator, VSI — the standard "basic T" arrangement, with the four primary instruments (airspeed, attitude, altitude, heading) forming the T.

Magnetic compass and heading references

• The direct reading magnetic compass aligns with the horizontal component of the earth's magnetic field; at high latitudes the field dips steeply so the horizontal component weakens and the compass becomes unreliable.
• Variation: angle between true north and magnetic north, from the chart (isogonals). Deviation: error from the aircraft's own magnetism, from the aircraft's compass deviation card after a compass swing.
• Sequence: True → Variation → Magnetic → Deviation → Compass; remembered as "CDMVT" — add west variation when going from true to magnetic ("west is best, east is least" applies in that true-to-magnetic direction; reverse the signs when converting magnetic to true).
• Turning errors (northern hemisphere): on a northerly heading the compass lags (under-reads the turn); on a southerly heading it leads — hence UNOS (Undershoot North, Overshoot South).
• Acceleration errors (northern hemisphere): acceleration on an easterly or westerly heading gives an apparent turn to the north, deceleration to the south — ANDS.
• A remote indicating compass (flux valve / flux gate) senses the field electrically at a wingtip or fin, away from aircraft magnetism, and slaves the heading display — the normal arrangement in airline aircraft.

Modern electronic instruments

• Air data computer (ADC): takes pitot, static and total air temperature inputs and computes CAS, TAS, Mach, altitude, vertical speed and SAT digitally, feeding the displays, autopilot, FMS and transponder.
• EFIS: the PFD (primary flight display) combines attitude, airspeed and altitude tapes, vertical speed, heading and flight director in one screen; the ND/MFD shows navigation, weather radar and terrain. ECAM (Airbus) or EICAS (Boeing) display engine and system data.
• Standby instruments are mandatory: an ISIS (integrated standby instrument system) or separate mechanical ASI, altimeter and attitude indicator on independent power.
• Advantages of glass cockpits: fewer mechanical parts, integrated alerting, decluttered display, reduced workload. Limitations: common-mode electrical failure, heavy reliance on sensor validity, and loss of scan habits — training emphasises partial-panel and degraded-mode flying.
• Related displays: Machmeter/VMO pointer, angle-of-attack indicator and stall-warning stick shaker, GPWS/EGPWS, TCAS and the flight data recorder feed.
• Indian regulatory frame: instrument fitment is governed by the Bharatiya Vayuyan Adhiniyam, 2024 (in force from 1 January 2025, replacing the former Aircraft Act, 1934) with the Aircraft Rules, 1937 and the Civil Aviation Requirements (CARs) issued by DGCA, which adopt ICAO Annex 6 and Annex 8; aircraft are certified to FAR/CS-23 or FAR/CS-25 standards. Instruments must be calibrated and the altimeter checked for the RVSM tolerance where applicable. (Rule numbers are periodically amended — quote the current CAR rather than a remembered rule number.)

Key points

• Pitot-static system - supplies total pressure to the ASI and static pressure to ASI, altimeter and VSI.
• ASI principle - measures dynamic pressure q = total − static, calibrated against ISA sea-level density.
• IAS → CAS - correct for instrument and position error; CAS → EAS compressibility; EAS → TAS density.
• TAS rule of thumb - roughly 2% greater than IAS per 1,000 ft of altitude.
• Why pilots fly IAS - stall and limiting speeds occur at fixed dynamic pressure, so at fixed IAS regardless of altitude.
• VNE red line - never exceed speed; white arc flap range, green normal range, yellow caution range.
• Altimeter - aneroid capsule referenced to static pressure; subscale sets QNH, QFE or 1013.25 hPa.
• 1 hPa ≈ 27-30 ft - and high-to-low pressure (or warmer-to-colder air) with an old setting means the altimeter over-reads.
• VSI - calibrated leak gives rate of climb with 6-9 s lag; IVSI removes most of the lag.
• Static blockage - altimeter freezes, VSI zero, ASI under-reads in climb and over-reads in descent.
• Gyro properties - rigidity in space and precession 90° from the applied force.
• Attitude indicator - earth gyro, spin axis vertical, two degrees of freedom; suffers acceleration and turn errors.
• Directional gyro - space gyro, no north-seeking; realign with the compass every 10-15 min for real and apparent wander.
• Apparent wander - 15°/h × sin(latitude) from earth rotation.
• Turn coordinator - rate gyro, one degree of freedom; standard rate turn 3°/s = 360° in 2 minutes; the ball shows balance, not bank.
• Compass conversions - True ± Variation = Magnetic ± Deviation = Compass; UNOS for turning error, ANDS for acceleration error.
• Radio altimeter - true height above terrain, independent of pressure and temperature, used below about 2,500 ft.
• EFIS and ADC - digital computation of all air data, displayed on PFD and ND, with mandatory independent standby instruments.

Formulas and examples

Example 1 — IAS to TAS. An aircraft shows 150 kt IAS at 10,000 ft; instrument plus position error is −3 kt; relative density σ = 0.7385 at 10,000 ft ISA.

• CAS = 150 − 3 = 147 kt. Compressibility is negligible at this speed, so EAS ≈ 147 kt.
• √σ = √0.7385 = 0.8594.
• TAS = 147 / 0.8594 = 171.1 kt.
• Rule-of-thumb check: 147 × (1 + 0.02 × 10) = 176 kt — close, and the rule always runs slightly high.

Example 2 — Mach number. TAS = 450 kt at FL350 where SAT = −54 °C.

• T = 273.15 − 54 = 219.15 K; a = √(1.4 × 287 × 219.15) = √(88,054) = 296.7 m/s.
• In knots: 296.7 / 0.5144 = 576.8 kt.
• M = 450 / 576.8 = 0.780.

Example 3 — Altimeter setting error. An aircraft flies from Delhi (QNH 1020 hPa) towards Jaipur where the actual QNH is 1006 hPa, with 1020 still set and the altimeter reading 5,000 ft.

• Pressure difference = 1020 − 1006 = 14 hPa; at 27 ft/hPa that is 14 × 27 = 378 ft.
• Pressure fell, so the aircraft is 378 ft lower than indicated — true altitude ≈ 4,622 ft. High to low, look out below.

Example 4 — Temperature error on approach. Indicated altitude 3,000 ft above an aerodrome, OAT at that level −5 °C, ISA temperature there about +9 °C (15 − 1.98 × 3).

• ISA deviation = −5 − 9 = −14 °C.
• Error = 4 × 14 × 3 = 168 ft, and because the air is colder the altimeter over-reads.
• True height ≈ 3,000 − 168 = 2,832 ft — the aircraft is lower than the instrument says.

Example 5 — Rate-one turn. Find the bank angle and radius for a 3°/s turn at 120 kt TAS.

• V = 120 × 0.5144 = 61.7 m/s; ω = 3°/s = 0.0524 rad/s.
• tan φ = ωV/g = 0.0524 × 61.7 / 9.81 = 0.330 → φ = 18.3°.
• Rule of thumb (120/10) + 7 = 19° — good agreement.
• Radius r = V/ω = 61.7 / 0.0524 = 1,177 m (about 0.64 NM).

Where it is used

• Airline and GA flight decks: Air India, IndiGo, SpiceJet and Akasa line pilots fly QNH/standard altimetry, Mach-limited cruise and PFD scans daily; the same theory is examined in DGCA CPL Technical General and Air Navigation papers.
• Flight training organisations: IGRUA Amethi, NFTI Gondia and the state flying clubs teach partial-panel and instrument flying on Cessna 172 and DA40 six-pack and G1000 panels.
• MRO and AME work: pitot-static leak checks, altimeter calibration and compass swings at Air India Engineering Services, AIESL, Air Works and Indamer; DGCA CAR 145 organisations hold the test equipment.
• Air traffic and airport operations: AAI controllers issue QNH and transition levels, verify Mode C pressure-altitude readouts, and apply cold-temperature corrections for minimum altitudes in winter at northern airports.
• Design, avionics and defence: HAL, Bharat Electronics and Honeywell/Collins Indian centres build air data computers, AHRS and standby instrument systems; integration testing at ASTE and CEMILAC.
• Drones and UAVs: barometric and MEMS inertial sensors in DGCA-certified RPAS use exactly these principles, with the same pressure and temperature limitations.

Interview questions

• Q: Why does a pilot fly indicated airspeed rather than true airspeed? A: Aerodynamic limits such as stall, flap and gear speeds depend on dynamic pressure, which the ASI measures directly, so those limits occur at the same IAS at every altitude.
• Q: What happens to the ASI, altimeter and VSI if the static port blocks in a climb? A: The altimeter freezes at the blockage level, the VSI reads zero, and the ASI under-reads because static pressure is trapped too high.
• Q: What is the difference between QNH and QFE? A: With QNH set the altimeter reads altitude above mean sea level; with QFE set it reads height above the aerodrome and shows zero on the runway.
• Q: An altimeter is set to 1013 while the actual QNH is 995 — are you higher or lower than indicated? A: Lower; the set pressure is 18 hPa higher than actual, so the aircraft is about 18 × 27 ≈ 486 ft below the indicated altitude.
• Q: Why is the VSI slow to respond, and how is that fixed? A: The calibrated leak needs time to build the differential pressure, giving 6-9 seconds of lag; an instantaneous VSI adds accelerometer-driven pumps that supply an immediate indication.
• Q: State the two properties of a gyroscope and name an instrument using each. A: Rigidity in space, used by the attitude and heading indicators; precession, used by the rate gyro in the turn indicator.
• Q: Why must a directional gyro be realigned in flight? A: It has no north-seeking property, and it drifts from bearing friction (real wander) plus earth rotation and transport (apparent wander), so it is reset against the magnetic compass every 10-15 minutes.
• Q: What does the ball in a turn coordinator tell you? A: Balance, not bank — it shows whether the turn is slipping or skidding, corrected by applying rudder towards the ball.
• Q: What are UNOS and ANDS? A: Compass turning error in the northern hemisphere (undershoot north, overshoot south) and acceleration error (accelerate north, decelerate south).
• Q: How does a radio altimeter differ from a pressure altimeter? A: It times a radio reflection to give true height above the terrain below, so it is unaffected by pressure setting or temperature, and is used only in the final few thousand feet.
• Q: Why does the altimeter over-read in air colder than ISA, and when is that dangerous? A: Cold air is denser, so pressure levels lie closer to the ground and the aircraft is lower than indicated — dangerous on non-precision approaches and minimum sector altitudes in a north Indian winter.
• Q: What is the purpose of standby instruments in a glass cockpit? A: They give attitude, airspeed and altitude from independent sensors and power, so a display or electrical failure does not leave the crew without basic flight information.

Common mistakes

• Treating IAS, CAS, EAS and TAS as interchangeable, or applying the density correction before the position-error correction.
• Getting the altimeter error direction backwards — forgetting that flying towards lower pressure or colder air means the aircraft is lower than indicated.
• Confusing variation (chart, geographic) with deviation (aircraft's own magnetism, from the deviation card), or adding west variation in the wrong direction.
• Reading the slip ball as a bank indicator instead of a balance indicator, and reporting bank angle from it.
• Saying the directional gyro points to north; it holds whatever heading it was set to and drifts.
• Mixing units in Mach and TAS problems — using °C instead of kelvin in a = √(γRT), or knots instead of m/s.
• Quoting a single transition altitude for all of India; it is published per aerodrome in the AIP.

Related topics

Avionics basics · Fundamentals of aerodynamics · Aircraft performance · Air navigation and flight planning · Meteorology for pilots · Aircraft systems: hydraulics, fuel and landing gear