Advanced Aircraft Instruments & Navigation Systems Study Notes

Pitot–Static System & Air Data Fundamentals

• Airspeed “family-tree”

  • IAS \rightarrow corrected for instrument/position pressure errors \Rightarrow CAS

  • CAS \rightarrow corrected for compressibility \Rightarrow EAS

  • EAS \rightarrow corrected for density ((\rho)) \Rightarrow TAS

  • Practical memory aid: “Ice Cold Beer Tastes” (IAS→CAS→EAS→TAS).

• Temperature definitions

  • Outside Air Temperature (OAT) ≡ Static Air Temperature (SAT).

  • Total Air Temperature (TAT) measured by a probe in the airflow.

  • Kinetic (ram-rise) heating elevates TAT above SAT:
    TAT = SAT + \text{Ram-Rise}

• Standard Atmosphere references

  • ISA Tropopause (≈ FL\,360) temperature held constant at -56.5\,^\circ!C.

  • Temperature deviation codes:

  • “ISA+” = warmer than standard, “ISA–” = colder.

• Absolute temperature

  • TK = T{!^\circ C} + 273

Altimeters & Associated Errors

• Sensitive Altimeter

  • Two aneroid capsules + vibration motor (reduces stiction → ↑ accuracy).

• Servo (Electro-mechanical) Altimeter

  • Electric servo keeps capsule centred → no lag & higher precision.

• Pitot/Static blockage mnemonics

  • PUDSOD (Blockage on Descent)

  • Pitot UNDER-reads, Static OVER-reads.

  • PUSO (Leak)

  • Pitot under-reads, Static over-reads anytime.

• Climb/Descent characteristics

  • Above tropopause: TAS virtually constant despite altitude change.

Mach Relationships & Air Data Computers (ADC / ADIRU)

• \text{Mach} = \dfrac{TAS}{LSS} (LSS = local speed of sound).

• ADC standard outputs:

  1. Barometric altitude

  2. Mach No.

  3. CAS

  4. TAS

  5. OAT

• ADIRU = “Air-Data Inertial Reference Unit” (integrates ADC + IRS).

Magnetic Compass & Turning Errors

• Deviation = Compass heading – Magnetic heading.

  • West deviation (−) ➔ “West is best.”

  • East deviation (+) ➔ “East is least.”

• UNOS turning rule (Northern Hemisphere)

  • Undershoot North, Overshoot South.

  • \text{UNOS} \Rightarrow \text{apparent turn cues in opposite direction}

• Acceleration/Deceleration

  • Accelerate → apparent turn toward poles.

  • Decelerate → apparent turn toward equator. (Both hemispheres.)

Gyroscopes, Drift & Topple

• Gyro wander

  • Horizontal plane = Drift
    \text{Drift\,(deg h}^{-1}) = 15 \times \sin(\text{Latitude})

  • Vertical plane = Topple
    \text{Topple\,(deg h}^{-1}) = 15 \times \cos(\text{Latitude})

Rate Instruments & AHRS

• Rate of turn \propto \dfrac{1}{TAS} ⇒ higher TAS → slower indicated rate.

• AHRS (Attitude & Heading Reference System) replaces multiple remote gyros.

  • Gyro-magnetic compass sequence: Free → Error detector → Amplifier → Torque motor (slaving).

  • Pendulous vanes prone to acceleration/pitch error.

• ADI = Attitude Display Indicator (attitude + flight director commands).

INS / IRS Architecture

• Core sensors:

  • Gyroscope ⇒ measures rotation.

  • Accelerometer ⇒ measures linear acceleration.

• Stable (gimbal) INS: 3 mechanical rate gyros + 2 accelerometers.

• Strap-down IRS: 3 Ring-Laser Gyros + 3 accelerometers (fixed to airframe).

  • Ring laser gyro: solid glass block, internal cavity, anode/cathode for light path.

• IRS principal outputs:

  1. True/Magnetic heading

  2. Track

  3. Drift angle

  4. Attitude & attitude rates

  5. Altitude change / vertical rate

  6. Groundspeed

  7. Present position (Lat/Long)

INS Mode-Selector Unit (MSU)

• OFF → Unit unpowered.

• STBY → Gyros spin-up, self-test & battery check (aircraft may be moved).

• ALIGN → Platform levelled & aligned to north; aircraft must remain stationary.

• NAV → Full navigation enabled; aircraft may move.

• ATT → Reduced mode (attitude/heading only) if alignment lost.

Autopilot & Flight-Director Systems

• Components

  1. Actuators

  2. Mode-control panel

  3. Flight-mode annunciator (FMA)

• Basic stabilisation modes

  • Pitch-angle hold

  • Bank-angle hold

• Guidance modes

  1. IAS/Mach hold

  2. Altitude hold

  3. Glide-slope intercept & tracking

• Flight Director (FD): computer generates pitch/roll bars; pilot or AP can follow.

  • Fly-by-wire aircraft: FD may be used at all times.

  • Conventional controls: FD normally used when AP disengaged.

• Yaw-damper active in both manual & automatic flight.

• Mach-trim system engages above a preset Mach.

• Pitch-trim motor removes steady control pressure.

GNSS Constellations & Integrity Concepts

• 4 Core GNSS:

  1. USA \rightarrow GPS (NAVSTAR)

  2. Russia \rightarrow GLONASS

  3. EU \rightarrow Galileo

  4. China \rightarrow BeiDou

• Pseudo-range (PR) = measured satellite range including clock bias.

• Augmentation acronyms

  • SBAS = Satellite-Based Augmentation

  • GBAS = Ground-Based Augmentation

  • ABAS = Aircraft-Based Augmentation

  • AAIM = Aircraft Autonomous Integrity Monitoring

  • RAIM = Receiver Autonomous Integrity Monitoring (GNSS-only)

  • FDE = Fault Detection & Exclusion

• Satellite requirements

  • 3 sats → 2-D fix

  • 4 sats → 3-D fix

  • 5 sats → RAIM/Fault Detection

  • 6+ sats → FDE capability

• Guidance capability

  • GBAS ⇒ Horizontal + Vertical

  • SBAS ⇒ Vertical guidance (APV/LPV)

Performance-Based Navigation (PBN)

• Three pillars:

  1. NAVAID infrastructure (e.g. GNSS)

  2. Navigation specification (RNAV / RNP)

  3. Navigation application (SID, STAR, en-route, approach)

• Key performance areas: Accuracy, Integrity, Continuity.

• Specification matrix

  • Oceanic/Remote: RNAV-10 (RNP-10), RNAV-4.

  • En-route continental: RNAV-5; RNP-2.

  • Arrival/Departure: RNAV-1; RNP-1.

  • Approach: RNP-APCH (LNAV / LNAV-VNAV).

  • Precision-like: LPV (SBAS), GLS (GBAS), ILS, PAR.

  • Special: RNP-AR APCH (Authorisation Required)

  • Helicopter/misc.: RNP 0.3.

• Path design terminology

  • RF = Radius-to-Fix leg

  • FRT = Fixed-Radius Transition (en-route)

Aeronautical Information Regulation & Control (AIRAC)

• Publication cycles:

  • Normal cycle: 28 days.

  • Intermediate change: 42 days.

  • Major change: 56 days.

Navigation Error Budget

• PDE = Path Definition Error

• FTE = Flight Technical Error

• NSE = Navigation System Error

• Total System Error:
  TSE = PDE + FTE + NSE

• Estimated Position Error (EPE) must ⩽ required accuracy for given nav spec.

Miscellaneous Quick-Reference

• LNAV (autopilot lateral mode) follows great-circle track.

• UNOS & ANDS memory aids:

  • UNOS = Undershoot North, Overshoot South (turns).

  • ANDS = Accelerate North, Decelerate South (compass acceleration; “poles/equator” in transcript wording).

• Rate Gyro vs Accelerometer

  • Gyro → rotational rates (deg s⁻¹)

  • Accelerometer → linear acceleration (m s⁻²)

• Stable Platform vs Strap-down

  • Stable (SEANS): mechanical gimbals isolate sensors.

  • Strap-down (IRS): sensors fixed; attitude computed algorithmically.

• 3-D Approach families

  • ILS / PAR (ground-based precision)

  • GLS (GBAS)

  • LPV (SBAS)

  • RNP APCH with Baro-VNAV