The pitot-static system
- Pitot probe — faces into the airflow and senses total (pitot) pressure = static + dynamic pressure. Probes are electrically heated to prevent icing; a blocked pitot is a classic cause of unreliable airspeed.
- Static ports — flush holes on both sides of the fuselage where the local pressure is close to the true ambient pressure. Left and right ports are cross-connected so that a sideslip, which raises the pressure on one side and lowers it on the other, cancels out. The area around the ports must be kept smooth, clean and undamaged — a dent or a blob of sealant causes errors.
- Alternate static source — a selector for use if the ports block. In an unpressurised aircraft it usually takes cabin air, which is slightly below ambient, so the altimeter and ASI read slightly high.
- Drains at low points remove water. Air data computers (ADC), or on modern aircraft air data modules next to the probes feeding ADIRUs, convert the pressures to altitude, airspeed, Mach number and vertical speed, and with total air temperature work out true airspeed and static air temperature.
- Leak checks are made with an air data (pitot-static) test set whenever a line is disconnected and at specified intervals; the system must hold the test pressure within a stated leak rate. Pressure is applied and released slowly so that capsules are not damaged, and pitot and static are never mixed up.
Airspeed indicator, altimeter and VSI
Airspeed indicator (ASI)
Measures dynamic pressure (½ρV²): pitot pressure into the capsule, static pressure into the case. It is calibrated for sea-level standard density, so it shows indicated airspeed (IAS). Corrected for instrument and position errors it becomes calibrated airspeed (CAS); for compressibility, equivalent airspeed (EAS); and for density, true airspeed (TAS). Jets also show Mach number (a Machmeter combines an airspeed capsule and an altitude capsule) and a red-and-white "barber pole" for VMO/MMO. Light-aircraft ASIs have colour arcs: white (flap operating range), green (normal), yellow (caution) and a red line at VNE.
Altimeter
A stack of sealed, evacuated capsules inside a case connected to static pressure: as the aircraft climbs, static pressure falls and the capsules expand, driving the pointers through a mechanism. A subscale sets the datum: QNH (altitude above mean sea level), QFE (height above the airfield) or 1013.25 hPa / 29.92 inHg (flight levels). Servo and encoding altimeters send altitude to the transponder (Mode C/S) and other systems. Errors include instrument, position, temperature and lag errors, and hysteresis.
Vertical speed indicator (VSI)
Static pressure reaches the capsule directly but the case through a calibrated leak (restrictor). In level flight the pressures are equal; in a climb or descent the case pressure lags, and the difference is proportional to the rate of change of altitude. The VSI therefore lags a few seconds; an instantaneous VSI (IVSI) adds small accelerometer dashpots that respond immediately to vertical acceleration.
| Blockage | ASI | Altimeter | VSI |
|---|---|---|---|
| Pitot blocked, drain open | Falls towards zero | Normal | Normal |
| Pitot and drain blocked | Acts like an altimeter: over-reads in a climb, under-reads in a descent | Normal | Normal |
| Static blocked | Under-reads in a climb, over-reads in a descent | Frozen at the blockage altitude | Reads zero |
| Alternate static (cabin air, unpressurised) | Slightly high | Slightly high | Brief climb indication when selected |
Gyroscopic instruments
A spinning rotor has two properties that instruments use:
- Rigidity in space (gyroscopic inertia) — the rotor's spin axis keeps its direction in space unless a force acts on it. Rigidity increases with rotor mass, speed and how far the mass is from the axis (radius).
- Precession — a force applied to the rim acts as if it were applied 90° further round in the direction of rotation. Rate gyros measure it; unwanted precession from friction or imbalance causes drift.
Gyros are driven by air (vacuum or pressure systems on light aircraft — Chapter 18) or electrically. Apparent drift also arises because the earth rotates beneath the gyro: a horizontal gyro appears to wander by up to 15° per hour × sin(latitude). Modern transport aircraft use ring-laser or fibre-optic gyros inside inertial reference units (Chapter 8) rather than spinning-rotor instruments, except perhaps in standby instruments.
| Instrument | Gyro type | Principle and notes |
|---|---|---|
| Attitude indicator (artificial horizon) | Vertical gyro, two gimbals | Rigidity holds the horizon; an erection system (pendulous vanes on air gyros, levelling switches and torque motors on electric gyros) keeps the spin axis vertical. Older units could topple beyond pitch and roll limits and had a caging knob or fast-erect button |
| Attitude director indicator (ADI) | Attitude from a gyro or IRS | Attitude display plus flight-director command bars, ILS deviations and rate of turn; on EFIS aircraft its functions are in the PFD |
| Direction indicator (DI / directional gyro) | Horizontal gyro, two gimbals | Gives a stable heading free from compass turning and acceleration errors, but is not north-seeking — it must be reset to the magnetic compass about every 15 minutes unless it is slaved |
| Horizontal situation indicator (HSI) | Slaved heading | A slaved compass card combined with a course pointer and course-deviation bar (VOR/localiser), TO/FROM indication and glideslope pointer |
| Turn-and-slip (turn-and-bank) indicator | Rate gyro, one gimbal (along the longitudinal axis), spring-restrained | Shows rate of turn; a "rate one" turn is 3° per second — 360° in 2 minutes. The slip ball (an inclinometer in a curved liquid-filled tube) shows whether the turn is balanced: "step on the ball" |
| Turn coordinator | Rate gyro with its gimbal axis canted about 30° | Responds to roll rate as well as yaw rate, so it indicates the start of a turn sooner; shows a small aircraft symbol, not a pointer |
Compass systems
Direct-reading (standby) compass
A magnetised float on a pivot in a liquid-filled bowl (the liquid damps oscillations; a bellows or diaphragm allows for expansion), read against a lubber line. Its errors:
- Variation — the angle between true and magnetic north at that place (a property of the earth, shown on charts).
- Deviation — caused by the aircraft's own magnetic fields (steel parts, electrical currents). It is reduced by compensator (corrector) magnets and the remainder recorded on a deviation card after a compass swing. CS 25.1327 allows no more than 10° on any heading.
- Dip, turning and acceleration errors — the field's vertical component tilts the float. In the northern hemisphere a compass undershoots north and overshoots south in turns (UNOS) and shows an apparent turn north when accelerating on east or west headings (ANDS: accelerate north, decelerate south).
Performed after installation or replacement of the compass or of significant magnetic equipment, after a lightning strike or heavy landing where the AMM requires, when deviation is suspected, and at periodic intervals. The aircraft is placed on a compass base (a site free of magnetic disturbance) with engines and electrical loads as in flight, turned through the cardinal and intermediate headings, the compensators are adjusted, and a new deviation card is made out. Use non-magnetic tools near the compass.
Remote-reading (slaved) compass
A flux valve (flux detector) — a three-legged soft-iron core with an excitation winding fed at 400 Hz and pick-off coils producing an 800 Hz signal — senses the direction of the horizontal component of the earth's field. It is mounted where the aircraft's own fields are weakest, usually in a wing tip or the fin. The compass coupler compares its output with the heading of a directional gyro and drives a slaving torque motor to precess the gyro into agreement. The gyro supplies stability; the flux valve supplies the north reference. Modern inertial reference systems compute heading directly, using stored magnetic variation tables to give magnetic heading.
Stall warning and angle-of-attack systems
- CS 25.207 requires a clear and distinctive stall warning that begins at a speed at least 5 knots or 5 % (whichever is greater) above the stall, with flaps and gear in any normal position.
- Light aircraft use a flapper (vane) switch on the leading edge — as the angle of attack rises the stagnation point moves down and under the wing, the airflow lifts the vane and closes the switch — or a pneumatic reed horn.
- Transport aircraft use angle-of-attack sensors: heated vane-type sensors that align themselves with the local airflow and drive a resolver or synchro, or conical (slotted) probes that sense a pressure difference. A stall warning computer (or the flight control or air data computers) combines AoA with flap/slat position and Mach number and drives the stick shaker — an electric motor with an eccentric weight on the control column — and an aural warning.
- Some types add stall protection: a stick pusher that pushes the column forward (common on T-tail aircraft prone to deep stall), or fly-by-wire angle-of-attack protection (Chapter 12).
- AoA vanes are fragile: they must not be pushed or bent, must move freely, must be heated, and are tested with a fixture that sets known angles. A damaged or mis-rigged vane can trigger false warnings — or none.
The glass cockpit
- Electronic flight instrument system (EFIS): the PFD shows attitude, flight-director bars, airspeed and altitude tapes, vertical speed, heading, flight-mode annunciations and ILS deviations; the ND shows heading, track, route, VOR/ILS and map modes, with weather radar, terrain and TCAS overlaid.
- Engine and alerting displays — ECAM (Airbus: engine/warning display plus system display with checklists) or EICAS (Boeing: engine indication and crew alerting) — show engine parameters, warnings, cautions and system synoptics.
- Displays are LCDs (CRTs on older types) driven by symbol generators or display-management computers, with reversion switching so that a pilot can move the PFD to another screen or select an alternate source.
- An integrated standby instrument system (ISIS) — or separate standby attitude, airspeed and altitude instruments and a standby compass — has its own sensors and power so that it survives a total main-display failure.
- Colour conventions (which vary between manufacturers): red for warnings and limits, amber for cautions, green for normal values and engaged modes, cyan/white for selected or scale data, magenta for targets or flight-plan data.
Indications of other aircraft systems
Every system reports to the crew through sensors: thermocouples (EGT), resistance temperature detectors (oil, fuel, air temperatures), pressure transducers, speed sensors and tachometer generators (N1, N2), capacitance probes (fuel quantity), synchros, LVDTs and RVDTs (surface, flap and valve positions), proximity sensors (doors, gear) and accelerometers (engine vibration). Their data travel over digital buses (ARINC 429, AFDX) to the display computers. Exceedances and failures generate crew alerts graded as warnings (red, with master warning), cautions (amber, master caution) and advisories, while the central maintenance computer records the fault messages for the engineer (Chapter 19).