Aircraft General KnowledgeInstrumentation

Three of the six basic instruments are pressure instruments. They have no moving parts of their own worth speaking of — everything they show comes from two pressures: total (pitot) pressure and static pressure. Know which instrument is fed by what, and every blockage question answers itself.

Construction

The pitot tube faces the airflow and senses total pressure (static + dynamic). Static ports sit flush with the fuselage, usually one each side so that sideslip errors cancel. Both lines are drained (water traps, drain holes) and the pitot is electrically heated against ice. The ASI capsule is fed pitot inside and static outside; its expansion is therefore proportional to dynamic pressure.

Blockage cases — the classic exam questions

• Pitot blocked, drain hole open: total pressure leaks away to static. Dynamic pressure collapses and the ASI falls towards zero. Altimeter and VSI unaffected.
• Pitot blocked, drain also blocked: trapped total pressure is now constant, so the ASI reacts only to changing static pressure. It behaves like an altimeter: over-reads in a climb, under-reads in a descent. This is the ice-in-the-pitot case.
• Static blocked: altimeter freezes at the blockage altitude, VSI reads zero, and the ASI under-reads in a climb, over-reads in a descent (trapped static is too high on the climb, so the measured difference is too small).
• Both blocked: ASI frozen, altimeter frozen, VSI zero.

Alternate static source

The alternate source usually vents to the cabin, where pressure is slightly lower than ambient static because of aerodynamic suction. Lower static means: altimeter over-reads, ASI over-reads, and the VSI shows a momentary climb as it is selected. Unpressurised aircraft with no alternate source: break the VSI glass as a last resort — the VSI then vents the static line to the cabin, and the altimeter and ASI read again while the VSI reads in reverse.

Airspeed error chain

IAS → (instrument + position/pressure error) → CAS → (compressibility) → EAS → (density) → TAS. Position error is largest at high angles of attack, i.e. low speed and high load. Manoeuvre-induced error is a transient caused by rapid attitude change disturbing airflow over the static ports.

A pressure altimeter is an aneroid barometer calibrated against the ICAO Standard Atmosphere (ISA). It measures pressure and displays the height in ISA that corresponds to that pressure. Every altimeter error follows from that one sentence.

Construction

One or more sealed, partially evacuated capsules sit in a case that is fed static pressure. As static pressure falls the capsules expand; the movement is amplified mechanically (or by a servo loop in a servo altimeter) to drive the pointers or drum. The subscale (Kollsman window) lets you set a datum pressure, typically 950–1050 hPa, which shifts the whole scale.

• QNH set → the instrument reads altitude above mean sea level.
• QFE set → it reads height above the aerodrome or threshold datum.
• 1013.25 hPa (QNE / standard) set → it reads pressure altitude, displayed as a flight level.

Near sea level, 1 hPa ≈ 27 ft; 1 mb of subscale change moves the pointers about 27 ft. A useful cross-check: aerodrome elevation × 1 hPa/27 ft is roughly the QNH–QFE difference.

Errors

Cold-weather correction

Rule of thumb: 4 ft per 1000 ft per degree Celsius of deviation from ISA, or about 4% of the height above the setting datum for each 10 °C below ISA. With an aerodrome at −20 °C ISA deviation, a 2000 ft procedure altitude is about 160 ft lower than it looks. Published minima on instrument charts are not temperature compensated by the chart; the corrections are applied by the pilot (or automatically, on some avionics, when a temperature-compensating altimetry function is armed).

Encoding and servo altimeters

A servo altimeter uses an electrical pick-off from the capsule driving a servomotor, which removes most of the friction and lag and gives finer resolution — hence its use at higher levels. An encoding altimeter supplies pressure altitude (always referenced to 1013.25 hPa, regardless of the subscale setting) to the transponder for Mode C reporting. Setting the subscale does not change what ATC sees; their system applies the local QNH.

Airspeed indicator

The ASI measures dynamic pressure, which is one half rho V squared. Because it is calibrated for ISA sea-level density, the indication is indicated airspeed, not true airspeed. At altitude, where density is lower, the same dynamic pressure corresponds to a higher TAS: TAS increases by roughly 1.5–2% per 1000 ft for a constant IAS.

IAS is the number that matters for handling, because lift and stall depend on dynamic pressure, not on TAS. That is why V-speeds are IAS. TAS is the number that matters for navigation.

Vertical speed indicator

The capsule receives static pressure directly; the case receives the same static pressure through a calibrated leak (metering unit). In level flight both are equal and the pointer reads zero. In a climb the capsule pressure falls immediately while the case pressure lags, and the resulting differential drives the pointer.

• Lag: the indication takes about 2–3 seconds to settle, and up to 6–9 seconds after a large change. A steady, accurate rate reading therefore trails the aircraft's actual performance.
• Trend versus rate: the first movement of the needle shows the trend and is instant enough to be useful; the settled value shows the rate.
• Instantaneous VSI (IVSI): adds accelerometer-driven dashpots that momentarily pump air into or out of the capsule, cancelling most of the lag. Its weakness is that it can give a false indication in a steep turn.
• A blocked static line makes the VSI read zero; a leak in the case makes it over-read.

A spinning rotor has two useful properties. Rigidity in space: it maintains its plane of rotation unless acted on. Precession: a force applied to the rim produces a movement 90° further round in the direction of rotation. Rigidity is what gives an attitude reference; precession is what makes rate instruments work and also what causes their errors.

Gyro stiffness increases with rotor mass, radius of gyration and rpm. Rotors are driven either by an engine-driven vacuum (suction) pump or electrically; a typical light IFR aeroplane uses vacuum for the attitude indicator and directional gyro, and electric power for the turn indicator, so that a single failure does not remove every gyro reference.

Attitude indicator

Erection systems keep the spin axis vertical. Because those systems sense apparent gravity, sustained accelerations mislead them, producing acceleration and turning errors: on a rapid acceleration the horizon bar shows a slight nose-up and right-turn tendency in a conventional design, and errors also appear on rolling out of a prolonged turn. Modern instruments limit these to a couple of degrees. Watch for the toppling limits of older instruments and for a slow, unannounced failure with decaying vacuum — the classic instrument-flying trap, because a dying AI drifts rather than flagging.

Directional gyro

The DG has no north-seeking capability; it holds a heading you set from the compass. It suffers:

• Real wander (mechanical imperfection, friction).
• Apparent wander due to earth rotation — 15° per hour times the sine of the latitude; a latitude nut compensates for a nominal latitude.
• Apparent wander due to transport when changing latitude.

Consequence: realign the DG with the compass every 10–15 minutes, in straight and level unaccelerated flight. Gyro-magnetic (slaved) compasses do this continuously from a flux valve, which is why they need no manual synchronisation.

Rate of turn indicator and turn coordinator

Precession is opposed by a calibrated spring, so deflection is proportional to yaw rate. A rate one turn is 3° per second, that is 180° per minute, so a half-standard-rate turn takes 6 minutes for 360°. Bank required for rate one is approximately TAS/10 + 7 (so about 17° at 100 kt). The instrument reads turn rate only — it says nothing about bank angle, and a turn coordinator, whose gimbal is canted, senses both roll and yaw so it responds initially to roll. The slip ball is not a gyro instrument at all: it is a simple pendulum showing the resultant of gravity and centrifugal acceleration. Ball out to the right means too little right rudder for the bank — step on the ball.

Limited panel: what to do when a gyro dies

A vacuum failure removes the attitude indicator and the directional gyro slowly and without a flag, which is why the failure is detected from the suction gauge (typically 4.5–5.5 inHg) and from cross-checks, not from a warning. On a limited panel:

• Pitch: altimeter and VSI, backed by the ASI. Set a known power and trim for a known speed.
• Bank and heading: turn indicator and slip ball, with the magnetic compass for the heading. Use timed turns at rate one — 3° per second, so a 90° turn takes 30 seconds — and roll out on time, then check the compass in unaccelerated flight.
• Keep it small: half-standard-rate turns, shallow climbs and descents, and no simultaneous changes of heading and level.
• Declare the failure, ask for a no-gyro or a straightforward vectored approach, and prefer an aerodrome with a 3D approach and better weather over the nearest one.

Solid-state and AHRS

Attitude and heading reference systems replace spinning rotors with MEMS rate sensors and accelerometers, corrected over the long term by a magnetometer and often by GNSS. There is no toppling limit and no vacuum system, but the unit needs electrical power and an alignment period, and its outputs are only as good as its power supply — hence the requirement for an independent standby attitude source in IFR aircraft.

The earth's magnetic field can be resolved into a horizontal component H, which is what a compass uses, and a vertical component Z. The angle between the total field and the horizontal is the angle of dip, near zero at the magnetic equator and 90° at the magnetic poles. As H weakens towards the poles, a direct reading compass becomes progressively more sluggish and less reliable.

• Variation — the angle between true and magnetic north; shown on charts by isogonals, and changing slowly with time.
• Deviation — the compass error caused by the aircraft's own magnetism, recorded on a compass deviation card, applied per heading.
• True to magnetic: west is best, east is least when converting from true to magnetic (add westerly variation).

Direct reading compass errors

The magnet assembly is pendulously suspended below its pivot so that dip does not tilt the card. That same pendulous suspension makes the compass sensitive to acceleration:

• Acceleration error: in the northern hemisphere, accelerating on an easterly or westerly heading gives an apparent turn towards north; decelerating gives an apparent turn towards south. Remember ANDS: Accelerate North, Decelerate South. Nil error on north or south headings.
• Turning error: turning through north the compass under-reads the turn (it lags, so you roll out early — undershoot the heading); turning through south it over-reads (leads, so you overshoot). Remember UNOS: Undershoot North, Overshoot South.

Both errors reverse in the southern hemisphere and both increase with latitude.

Serviceability and handling

Check the compass for liquid, bubbles, discolouration and a card that is free to move; a compass swing establishes deviation and is repeated after any significant change to the aircraft's electrical or magnetic state, after a heavy landing, or when the aircraft has been parked on the same heading for a long period. Keep headsets, phones and metal objects clear of the instrument.

Flux valve and slaved compass systems

A flux valve (flux gate detector) is a static, no-moving-parts sensor: a laminated core is excited by an AC coil, and the earth's field superimposes a signal on the harmonics from which the direction of H is derived. It is pendulously mounted, usually in a wingtip or tail away from magnetic interference. Its output slaves the directional gyro: the gyro gives short-term stability while the flux valve provides long-term magnetic reference. This is the gyro-magnetic or remote indicating compass driving an HSI, and it removes the need for manual DG synchronisation. In slaving failure the system reverts to a free DG (DG mode), which must then be synchronised manually and will drift.

From ADI/HSI to EFIS

The ADI (attitude director indicator) is an attitude indicator with flight director command bars added; the HSI (horizontal situation indicator) combines a compass card with a course deviation indicator, a to/from indication and often a glideslope pointer, so that track and heading are read in one place. An EFIS presents the same information on display units:

• PFD (primary flight display): attitude filling the display, with airspeed and altitude tapes, vertical speed, heading strip, selected values (bugs), flight director, and mode annunciations. Rate-of-change trend vectors are typical.
• ND / MFD (navigation or multifunction display): map or arc modes, flight plan track, navaids, terrain and weather overlays, engine and systems pages.

Exam-relevant limitations: display de-cluttering and failure flags (a failed parameter is removed or crossed, never left showing a plausible wrong value), the need for independent standby instruments (attitude, airspeed, altitude and a compass) on a separate power supply, screen brightness and reflectivity in bright sunlight, and the fact that a single display can be reverted to composite mode. Tape displays give excellent trend information but are poorer than round dials for absolute-value scanning, which is why bugs and trend vectors matter.

Radio altimeter

A low-power frequency-modulated continuous wave transmitter in the 4200–4400 MHz band measures height above the surface immediately below, usually up to 2500 ft. It reads height above terrain, not altitude, and is the source for many warnings and for decision-height alerts on precision approaches.

GPWS / TAWS

Terrain awareness and warning systems compare position and height with a terrain and obstacle database (TAWS) and use radio altimeter and air data inputs (GPWS). The classical GPWS modes are worth knowing:

A hard warning ("TERRAIN TERRAIN PULL UP") in IMC requires an immediate maximum-performance climb; a caution requires correction and cross-check. Nuisance warnings are avoided by using the correct approach configuration and by inhibiting where the procedure and the aircraft flight manual allow.

ACAS / TCAS and transponders

An airborne collision avoidance system interrogates other aircraft's transponders. ACAS I / TCAS I gives traffic advisories only; ACAS II / TCAS II gives traffic advisories (TA) and resolution advisories (RA) in the vertical plane only, and requires Mode S. Rules that get examined: an RA is followed even against an ATC clearance, ATC is informed as soon as practicable, and the system depends on other aircraft having an operating transponder with altitude reporting — a non-transponding aircraft is invisible to it.