CAA NZ ATPL Instruments & Navaids key terms

The pressure of undisturbed ambient air surrounding the aircraft, sensed by a flush static port. It is the reference against which pitot pressure is compared and drives the altimeter, airspeed indicator and vertical speed indicator.

The total pressure sensed by a forward-facing probe that brings air to rest. It equals static pressure plus dynamic pressure, and the difference between the two is what the airspeed indicator converts into indicated airspeed.

The error in sensed static pressure caused by airflow disturbance around the fuselage at the static port location. It varies with airspeed, angle of attack and configuration, and is corrected to give calibrated airspeed.

A partial or total obstruction of the pitot probe, usually by ice or insects. If the drain hole is also blocked, the trapped pressure causes the airspeed indicator to behave like an altimeter, reading high in the climb and low in the descent.

A secondary static port, usually inside the cabin, selected when the normal static system is blocked or damaged. Indicated airspeed and altitude then carry errors, and the aircraft must fly with the appropriate corrections applied.

An electronic unit that receives pitot and static pressures plus temperature inputs and computes pressure altitude, indicated and true airspeed, vertical speed, Mach number and temperature for display and for other aircraft systems.

A small solid-state sensor that converts pitot and static pressures into digital pressure values. Several modules feed the air data computer or avionics buses, giving redundancy and allowing cross-comparison between sources.

The ratio of true airspeed to the local speed of sound. It is computed from pitot and static pressure alone, so it remains available even when outside air temperature is not, and it governs high-speed handling and overspeed warnings.

Indicated airspeed corrected for instrument error and position error. It approximates the speed the aircraft would show in incompressible flow at sea level, and is the value used for most published handling and approach speeds.

The actual speed of the aircraft through the air mass, obtained by correcting calibrated airspeed for altitude (density) and temperature, with a compressibility correction at high Mach numbers. It is used for navigation and flight planning.

The altitude indicated when the subscale is set to the standard datum of 1013.25 hPa. It is used for flight levels above the transition altitude, for performance calculations, and as the input to true airspeed and Mach computations.

A sensitive altimeter in which a pressure capsule drives a servo that positions the pointers and also supplies an electrical altitude signal to other systems, such as the autopilot or the altitude alerting function.

A warning function that annunciates when the aircraft approaches or deviates from a selected altitude, typically with a caution tone and visual message, and often with a soft aural alert at the capture point.

The property of a spinning rotor that resists any change in the orientation of its spin axis in space. It is the basis of the attitude indicator and the heading indicator, whose indications remain fixed as the aircraft manoeuvres around them.

The apparent movement of a spinning gyroscope's spin axis in response to an applied torque, occurring at 90 degrees to that torque in the direction of rotation. It is exploited in rate gyros and causes real drift in attitude and heading instruments.

A gyroscope deliberately restrained so that it precesses in proportion to the rate of turn about an axis. Its output drives the turn indicator, and in many aircraft the turn coordinator also senses roll rate.

The apparent wander of a gyroscope caused by the Earth's rotation and by the aircraft's movement over the Earth's surface. It is corrected in heading indicators by a slaving system or periodically realigned by the pilot.

A directional gyro whose drift is continuously corrected by comparing its output with a magnetic heading reference from a flux valve or magnetometer. The result is a stable, drift-free heading display.

A remote magnetic sensor, usually mounted in a wingtip or tail, that detects the direction of the Earth's magnetic field and supplies a synchronising signal to the slaved heading system.

A self-contained navigation system using ring laser or fibre optic gyroscopes and accelerometers to measure angular rates and accelerations, integrating them to give attitude, heading, velocity and position without external radio aids.

A rotation sensor in which two laser beams travel in opposite directions around a closed cavity. Rotation produces a frequency difference between the beams proportional to angular rate, giving a strapdown inertial system with no moving parts.

Design of an inertial platform so that its natural period of oscillation is about 84 minutes, matching a pendulum of Earth radius. This makes the platform insensitive to horizontal accelerations and keeps it locally level during flight.

The angular difference between true north and magnetic north at a given place, shown on charts as easterly or westerly. It must be applied to convert between true and magnetic courses and headings.

The error in a magnetic compass caused by the aircraft's own magnetic fields, changing with heading. It is recorded on a compass swing card and applied to convert between magnetic heading and compass heading.

An apparent compass error in turns caused by the vertical component of the Earth's magnetic field acting on a compass whose pivot is not perfectly balanced, producing errors that vary with hemisphere and heading.

The compass error in a turn caused by the combination of the vertical component of the Earth's field and the aircraft's rate of turn, leading to under-reading in a northerly turn and over-reading in a southerly turn.

A compass error during acceleration or deceleration on easterly or westerly headings, caused by the compass card tilting about its pivot. It gives a false turn indication and is corrected by reference to the attitude indicator.

A calibration procedure in which the aircraft is aligned on a series of known headings and the resulting deviation recorded, producing a deviation card that is placarded near the compass.

A magnetic bearing from a VOR station, expressed in degrees from magnetic north. A selected radial is used with the course deviation indicator to fly towards or away from the station.

Course deviation indicator: the vertical needle or bar in a navigation display that shows displacement from a selected course or radial, with full-scale deflection typically representing about 10 degrees for a VOR.

The region above a VOR or NDB where the received signal provides no reliable bearing information, because the aircraft is nearly overhead. The navigation instrument gives erratic or ambiguous indications until clear of the zone.

The line-of-sight distance measured by DME between aircraft and ground transponder, which exceeds the true horizontal ground distance. The error is significant at high altitude and short range, and is negligible at long range.

The relative bearing indicated by an automatic direction finder to a non-directional beacon or broadcast station. It is relative to the aircraft's heading, so heading must be added to obtain the magnetic bearing to the station.

The angle between the aircraft's nose and a line to a station, measured clockwise from the nose from 0 to 360 degrees. Magnetic bearing to the station equals relative bearing plus magnetic heading.

The vertical guidance component of an ILS, normally on a UHF frequency paired with the localiser and defining a path, typically about 3 degrees, to the runway. Its deviations are displayed on the same indicator as the localiser.

The lateral guidance component of an ILS, transmitted on a VHF frequency and aligned with the runway centreline. Deviation sensitivity increases with distance from the runway to keep the course width constant at the threshold.

A defined zone around ILS localiser and glideslope antennas in which aircraft or vehicles can disturb the signal through reflection or obstruction. It is protected during low visibility approaches and must not be entered without clearance.

Area navigation that allows an aircraft to fly any desired path within the coverage of navigation aids or a self-contained system, rather than tracking directly to or from ground stations.

Required navigation performance: RNAV operations with a specified lateral accuracy value plus on-board performance monitoring and alerting. It tells the crew whether the system is achieving the required accuracy for the airspace.

The framework of specifications for navigation based on required accuracy rather than on specific sensors. It comprises RNAV and RNP specifications and allows approval for operations in particular airspace or on particular procedures.

Receiver autonomous integrity monitoring: a GNSS receiver function that checks consistency between satellites to detect a faulty signal, and with sufficient satellites to exclude it. It is essential for sole-means GNSS navigation.

A GNSS integrity function that not only detects an anomalous satellite signal but also removes it from the position solution, requiring more satellites in view than detection alone.

Traffic alert and collision avoidance system: airborne equipment that interrogates nearby transponders, displays traffic, and issues traffic advisories and resolution advisories with vertical escape commands when a collision threat exists.

A TCAS command telling the crew to climb, descend or maintain vertical speed to resolve a predicted collision. It must be followed promptly, and manoeuvring is limited to the vertical plane unless the crew has visual contact.

A TCAS alert indicating nearby traffic that may become a threat, shown as a symbol and often accompanied by an aural message. It requires the crew to look for the traffic but does not command a manoeuvre.

Enhanced ground proximity warning system: a terrain awareness system using a terrain database and aircraft position to predict impact and give visual and aural warnings, with alerts graded by urgency.

A GPWS alert based on radio altimeter height and landing gear or flap position, warning of insufficient terrain clearance when the aircraft is not configured for landing.

Airborne radar that transmits pulses and displays returns from precipitation, giving bearing and range to hazardous weather. Tilt management and gain control are needed to interpret returns correctly and avoid painting the ground.

The weakening of a radar signal as it passes through heavy precipitation, which can hide more distant and more severe weather behind the near return, so that the nearer storm appears as the greater threat.

Automatic flight control equipment that moves the flight controls to hold selected attitude, heading, altitude or navigation path, reducing crew workload and providing precise tracking during instrument flight.

A display of computed steering commands on the attitude indicator, telling the pilot how to manoeuvre to follow a selected mode. It can be used with or without the autopilot engaged.

The display area, usually at the top of the primary flight display, showing which autopilot and autothrottle modes are active, armed or engaged, so the crew can confirm what the automation is doing.

A computer system that stores navigation data, builds and flies routes, computes performance and predictions, and provides guidance to the autopilot. It is the central navigation and flight planning tool in modern airliners.

The electronic display that integrates attitude, airspeed, altitude, vertical speed, heading and flight director or autopilot information on one screen, replacing the traditional separate mechanical instruments.

A defined geographical position, identified by name or coordinates, that forms part of a navigation route. Waypoints may be defined by radio aids or by latitude and longitude for area navigation.

A line on a chart joining places of equal magnetic variation. Isogonals show how variation changes across a region and help convert between true and magnetic directions during flight planning.

The altitude above which the aircraft sets standard pressure and operates in terms of flight levels. Below it, the altimeter is set to the local pressure setting, and the changeover occurs at the transition level.