Navigation Instruments and Systems
What the cockpit uses
- Direct-reading magnetic compass: simple and independent, but suffers turning and acceleration errors and dip at high latitude.
- Gyro-magnetic system: a flux valve or magnetometer slaved to a directional gyro, giving a stable magnetic heading and feeding the HSI and flight director.
- INS/IRS: self-contained inertial systems aligned on the ground, then drifting slowly in flight. Schuler-tuned platforms limit the effect of acceleration, but position error still grows with time.
- Air data computer: supplies IAS, TAS, Mach, altitude and temperature to the navigation and flight guidance systems.
- FMS: merges sensors, computes LNAV and VNAV, compares ANP with RNP and provides the alert if performance falls short.
- Doppler and radar altimeters, EGPWS/TAWS, TCAS, ADS-B and standby instruments complete the picture.
Rules and behaviour
Alignment of an inertial system needs the aircraft stationary and a known position; it takes several minutes and cannot be done in flight. Inertial position error grows at around 1 to 2 nm per hour, so it is updated by radio or GNSS aids. Near the poles a magnetic reference becomes unreliable and true or grid references are used. Never rely on a single sensor when the specification requires monitoring and alerting.
Worked example
An IRS drifts 1.5 nm per hour. After 4 hours the position error is 6 nm, plus 2 nm from the alignment position, giving about 8 nm. On an RNP 10 route that is still acceptable, but on an RNP 4 route it exceeds the containment and the position must be updated or the capability reported as degraded.
Traps
Confusing the magnetic heading output of an AHRS with true heading; expecting a gyro compass to be usable close to the magnetic poles; forgetting alignment constraints; and treating a raw IRS position as adequate when RNP monitoring is required.