FMS sensors, databases and path execution

Which sensor is the FMS trusting now?

A navigation solution can combine several inputs with different failure modes.

Measurement versus integrated solution

An FMS integrates inputs such as GNSS, radio navigation and inertial information according to its architecture. Inertial systems can propagate position without external radio signals, but accumulated error grows without suitable updating. Radio or GNSS updates can reduce that drift when their quality and geometry meet the installed logic.

Two FMC displays may share sensors or data. Their agreement does not automatically supply independent confirmation. Check the active sensor solution, degraded modes, update sources and achieved performance using the aircraft procedure. A valid-looking route line is an output of the chain, not evidence that every input is healthy.

A transparent reversion model

The laboratory assigns synthetic uncertainty-growth rates to inertial-only operation and a lower bound to suitable radio updating. The numbers are deliberately supplied exercise data rather than an aircraft performance model. Increasing time after an update shows why an initially acceptable solution can later exceed a requirement.

Watch uncertainty cross the required value

The supplied inertial-only model starts at 0.1 NM uncertainty and grows linearly at 0.02 NM per minute. A synthetic requirement of 0.5 NM is reached after (0.5 − 0.1)/0.02 = 20 minutes. At 25 minutes the model gives 0.6 NM. The position line can remain smooth throughout this change, so smoothness does not indicate that the required performance remains available.

Select suitable radio updating in the laboratory and its explicitly supplied uncertainty bound changes. This does not claim that every DME/DME system immediately resets drift to the same value. Real systems assess geometry, measurement quality, state history and their approved algorithms. The exercise teaches the decision structure: identify the remaining sensors, read achieved performance and alerts, then determine whether the intended operation is still supported.

Why does loading the right named procedure still need checking?

Database, runway, transition and leg type define the actual path.

Path definition and leg types

The navigation database encodes waypoints and path/terminator instructions. A track-to-fix leg describes a track ending at a fix; a direct-to-fix leg constructs a direct path to its fix; a course-to-fix leg specifies a course ending there. They can produce different geometry even when the next waypoint name is identical.

Fly-by waypoints anticipate the turn; fly-over waypoints require passage before the turn starts. A radius-to-fix leg defines a constant-radius path about a centre. Aircraft speed and turn capability still matter, and the system must support the encoded function.

Verification catches the wrong path

Check database applicability, correct procedure, runway and transition, then compare the coded sequence and constraints with the current chart using the approved procedure. An unexpected discontinuity, turn direction or track should be resolved rather than patched by guessing coordinates. The laboratory compares fly-by and fly-over geometry without pretending to reproduce every FMS algorithm.

Fly-by and fly-over reach different places

Consider an eastbound leg followed by a northbound leg at a named waypoint. A fly-by path begins a suitable turn before the waypoint so that it can join the outbound leg smoothly. A fly-over path reaches the waypoint first and then turns, initially carrying the aircraft east of the inbound line. Both can use the same named point while requiring different trajectories.

At higher groundspeed, the turn radius needed for a given bank increases approximately with speed squared. Doubling speed at fixed bank therefore quadruples the radius in an ideal coordinated level turn. An FMS must manage the encoded path within its supported functions and limits; a schematic display may also simplify the drawn turn. Compare the loaded sequence, turn direction and constraints with the current chart rather than validating only the waypoint names.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.