Height below the aircraft is not terrain ahead

What surface does a radio altimeter measure?

A ground-relative range is a different reference from barometric altitude.

Round-trip timing and the measured surface

A radio altimeter transmits radio energy and measures the return from the surface below. Pulse timing or frequency-modulated continuous-wave techniques infer a range from the two-way propagation delay or associated frequency difference. In a simple vertical timing model, height = c × round-trip time / 2. The factor of two accounts for the outward and return path; forgetting it doubles the inferred height.

The display provides low-altitude height information within the installed system’s operating range and geometry. It is not a barometric altitude above mean sea level, nor a guarantee of clearance from obstacles ahead. Bank, pitch, antenna pattern, terrain slope, reflective surfaces and interference can affect which return is accepted or its validity. A sudden terrain change can alter radio height even if the aircraft remains at constant barometric altitude.

One source can support several functions

Radio height may feed automatic landing, thrust-retard or flare functions, terrain warning, altitude callouts and traffic-alert inhibition logic according to the aircraft design. An erroneous but accepted height can therefore have consequences beyond the visible number. Independent channels and comparison logic improve monitoring but their response is type-specific. A flagged loss and a wrong low indication can lead to different downstream states.

Understand the source used by each function and apply the approved failure procedure before making selections that can remove another useful channel. Compare the radio-height trend with the terrain and other flight data; do not substitute barometric altitude directly without accounting for the ground reference. The laboratory changes surface elevation while barometric altitude is fixed, showing why the two indications can legitimately move differently. It remains a geometric teaching model rather than a real antenna or propagation simulation.

Why can look-ahead warning act before height starts falling?

Position, terrain data and predicted path add a forward view.

Reactive and predictive information

Conventional ground-proximity warning uses combinations such as radio height, vertical rate, configuration and approach deviation to detect dangerous relationships with terrain. A high descent rate, excessive terrain closure or inappropriate configuration near the ground can trigger different modes. These are condition-based protections with limits: terrain that rises abruptly ahead can leave less warning than a gradual closure directly below.

A terrain-awareness system adds a terrain database, position estimate and predicted flight path to look ahead. It can detect a future conflict while present radio height remains comfortable. This depends on correct position, data and modelling; it is not a surveyed obstacle-clearance guarantee. A database display, caution and warning have different meanings and associated procedures, and not every map colour represents the same height reference across systems.

Time to conflict depends on closure

For a simplified constant closure, time available equals remaining distance divided by groundspeed. At 180 kt, 3 NM takes one minute. Faster groundspeed shortens that time; a steep descent can reduce vertical margin simultaneously. A large instantaneous terrain clearance does not prove that the forward path is safe if the aircraft is closing rapidly towards rising terrain.

Use the prescribed terrain-warning response promptly when the actual system calls for it; this lab does not select a real escape manoeuvre or threshold. Set a synthetic hill, horizontal speed and descent rate, then follow the projected path. The graphic and time readout use the same geometry so the effect of speed and descent is causal. It also makes the limitation explicit: changing the forecast path or position changes the prediction, while a wrong accepted position can make the prediction wrong.

Can you explain and apply the mechanism?

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