Vertical Navigation

FMS Vertical Navigation (avionics/flight-management/vertical-navigation)

Use when the task is flight management system vertical navigation: the vertical descent profile from cruise down to an arrival constraint, its gradient and flight path angle, and altitude constraint checks along the path.

Domain quick reference

• Vertical navigation (VNAV) computes the descent profile between altitude constraints, not the lateral route geometry.
• Top of descent (TOD) is the point where the descent must start so the aircraft reaches the target altitude on the planned gradient.
• Descent gradient is the altitude loss per nautical mile along the path, commonly about 318 ft/nm for a 3 degree flight path angle (1 nm = 6076.1154 ft).
• Flight path angle is the descent angle derived from the gradient.
• Altitude constraints at waypoints are stated as AT or AT OR ABOVE values that the computed path must satisfy.
• An FMS executes this vertical profile under the airborne software lifecycle discipline of DO-178C.

Workflow

1. Collect the cruise altitude, the target constraint altitude, and the planned descent gradient.
2. Compute the top of descent distance with tod_distance.
3. Derive the gradient from altitudes and distance with descent_gradient when the gradient is not given.
4. Convert the gradient to a flight path angle with fpa_deg.
5. Step down the path with altitude_at and check each waypoint constraint with constraint_ok.

Pitfalls

• Mixing units: altitudes in feet with distances in nautical miles requires gradient in feet per nautical mile, not percent.
• Starting the descent late, which forces a steeper gradient than the aircraft can fly.
• Treating an AT OR ABOVE constraint as an exact crossing altitude.
• Computing a descent that ends below zero altitude.
• Confusing vertical navigation with lateral flight plan building (that is the flight-planning leaf).

Behavior contract (gate 3)

The TOD, gradient, flight path angle, and constraint logic is exercised by the gate 3 contract test: scripts/test_vertical_navigation_logic.py against scripts/vertical_navigation_logic.py (stdlib unittest, offline). Run: python3 scripts/test_vertical_navigation_logic.py

Compliance

• Standards referenced, not reproduced: DO-178C text is proprietary (RTCA); the VNAV computations here are common methodology, summary-only per standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.