Guidance and Speed Management

Overview

GuidanceActor is the FMGC subsystem that runs once per second, driven by a GuidanceInput message assembled and delivered by KernelActor. On each cycle it computes the active lateral guidance data — cross-track error (XTE), heading-to-FCU, magnetic track and distance to the active waypoint — alongside managed speed guidance for the current flight phase, DECEL point placement for the approach transition, and radio navigation autotune. Results are published to the Akka.NET EventStream as a GuidanceResult and sent directly to external simulator modules (FCU, speed tape display) via INET registers through INetActor.

GuidanceInput Assembly

KernelActor assembles a GuidanceInput single-message bundle each tick (after receiving a FlightPlanSnapshot from FlightPlanActor via Ask+PipeTo) and sends it to GuidanceActor. The bundle contains three fields:

Aircraft As AircraftState — immutable snapshot of the 18-field INET register cache
Trajectory As TrajectoryResult — the latest trajectory computation result (may be Nothing until first computation completes)
FlightPlan As FlightPlanSnapshot — current primary flight plan snapshot

AircraftState Fields and Source Registers

Field	Register	Scaling	Unit
Latitude	1703	raw / 10000	Decimal degrees
Longitude	1704	raw / 10000	Decimal degrees
TrueTrack	1768	raw / 10	Degrees
TrueHeading	1706	raw / 10	Degrees
MagHeading	— (derived)	TrueHeading − MagVariation	Degrees
MagneticVariation	1705	raw / 10	Degrees
TasKts	1025	raw	Knots
GroundSpeedKts	1766	raw	Knots
PressureAltFt	1099	raw	Feet
AltitudeFt	1009	raw	Feet
WindDirectionDeg	1718	raw / 10	Degrees true
WindSpeedKts	1717	raw	Knots
FlapLever	2345	raw	0–4
ThrustDetent	2381	raw	Integer
OnGround	1001	1 = on ground	Boolean
ManagedHdg	2284	bit 16 set	Boolean
GrossWeightKg	1082	raw	kg
OatDegC	1071	raw	°C

Guidance Computation Cycle

Active Leg Sequencing

GuidanceActor maintains a private _activeWptIndex integer tracking which leg in the flight plan list is currently active. On each cycle, it calculates bearing and distance from the aircraft position to the active waypoint using GeoCalculations.Bearing and GeoCalculations.DistanceNm.

Waypoint passage is detected by HasPassedAbeam, which compares the inbound track angle of the leg against the current bearing to the waypoint. When the bearing difference indicates the aircraft has passed abeam, _activeWptIndex is incremented and the cycle returns immediately — the next tick recalculates from scratch with the new active waypoint. This is a faithful port of the VB6 clsGuidance.cls early-exit fix from 2025-12-20.

A _suppressOffset flag is set on passage and holds XTE at zero until the aircraft is back within 0.5 NM of the new leg track, preventing spurious XTE spikes during the leg transition.

XTE uses a track-angle difference formula with a 6-degree dead band (port of VB6 Calc_Crosstrack_Error), ensuring the aircraft must be more than 6 degrees off track before a non-zero XTE is reported.

Speed Guidance

ComputeSpeedGuidance runs at the start of every guidance cycle, regardless of whether a flight plan is loaded. It computes all speed values from the current AircraftState and sends changed values to the FCU speed tape registers.

Managed Speed Computation per Phase

Flight Phase	Commanded Speed	Mach Mode Trigger
Preflight / Takeoff	DefaultClimbIas (300 kt) limited below 10,000 ft to 250 kt	No
Climb	DefaultClimbIas (300 kt) limited below 10,000 ft to 250 kt	When IAS equivalent Mach >= 0.78
Cruise	DefaultCruiseIas (310 kt)	When IAS equivalent Mach >= 0.78
Descent	DefaultDescentIas (280 kt) limited below 10,000 ft to 250 kt	When IAS equivalent Mach >= 0.78
Approach / GoAround	VApp	No

VLS Protection

VlsCheckedSpeed ensures the commanded speed never falls below VLS for the current flap/weight configuration. VLS is derived from CalcVlsForConfig, which interpolates the VSS weight table (6 flap configs x 8 weight bands) and applies a multiplier depending on flap configuration. The ground state (weight-on-wheels) suppresses VLS protection to allow taxi operations.

SpeedGuidanceResult

SpeedGuidanceResult is a Friend Class that groups all outputs of ComputeSpeedGuidance to avoid ByRef return parameters:

CmdSpeedKts As Integer — commanded managed speed
VlsKts As Integer — VLS for current config
GreendotKts As Integer — green dot (best-lift) speed
SSpeedKts As Integer — S-speed (flaps retraction speed)
FSpeedKts As Integer — F-speed (slats retraction speed)
VAppKts As Integer — approach speed
IsMachMode As Boolean — true when Mach display applies

CalcDecelDistanceNm

CalcDecelDistanceNm(fromSpeedKts, toSpeedKts, decelRateFtPerSecSq) computes the distance in nautical miles required to decelerate from one speed to another. The deceleration rate is passed as a parameter at call sites with the value 0.05 * 32.174 ft/s² (0.05 g), matching the VB6 performance model. This parameterisation allows unit tests to inject a different rate.

DECEL Point Placement

The (DECEL) pseudo-waypoint is placed by ComputedFlightPlanBuilder.InsertDecelMarker based on the trajectory descent profile data. It marks the point where approach deceleration begins — the aircraft is still at full descent speed (e.g. 250 kt) at DECEL.

Trajectory-driven placement: The trajectory descent profile models a distance-based deceleration zone using 0.1 g horizontal deceleration rate (see Trajectory: Approach Deceleration). InsertDecelMarker scans the descent profile points in forward order (T/D to ADES) and finds the last point still at full IAS before the first drop exceeding 3 kt below FL100. This point becomes the (DECEL) marker position, with all predicted values (altitude, IAS, GS, fuel, ETA) taken directly from the trajectory profile point.

Guidance runtime: During the Descent flight phase, CheckDecelPassage scans the leg list for the (DECEL) pseudo-waypoint. Since the marker now always has coordinates from the trajectory, the actor computes distance from the aircraft to that position. When passage is confirmed and FCU managed heading mode is active, ActivateApproachPhase is called, setting _approachPhaseActive = True and triggering the flight phase transition to Approach.

Radio Nav Autotune

RadioNavModule (a Friend Module with pure functions) performs radio navigation autotune on every guidance cycle.

VOR/NDB Continuous Autotune

FindAheadVorNdb scans up to 5 legs forward from the active waypoint index looking for a leg with NavaidType = "VOR" or "NDB" and Frequency > 0. The first match is formatted as a compound register value and sent to register 5339 on change.

Format for VOR: "02446FV1{freq}" where freq has the decimal point removed (e.g., 113.60 MHz → "02446FV111360"). Format for NDB: "02446FA1{freq}" where freq is the ADF frequency in kHz without decimal point.

ILS Approach-Phase-Only Autotune

FindAheadIls is guarded: it returns Nothing unless approachPhaseActive = True. When active, it scans the last 10 legs of the flight plan for a leg with NavaidType = "ILS". The ILS frequency and course are then sent to register 5111:

Frequency: "&h03502{bcd5}" — 5-digit BCD-encoded MHz value (e.g., 110.30 MHz → BCD digits 1, 0, 3, 0 → &h03502 + k)
Course: "&h0c4e2{course5}" — 5-digit decimal course (e.g., 274° → &h0c4e200274)

Autotune Send-on-Change

Both VOR/NDB and ILS register values are cached in OldGuidanceCache (VorNdbRegisterValue, IlsFreqRegisterValue, IlsCourseRegisterValue) and only sent when the value has changed, preventing unnecessary INET traffic.

Flight Phase State Machine

GuidanceActor tracks the current phase as a GuidanceFlightPhase enum (Integer backing, matching VB6 FLIGHTPHASE_* constants):

Phase	Value	Transition In	Transition Out
Preflight	0	Initial state	On takeoff roll
Takeoff	1	(manual/external trigger)	Altitude > 1,500 ft AGL
Climb	2	AltitudeFt > AccelAlt (1,500 ft) from Takeoff or GoAround	Within 200 ft of cruise altitude from TrajectoryResult
Cruise	3	Climb → Cruise on altitude capture	(manual T/D activation via MCDU)
Descent	4	(manual T/D via MCDU/PERF page)	DECEL passage detected
Approach	5	DECEL passage or manual activation	TOGA thrust detected (ThrustDetent = 7)
GoAround	6	TOGA thrust in Approach phase	AltitudeFt > AccelAlt → back to Climb

Source Files

A320_FMGC/Guidance/GuidanceActor.vb — main guidance cycle, speed guidance, DECEL detection, flight phase state machine, send-on-change register outputs
A320_FMGC/Guidance/GuidanceModels.vb — AircraftState, GuidanceInput, GuidanceResult, OldGuidanceCache, HoldStateMachine, GuidanceFlightPhase enum
A320_FMGC/Guidance/RadioNavModule.vb — VOR/NDB/ILS autotune, register format helpers (registers 5339, 5111, 2605)
A320_FMGC/Kernel/KernelActor.vb — GuidanceInput assembly, AircraftState construction from 18-field register cache