fix(fms): fix vnav crash on steep approaches (#8766)

Handle steep approaches better
flybywiresim · Jul 28, 2024 · 7bd5b1d · 7bd5b1d
1 parent 6c7c3aa
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Showing 2 changed files with 59 additions and 16 deletions.
diff --git a/.github/CHANGELOG.md b/.github/CHANGELOG.md
@@ -106,8 +106,9 @@
 1. [EFB] Added a takeoff performance calculator - @donstim (donbikes), @tracernz (Mike)
 1. [MCDU] Removed V-speed auto-fill function - @tracernz (Mike)
 1. [PFD] Implement alerts within artificial horizon (ROP, ROW, OANS, stall, windshear) @flogross89 (Flo)
-1. [MCDU] Fixed ZFW Autofill with lbs during boarding @ShreyasKallingal 
+1. [MCDU] Fixed ZFW Autofill with lbs during boarding @ShreyasKallingal
 1. [FWC] Fix NW STRG DISC turning amber too soon - @adoggman (Andrew)
+1. [FMS] Fix VNAV crash for steep approaches - @BlueberryKing (BlueberryKing)
 
 ## 0.11.0
 

diff --git a/fbw-a32nx/src/systems/fmgc/src/guidance/vnav/descent/ApproachPathBuilder.ts b/fbw-a32nx/src/systems/fmgc/src/guidance/vnav/descent/ApproachPathBuilder.ts
@@ -348,15 +348,14 @@ export class ApproachPathBuilder {
     const decelerationSequence = new TemporaryCheckpointSequence(lastCheckpoint);
 
     const parameters = this.observer.get();
-    const { managedDescentSpeedMach } = parameters;
-
-    for (
-      let i = 0;
-      i < 10 &&
-      decelerationSequence.lastCheckpoint.reason !== VerticalCheckpointReason.Decel &&
-      decelerationSequence.lastCheckpoint.distanceFromStart - targetDistanceFromStart > 1e-4; // We really only want to prevent floating point errors here
-      i++
-    ) {
+
+    let lastAccelerationCheckpointIndex = 1;
+
+    const isDoneDeclerating = () =>
+      decelerationSequence.lastCheckpoint.reason === VerticalCheckpointReason.Decel ||
+      decelerationSequence.lastCheckpoint.distanceFromStart - targetDistanceFromStart <= 1e-4; // We really only want to prevent floating point errors here
+
+    for (let i = 0; i < 10 && !isDoneDeclerating(); i++) {
       const { distanceFromStart, altitude, speed, remainingFuelOnBoard } = decelerationSequence.lastCheckpoint;
 
       const speedConstraint = speedProfile.getMaxDescentSpeedConstraint(distanceFromStart - 1e-4);
@@ -387,16 +386,24 @@ export class ApproachPathBuilder {
           altitude,
           Math.max(speedConstraint.maxSpeed, speed), // If constraint speed is less than the current speed, don't try to decelerate to it (because we'll end up going the wrong way)
           speed,
-          managedDescentSpeedMach,
+          parameters.managedDescentSpeedMach,
           remainingFuelOnBoard,
           windProfile.getHeadwindComponent(distanceFromStart, altitude),
           AircraftConfigurationProfile.getBySpeed(speed, parameters),
         );
 
-        if (decelerationStep.distanceTraveled > 0) {
-          throw new Error('[FMS/VNAV] Deceleration step distance should not be positive');
+        if (decelerationStep.error !== undefined || decelerationStep.distanceTraveled > 0) {
+          // Reset
+          decelerationSequence.checkpoints.splice(
+            lastAccelerationCheckpointIndex,
+            decelerationSequence.checkpoints.length,
+          );
+
+          break;
         }
 
+        lastAccelerationCheckpointIndex = decelerationSequence.length;
+
         if (decelerationStep.distanceTraveled < 1e-4) {
           // We tried to declerate, but it took us beyond targetDistanceFromStart, so we scale down the step
           const scaling = Math.min(1, remainingDistance / decelerationStep.distanceTraveled);
@@ -420,7 +427,7 @@ export class ApproachPathBuilder {
             altitude,
             -remainingDistanceToConstraint,
             speedConstraint.maxSpeed,
-            managedDescentSpeedMach,
+            parameters.managedDescentSpeedMach,
             remainingFuelOnBoard - decelerationStep.fuelBurned,
             windProfile.getHeadwindComponent(distanceFromStart, altitude),
             AircraftConfigurationProfile.getBySpeed(speedConstraint.maxSpeed, parameters),
@@ -442,17 +449,28 @@ export class ApproachPathBuilder {
         const targetSpeed = Math.min(flapTargetSpeed, limitingSpeed);
         const config = AircraftConfigurationProfile.getBySpeed(speed, parameters);
 
-        // TODO: Handle case where we need speedbrakes
         const decelerationStep = this.fpaStrategy.predictToSpeed(
           altitude,
           targetSpeed,
           speed,
-          managedDescentSpeedMach,
+          parameters.managedDescentSpeedMach,
           remainingFuelOnBoard,
           windProfile.getHeadwindComponent(distanceFromStart, altitude),
           config,
         );
 
+        if (decelerationStep.error !== undefined || decelerationStep.distanceTraveled > 0) {
+          // Reset
+          decelerationSequence.checkpoints.splice(
+            lastAccelerationCheckpointIndex,
+            decelerationSequence.checkpoints.length,
+          );
+
+          break;
+        }
+
+        lastAccelerationCheckpointIndex = decelerationSequence.length;
+
         if (decelerationStep.distanceTraveled < remainingDistance) {
           const scaling = Math.min(1, remainingDistance / decelerationStep.distanceTraveled);
           this.scaleStepBasedOnLastCheckpoint(decelerationSequence.lastCheckpoint, decelerationStep, scaling);
@@ -469,6 +487,30 @@ export class ApproachPathBuilder {
       }
     }
 
+    if (!isDoneDeclerating()) {
+      const config = AircraftConfigurationProfile.getBySpeed(decelerationSequence.lastCheckpoint.speed, parameters);
+
+      // Fly constant speed instead
+      const constantSpeedStep = this.fpaStrategy.predictToDistance(
+        decelerationSequence.lastCheckpoint.altitude,
+        targetDistanceFromStart - decelerationSequence.lastCheckpoint.distanceFromStart,
+        decelerationSequence.lastCheckpoint.speed,
+        parameters.managedDescentSpeedMach,
+        decelerationSequence.lastCheckpoint.remainingFuelOnBoard,
+        windProfile.getHeadwindComponent(
+          decelerationSequence.lastCheckpoint.distanceFromStart,
+          decelerationSequence.lastCheckpoint.altitude,
+        ),
+        config,
+      );
+
+      decelerationSequence.addDecelerationCheckpointFromStep(
+        constantSpeedStep,
+        FlapConfigurationProfile.getFlapCheckpointReasonByFlapConf(config.flapConfig),
+        FlapConfigurationProfile.getApproachPhaseTargetSpeed(config.flapConfig, parameters),
+      );
+    }
+
     return decelerationSequence;
   }
 }