diff --git a/OpenRA.Mods.Common/Activities/Air/Fly.cs b/OpenRA.Mods.Common/Activities/Air/Fly.cs
index 07fd021b00..70ada20612 100644
--- a/OpenRA.Mods.Common/Activities/Air/Fly.cs
+++ b/OpenRA.Mods.Common/Activities/Air/Fly.cs
@@ -37,6 +37,8 @@ namespace OpenRA.Mods.Common.Activities
 
 		public static void FlyToward(Actor self, Plane plane, int desiredFacing, WDist desiredAltitude)
 		{
+			desiredAltitude = new WDist(plane.CenterPosition.Z) + desiredAltitude - self.World.Map.DistanceAboveTerrain(plane.CenterPosition);
+
 			var move = plane.FlyStep(plane.Facing);
 			var altitude = plane.CenterPosition.Z;
 
@@ -71,7 +73,8 @@ namespace OpenRA.Mods.Common.Activities
 			var desiredFacing = Util.GetFacing(d, plane.Facing);
 
 			// Don't turn until we've reached the cruise altitude
-			if (plane.CenterPosition.Z < plane.Info.CruiseAltitude.Length)
+			var targetAltitude = plane.CenterPosition.Z + plane.Info.CruiseAltitude.Length - self.World.Map.DistanceAboveTerrain(plane.CenterPosition).Length;
+			if (plane.CenterPosition.Z < targetAltitude)
 				desiredFacing = plane.Facing;
 
 			FlyToward(self, plane, desiredFacing, plane.Info.CruiseAltitude);
diff --git a/OpenRA.Mods.Common/Activities/Air/HeliFly.cs b/OpenRA.Mods.Common/Activities/Air/HeliFly.cs
index 5b4289f142..c4b128c993 100644
--- a/OpenRA.Mods.Common/Activities/Air/HeliFly.cs
+++ b/OpenRA.Mods.Common/Activities/Air/HeliFly.cs
@@ -38,6 +38,8 @@ namespace OpenRA.Mods.Common.Activities
 
 		public static bool AdjustAltitude(Actor self, Helicopter helicopter, WDist targetAltitude)
 		{
+			targetAltitude = new WDist(helicopter.CenterPosition.Z) + targetAltitude - self.World.Map.DistanceAboveTerrain(helicopter.CenterPosition);
+
 			var altitude = helicopter.CenterPosition.Z;
 			if (altitude == targetAltitude.Length)
 				return false;
@@ -85,7 +87,8 @@ namespace OpenRA.Mods.Common.Activities
 			// The next move would overshoot, so just set the final position
 			if (dist.HorizontalLengthSquared < move.HorizontalLengthSquared)
 			{
-				helicopter.SetPosition(self, pos + new WVec(0, 0, helicopter.Info.CruiseAltitude.Length - pos.Z));
+				var targetAltitude = helicopter.CenterPosition.Z + helicopter.Info.CruiseAltitude.Length - self.World.Map.DistanceAboveTerrain(helicopter.CenterPosition).Length;
+				helicopter.SetPosition(self, pos + new WVec(0, 0, targetAltitude - pos.Z));
 				return NextActivity;
 			}