diff --git a/OpenRA.Game/GameRules/UserSettings.cs b/OpenRA.Game/GameRules/UserSettings.cs
index 44a51a5bb8..386ac6478c 100644
--- a/OpenRA.Game/GameRules/UserSettings.cs
+++ b/OpenRA.Game/GameRules/UserSettings.cs
@@ -23,8 +23,8 @@ namespace OpenRA.GameRules
 	public class UserSettings
 	{
 		// Debug settings
-		public bool UnitDebug = false;
-		public bool PathDebug = false;
+		public bool UnitDebug = true;
+		public bool PathDebug = true;
 		public bool PerfDebug = true;
 		public bool IndexDebug = false;
 		public bool RecordSyncReports = true;
diff --git a/OpenRA.Game/OpenRA.Game.csproj b/OpenRA.Game/OpenRA.Game.csproj
index dd0b829c82..b81e6480f5 100755
--- a/OpenRA.Game/OpenRA.Game.csproj
+++ b/OpenRA.Game/OpenRA.Game.csproj
@@ -242,6 +242,8 @@
     <Compile Include="Widgets\ImageWidget.cs" />
     <Compile Include="Traits\SharesCell.cs" />
     <Compile Include="Traits\World\AircraftInfluence.cs" />
+    <Compile Include="Traits\World\HazardLayer.cs" />
+    <Compile Include="Traits\Hazardous.cs" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\OpenRA.FileFormats\OpenRA.FileFormats.csproj">
diff --git a/OpenRA.Game/PathFinder.cs b/OpenRA.Game/PathFinder.cs
index 1384aaa0dc..29ae47f370 100644
--- a/OpenRA.Game/PathFinder.cs
+++ b/OpenRA.Game/PathFinder.cs
@@ -42,7 +42,7 @@ namespace OpenRA
 			for( int x = 0 ; x < map.MapSize.X ; x++ )
 				for( int y = 0 ; y < map.MapSize.Y ; y++ )
 					for (var umt = UnitMovementType.Foot; umt <= UnitMovementType.Fly; umt++ )
-						passableCost[(int)umt][ x, y ] = (umt == UnitMovementType.Fly) ? 0f : ( world.Map.IsInMap( x, y ) )
+						passableCost[(int)umt][ x, y ] = (umt == UnitMovementType.Fly) ? 1f : ( world.Map.IsInMap( x, y ) )
 							? (float)Rules.TerrainTypes[world.TileSet.GetTerrainType(world.Map.MapTiles[x, y])]
 								.GetCost(umt)
 							: float.PositiveInfinity;
diff --git a/OpenRA.Game/PathSearch.cs b/OpenRA.Game/PathSearch.cs
index 9c56a9d061..814555d5cc 100755
--- a/OpenRA.Game/PathSearch.cs
+++ b/OpenRA.Game/PathSearch.cs
@@ -33,7 +33,6 @@ namespace OpenRA
 		public CellInfo[ , ] cellInfo;
 		public PriorityQueue<PathDistance> queue;
 		public Func<int2, float> heuristic;
-		public UnitMovementType umt;
 		Func<int2, bool> customBlock;
 		public bool checkForBlocked;
 		public Actor ignoreBuilding;
@@ -48,9 +47,6 @@ namespace OpenRA
 			world = self.World;
 			cellInfo = InitCellInfo();
 			queue = new PriorityQueue<PathDistance>();
-			
-			umt = self.traits.Get<Mobile>().GetMovementType();
-
 			buildingInfluence = world.WorldActor.traits.Get<BuildingInfluence>();
 		}
 
@@ -88,11 +84,13 @@ namespace OpenRA
 
 		public int2 Expand( World world, float[][ , ] passableCost )
 		{
+			var	umt = self.traits.Get<Mobile>().GetMovementType();
+
 			var p = queue.Pop();
 			cellInfo[ p.Location.X, p.Location.Y ].Seen = true;
 			
 			var thisCost = passableCost[(int)umt][p.Location.X, p.Location.Y]*
-						   world.WorldActor.traits.WithInterface<ICustomTerrain>().Aggregate(1f, (a, x) => a * x.GetCost(p.Location,umt));
+						   world.WorldActor.traits.WithInterface<ICustomTerrain>().Aggregate(1f, (a, x) => a * x.GetCost(p.Location,self));
 			
 			if (thisCost == float.PositiveInfinity) 
 				return p.Location;
@@ -107,7 +105,7 @@ namespace OpenRA
 
 				var costHere = passableCost[(int)umt][newHere.X, newHere.Y]*
 						 	   world.WorldActor.traits.WithInterface<ICustomTerrain>()
-							   .Aggregate(1f, (a, x) => a * x.GetCost(newHere,umt));
+							   .Aggregate(1f, (a, x) => a * x.GetCost(newHere,self));
 			
 				if (costHere == float.PositiveInfinity)
 					continue;
diff --git a/OpenRA.Game/Traits/Hazardous.cs b/OpenRA.Game/Traits/Hazardous.cs
new file mode 100644
index 0000000000..615472cb64
--- /dev/null
+++ b/OpenRA.Game/Traits/Hazardous.cs
@@ -0,0 +1,52 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007,2009,2010 Chris Forbes, Robert Pepperell, Matthew Bowra-Dean, Paul Chote, Alli Witheford.
+ * This file is part of OpenRA.
+ * 
+ *  OpenRA is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ * 
+ *  OpenRA is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ * 
+ *  You should have received a copy of the GNU General Public License
+ *  along with OpenRA.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#endregion
+
+using System.Linq;
+using System.Collections.Generic;
+
+namespace OpenRA.Traits
+{
+	class AntiAirInfo : ITraitInfo
+	{
+		public readonly float Badness = 1000f;
+		public object Create( ActorInitializer init ) { return new AntiAir( init.self ); }
+	}
+	
+	class AntiAir : IProvideHazard
+	{
+		public AntiAir(Actor self)
+		{
+			self.World.WorldActor.traits.Get<HazardLayer>().Add( self, this );
+		}
+		
+		public IEnumerable<HazardLayer.Hazard> HazardCells(Actor self)
+		{
+			var info = self.Info.Traits.Get<AntiAirInfo>();
+			return self.World.FindTilesInCircle(self.Location, (int)self.GetPrimaryWeapon().Range).Select(
+			      	t => new HazardLayer.Hazard(){location = t, type = "antiair", intensity = info.Badness});
+		}
+	}
+	
+	class AvoidsAAInfo : TraitInfo<AvoidsAA> {}
+	class AvoidsAA : IAvoidHazard
+	{
+		public string Type { get { return "antiair"; } }
+	}
+}
diff --git a/OpenRA.Game/Traits/TraitsInterfaces.cs b/OpenRA.Game/Traits/TraitsInterfaces.cs
index b694e60eca..e1ee021cdb 100644
--- a/OpenRA.Game/Traits/TraitsInterfaces.cs
+++ b/OpenRA.Game/Traits/TraitsInterfaces.cs
@@ -22,6 +22,7 @@ using System.Collections.Generic;
 using System.Drawing;
 using OpenRA.GameRules;
 using OpenRA.Graphics;
+using OpenRA.Traits;
 
 namespace OpenRA.Traits
 {
@@ -53,11 +54,13 @@ namespace OpenRA.Traits
 	public interface INotifyCapture { void OnCapture(Actor self, Actor captor); }
 	public interface IAcceptSpy { void OnInfiltrate(Actor self, Actor spy); }
 	public interface INotifyEnterCell { void OnEnterCell(Actor self, int2 cell); }
+	public interface IProvideHazard { IEnumerable<HazardLayer.Hazard> HazardCells(Actor self); }
+	public interface IAvoidHazard { string Type { get; } }
 
 	public interface ICustomTerrain
 	{
-		float GetCost(int2 p, UnitMovementType umt);
-		float GetSpeedModifier(int2 p, UnitMovementType umt);
+		float GetCost(int2 p, Actor forActor);
+		float GetSpeedModifier(int2 p, Actor forActor);
 	}
 
 	public interface IDisable { bool Disabled { get; } }
diff --git a/OpenRA.Game/Traits/Util.cs b/OpenRA.Game/Traits/Util.cs
index f34cb5a14f..f87bb9391c 100755
--- a/OpenRA.Game/Traits/Util.cs
+++ b/OpenRA.Game/Traits/Util.cs
@@ -149,7 +149,7 @@ namespace OpenRA.Traits
 				var tt = self.World.GetTerrainType(self.Location);
 				terrain = Rules.TerrainTypes[tt].GetSpeedModifier(umt)*self.World.WorldActor.traits
 					.WithInterface<ICustomTerrain>()
-					.Select(t => t.GetSpeedModifier(self.Location, umt))
+					.Select(t => t.GetSpeedModifier(self.Location, self))
 					.Product();
 			}
 			var modifier = self.traits
diff --git a/OpenRA.Game/Traits/World/AircraftInfluence.cs b/OpenRA.Game/Traits/World/AircraftInfluence.cs
new file mode 100644
index 0000000000..dcc23abf64
--- /dev/null
+++ b/OpenRA.Game/Traits/World/AircraftInfluence.cs
@@ -0,0 +1,97 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007,2009,2010 Chris Forbes, Robert Pepperell, Matthew Bowra-Dean, Paul Chote, Alli Witheford.
+ * This file is part of OpenRA.
+ * 
+ *  OpenRA is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ * 
+ *  OpenRA is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ * 
+ *  You should have received a copy of the GNU General Public License
+ *  along with OpenRA.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#endregion
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+using OpenRA.FileFormats;
+
+namespace OpenRA.Traits
+{
+	public class AircraftInfluenceInfo : ITraitInfo
+	{
+		public object Create( ActorInitializer init ) { return new AircraftInfluence( init.world ); }
+	}
+
+	public class AircraftInfluence : ITick
+	{
+		List<Actor>[,] influence;
+		Map map;
+
+		public AircraftInfluence( World world )
+		{
+			map = world.Map;
+			influence = new List<Actor>[world.Map.MapSize.X, world.Map.MapSize.Y];
+			for (int i = 0; i < world.Map.MapSize.X; i++)
+				for (int j = 0; j < world.Map.MapSize.Y; j++)
+					influence[ i, j ] = new List<Actor>();
+
+			world.ActorRemoved += a => Remove( a, a.traits.GetOrDefault<IOccupyAir>() );
+		}
+
+		public void Tick( Actor self )
+		{
+			SanityCheck( self );
+		}
+
+		[Conditional( "SANITY_CHECKS" )]
+		void SanityCheck( Actor self )
+		{
+			for( int x = 0 ; x < self.World.Map.MapSize.X ; x++ )
+				for( int y = 0 ; y < self.World.Map.MapSize.Y ; y++ )
+					if( influence[ x, y ] != null )
+						foreach (var a in influence[ x, y ])
+							if (!a.traits.Get<IOccupyAir>().OccupiedAirCells().Contains( new int2( x, y ) ) )
+								throw new InvalidOperationException( "AIM: Sanity check failed A" );
+
+			foreach( var t in self.World.Queries.WithTraitMultiple<IOccupyAir>() )
+				foreach( var cell in t.Trait.OccupiedAirCells() )
+					if (!influence[cell.X, cell.Y].Contains(t.Actor))
+						throw new InvalidOperationException( "AIM: Sanity check failed B" );
+		}
+
+		Actor[] noActors = { };
+		public IEnumerable<Actor> GetUnitsAt( int2 a )
+		{
+			if (!map.IsInMap(a)) return noActors;
+			return influence[ a.X, a.Y ];
+		}
+
+		public void Add( Actor self, IOccupyAir unit )
+		{
+			foreach( var c in unit.OccupiedAirCells() )
+				influence[c.X, c.Y].Add(self);
+		}
+
+		public void Remove( Actor self, IOccupyAir unit )
+		{
+			if (unit != null)
+				foreach (var c in unit.OccupiedAirCells())
+					influence[c.X, c.Y].Remove(self);
+		}
+
+		public void Update(Actor self, IOccupyAir unit)
+		{
+			Remove(self, unit);
+			if (!self.IsDead) Add(self, unit);
+		}
+	}
+}
diff --git a/OpenRA.Game/Traits/World/HazardLayer.cs b/OpenRA.Game/Traits/World/HazardLayer.cs
new file mode 100644
index 0000000000..952eca497c
--- /dev/null
+++ b/OpenRA.Game/Traits/World/HazardLayer.cs
@@ -0,0 +1,99 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007,2009,2010 Chris Forbes, Robert Pepperell, Matthew Bowra-Dean, Paul Chote, Alli Witheford.
+ * This file is part of OpenRA.
+ * 
+ *  OpenRA is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ * 
+ *  OpenRA is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ * 
+ *  You should have received a copy of the GNU General Public License
+ *  along with OpenRA.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#endregion
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+using OpenRA.FileFormats;
+
+namespace OpenRA.Traits
+{
+	public class HazardLayerInfo : ITraitInfo
+	{
+		public object Create( ActorInitializer init ) { return new HazardLayer( init.world ); }
+	}
+
+	public class HazardLayer : ICustomTerrain
+	{
+		List<Pair<Actor, Hazard>>[,] hazards;
+		Map map;
+
+		public HazardLayer( World world )
+		{
+			//System.Console.WriteLine("Created HazardLayer");
+			map = world.Map;
+			hazards = new List<Pair<Actor, Hazard>>[world.Map.MapSize.X, world.Map.MapSize.Y];
+			for (int i = 0; i < world.Map.MapSize.X; i++)
+				for (int j = 0; j < world.Map.MapSize.Y; j++)
+					hazards[ i, j ] = new List<Pair<Actor, Hazard>>();
+
+			world.ActorRemoved += a => Remove( a, a.traits.GetOrDefault<IProvideHazard>() );
+		}
+
+		
+		public float GetSpeedModifier(int2 p, Actor forActor)
+		{
+			return 1f;
+		}
+		
+		public float GetCost(int2 p, Actor forActor)
+		{
+			//System.Console.WriteLine("GetCost for {0}", forActor.Info.Name);
+
+			var avoid = forActor.traits.WithInterface<IAvoidHazard>().Select(h => h.Type).ToList();
+			var intensity = hazards[p.X,p.Y].Aggregate(1f,(a,b) => a + (avoid.Contains(b.Second.type) ? b.Second.intensity : 0f));			
+			//System.Console.WriteLine("Avoid {0} cost {1}", avoid.Aggregate("",(a,b) => a+","+b), intensity);
+
+			return intensity;
+		}
+		
+		public void Add( Actor self, IProvideHazard hazard )
+		{
+			//System.Console.WriteLine("Adding hazard {0}", self.Info.Name);
+
+			foreach( var h in hazard.HazardCells(self) )
+			{
+			//	System.Console.WriteLine("\t{0} {1} {2}", h.location, h.type, h.intensity);
+				hazards[h.location.X, h.location.Y].Add(Pair.New(self, h));
+			}
+		}
+
+		public void Remove( Actor self, IProvideHazard hazard )
+		{
+			if (hazard != null)
+				foreach (var h in hazard.HazardCells(self))
+					hazards[h.location.X, h.location.Y].Remove(Pair.New(self,h));
+		}
+
+		public void Update(Actor self, IProvideHazard hazard)
+		{
+			Remove(self, hazard);
+			if (!self.IsDead) Add(self, hazard);
+		}
+		
+		public struct Hazard
+		{
+			public int2 location;
+			public string type;
+			public float intensity;
+		}
+	}
+}
diff --git a/OpenRA.Game/Traits/World/ResourceLayer.cs b/OpenRA.Game/Traits/World/ResourceLayer.cs
index 642204528d..002c698535 100644
--- a/OpenRA.Game/Traits/World/ResourceLayer.cs
+++ b/OpenRA.Game/Traits/World/ResourceLayer.cs
@@ -93,17 +93,21 @@ namespace OpenRA.Traits
 						content[x, y].density = GetIdealDensity(x, y);
 		}
 		
-		public float GetSpeedModifier(int2 p, UnitMovementType umt)
+		public float GetSpeedModifier(int2 p, Actor forActor)
 		{
 			if (content[p.X,p.Y].type == null)
 				return 1.0f;
+			
+			var	umt = forActor.traits.Get<Mobile>().GetMovementType();
 			return content[p.X,p.Y].type.GetSpeedModifier(umt);
 		}
 		
-		public float GetCost(int2 p,UnitMovementType umt)
+		public float GetCost(int2 p, Actor forActor)
 		{
 			if (content[p.X,p.Y].type == null)
 				return 1.0f;
+			
+			var	umt = forActor.traits.Get<Mobile>().GetMovementType();
 			return content[p.X,p.Y].type.GetCost(umt);
 		}
 		
diff --git a/OpenRA.Mods.Cnc/MobileAir.cs b/OpenRA.Mods.Cnc/MobileAir.cs
new file mode 100644
index 0000000000..dbd2e0f5dc
--- /dev/null
+++ b/OpenRA.Mods.Cnc/MobileAir.cs
@@ -0,0 +1,100 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007,2009,2010 Chris Forbes, Robert Pepperell, Matthew Bowra-Dean, Paul Chote, Alli Witheford.
+ * This file is part of OpenRA.
+ * 
+ *  OpenRA is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ * 
+ *  OpenRA is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ * 
+ *  You should have received a copy of the GNU General Public License
+ *  along with OpenRA.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#endregion
+
+using System.Collections.Generic;
+using System.Linq;
+using OpenRA.GameRules;
+using OpenRA.Traits;
+
+namespace OpenRA.Traits
+{
+	public class MobileAirInfo : MobileInfo
+	{
+		public readonly int CruiseAltitude = 20;
+		public readonly float InstabilityMagnitude = 2.0f;
+		public readonly int InstabilityTicks = 5;	
+		public readonly bool LandWhenIdle = true;
+		
+		public override object Create(ActorInitializer init) { return new MobileAir(init, this); }
+	}
+	
+	public class MobileAir : Mobile, ITick, IOccupyAir
+	{
+		MobileAirInfo Info;
+		public MobileAir (ActorInitializer init, MobileAirInfo info)
+			: base(init)
+		{
+			Info = info;
+		}
+
+		public override void AddInfluence()
+		{
+			self.World.WorldActor.traits.Get<AircraftInfluence>().Add( self, this );
+		}
+		
+		public override void RemoveInfluence()
+		{
+			self.World.WorldActor.traits.Get<AircraftInfluence>().Remove( self, this );
+		}
+		
+		public override bool CanEnterCell(int2 p, Actor ignoreBuilding, bool checkTransientActors)
+		{
+			if (!checkTransientActors)
+				return true;
+			
+			return self.World.WorldActor.traits.Get<AircraftInfluence>().GetUnitsAt(p).Count() == 0;
+		}
+				
+		public override IEnumerable<int2> OccupiedCells()
+		{
+			// Todo: do the right thing when landed
+			return new int2[] {};
+		}
+		
+		public int2 TopLeft { get { return toCell; } }
+
+		public IEnumerable<int2> OccupiedAirCells()
+		{
+			return (fromCell == toCell)
+				? new[] { fromCell }
+				: CanEnterCell(toCell)
+					? new[] { toCell }
+					: new[] { fromCell, toCell };
+		}
+		
+		int offsetTicks = 0;
+		public void Tick(Actor self)
+		{
+			var unit = self.traits.Get<Unit>();
+			//if (unit.Altitude <= 0)
+			//	return;
+			
+			if (unit.Altitude < Info.CruiseAltitude)
+				unit.Altitude++;
+			
+			if (--offsetTicks <= 0)
+			{
+				self.CenterLocation += Info.InstabilityMagnitude * self.World.SharedRandom.Gauss2D(5);
+				unit.Altitude += (int)(Info.InstabilityMagnitude * self.World.SharedRandom.Gauss1D(5));
+				offsetTicks = Info.InstabilityTicks;
+			}
+		}
+	}
+}
\ No newline at end of file
diff --git a/OpenRA.Mods.RA/BridgeLayer.cs b/OpenRA.Mods.RA/BridgeLayer.cs
index 3a6823329f..e0ab1e91dc 100644
--- a/OpenRA.Mods.RA/BridgeLayer.cs
+++ b/OpenRA.Mods.RA/BridgeLayer.cs
@@ -90,14 +90,17 @@ namespace OpenRA.Mods.RA
 			}
 		}
 		
-		public float GetCost(int2 p, UnitMovementType umt)
+		public float GetCost(int2 p, Actor forActor)
 		{
+			var	umt = forActor.traits.Get<Mobile>().GetMovementType();
 			if (customTerrain[p.X, p.Y] != null)
 				return customTerrain[p.X,p.Y].GetCost(p,umt);
 			return 1f;
 		}
-		public float GetSpeedModifier(int2 p, UnitMovementType umt)
+		
+		public float GetSpeedModifier(int2 p, Actor forActor)
 		{
+			var	umt = forActor.traits.Get<Mobile>().GetMovementType();
 			if (customTerrain[p.X, p.Y] != null)
 				return customTerrain[p.X,p.Y].GetSpeedModifier(p,umt);
 			return 1f;
diff --git a/mods/cnc/structures.yaml b/mods/cnc/structures.yaml
index 95388f8e8c..b28fef0d1e 100644
--- a/mods/cnc/structures.yaml
+++ b/mods/cnc/structures.yaml
@@ -519,6 +519,8 @@ SAM:
 	AttackTurreted:
 		PrimaryWeapon: Nike
 	AutoTarget:
+	AntiAir:
+		Badness: 10
 	-RenderBuilding:
 	RenderRangeCircle:
 	
diff --git a/mods/cnc/system.yaml b/mods/cnc/system.yaml
index 68c9473bcd..52904f4d2e 100644
--- a/mods/cnc/system.yaml
+++ b/mods/cnc/system.yaml
@@ -48,6 +48,7 @@ World:
 	BuildingInfluence:
 	UnitInfluence:
 	AircraftInfluence:
+	HazardLayer:
 #	BridgeLoadHook:
 	Theater@DESERT:
 		Name:Desert
diff --git a/mods/cnc/vehicles.yaml b/mods/cnc/vehicles.yaml
index 931e42fa05..070273e3a5 100644
--- a/mods/cnc/vehicles.yaml
+++ b/mods/cnc/vehicles.yaml
@@ -407,6 +407,7 @@ TRAN:
 		Sight: 8
 		Speed: 15
 #	Helicopter:
+	AvoidsAA:
 	MobileAir:
 		MovementType: Fly
 	RenderUnitRotor: