diff --git a/boot/aircraft.ks b/boot/aircraft.ks
new file mode 100644
index 0000000..6a04de6
--- /dev/null
+++ b/boot/aircraft.ks
@@ -0,0 +1,10 @@
+RunOncePath("0:/lib/boot").
+
+parameter debug is false.
+
+Bootstrap(
+"/boot/aircraft",
+"/prog/aircraft",
+List(), // no additional program files
+debug
+).
diff --git a/lib/boot.ks b/lib/boot.ks
index cb0be86..3211f7e 100644
--- a/lib/boot.ks
+++ b/lib/boot.ks
@@ -78,7 +78,9 @@ function addLibs {
     if line:Contains("RunOncePath") {
       local start is line:Find(char(34)).
       local end is line:FindLast(char(34)).
-      libs:Add(line:Substring(start + 1, end - start - 1)).
+      local libFile is line:Substring(start + 1, end - start - 1).
+      libs:Add(libFile).
+      addLibs(libs, libFile).
     }
   }
 }
diff --git a/lib/flap_control.ks b/lib/flap_control.ks
new file mode 100644
index 0000000..cffcfc2
--- /dev/null
+++ b/lib/flap_control.ks
@@ -0,0 +1,32 @@
+
+function SetFlapAngle {
+  parameter angle.
+
+  local flaps is getFlaps().
+  if flaps:LENGTH = 0 { return. }
+  for flap in flaps {
+    flap:GetModule("ModuleControlSurface"):SetField("deploy angle", angle).
+    print("DEBUG: Set flap angle to " + angle).
+  }
+}
+
+function SetFlaps {
+  parameter extend is true.
+  local flaps is getFlaps().
+  if flaps:LENGTH = 0 { return. }
+  for flap in flaps {
+    flap:GetModule("ModuleControlSurface"):SetField("deploy", extend).
+    print("DEBUG: Set flap deployment to " + extend).
+  }
+}
+
+function getFlaps {
+  local flaps is SHIP:PartsTagged("flap").
+  for flap in flaps {
+    if flap:MODULES:Find("ModuleControlSurface") = -1 {
+      print "WARNING: Flap is not a control surface. Aborting operation.".
+      return List().
+    }
+  }
+  return flaps.
+}
diff --git a/lib/launch_rocket.ks b/lib/launch_rocket.ks
index 7842828..8dbf6b2 100644
--- a/lib/launch_rocket.ks
+++ b/lib/launch_rocket.ks
@@ -1,5 +1,6 @@
 RunOncePath("lib/throttle").
 RunOncePath("lib/navigation").
+RunOncePath("lib/navball").
 
 // Calculate the direction to lock during ascent.
 function getClampedDir {
@@ -8,18 +9,12 @@ function getClampedDir {
   // face just beneath prograde, but hold a solid eastern heading and don't
   // rotate the ship
   local newHeading is lookdirup(SHIP:SRFPROGRADE:FOREVECTOR, heading(90, 0, 270):TOPVECTOR).
-  if getPitch(newHeading:FOREVECTOR) < minPitch {
+  if GetPitch(newHeading:FOREVECTOR) < minPitch {
     set newHeading to heading(90, minPitch, 270).
   }
   return newHeading.
 }
 
-// Returns the navball pitch of a given vector.
-function getPitch {
-  parameter v is SHIP:FACING:FOREVECTOR.
-  return 90 - vectorangle(SHIP:UP:FOREVECTOR, v).
-}
-
 // Given a target (end) pitch, a target duration, and a start time,
 // returns the correct current heading.
 function pitchProgram {
diff --git a/lib/navball.ks b/lib/navball.ks
new file mode 100644
index 0000000..7c53383
--- /dev/null
+++ b/lib/navball.ks
@@ -0,0 +1,9 @@
+// Returns the navball pitch of a given vector.
+function GetPitch {
+  parameter v is SHIP:FACING:FOREVECTOR.
+  return 90 - vectorangle(SHIP:UP:FOREVECTOR, v).
+}
+
+function GetHeading {
+  return mod(360 - LatLng(90,0):BEARING, 360).
+}
diff --git a/lib/stabilize_aircraft.ks b/lib/stabilize_aircraft.ks
new file mode 100644
index 0000000..b7dcee2
--- /dev/null
+++ b/lib/stabilize_aircraft.ks
@@ -0,0 +1,34 @@
+RunOncePath("lib/navball").
+
+// Fly level toward the current compass heading.
+function HoldHorizon {
+  parameter Kp is 0.02.
+  parameter Ki is 0.04.
+  parameter Kd is 0.0066.
+
+  SAS off.
+
+  local pitchPID is PidLoop(Kp, Ki, Kd).
+
+  // local rollPID is PidLoop(Kp, Ki, Kd).
+  // lock rollVec to VXCL(SHIP:FACING:FOREVECTOR, SHIP:FACING:TOPVECTOR).
+  // lock upVec to VXCL(SHIP:FACING:FOREVECTOR, SHIP:UP:FOREVECTOR).
+
+  local h is GetHeading().
+
+  local p is 0.
+  lock p to pitchPID:Update(TIME:SECONDS, SHIP:VERTICALSPEED).
+  lock STEERING to Heading(h, p, 0).
+
+  // until done {
+  //   set SHIP:CONTROL:PITCH to pitchPID:Update(TIME:SECONDS, GetPitch(SHIP:SRFPROGRADE:FOREVECTOR)).
+  //   set SHIP:CONTROL:ROLL to rollPID:Update(TIME:SECONDS, VAng(rollVec, upVec)).
+  // }
+
+  wait until done.
+  set done to false.
+  unlock STEERING.
+  SAS on.
+  // set SHIP:CONTROL:PITCH to 0.
+  // set SHIP:CONTROL:ROLL to 0.
+}
diff --git a/lib/ui.ks b/lib/ui.ks
index a472a52..604e322 100644
--- a/lib/ui.ks
+++ b/lib/ui.ks
@@ -53,3 +53,25 @@ function MakeMenu {
     set top:AddButton(execLabel):onClick to callback:Bind(optionList).
   }
 }
+
+function AddStockButtons {
+  parameter row.
+  parameter bootname.
+
+  local btn is row:AddButton("Term").
+  set btn:TOGGLE to true.
+  set btn:ONTOGGLE to {
+    parameter d.
+
+    if d {
+      CORE:PART:GETMODULE("kOSProcessor"):DOEVENT("Open Terminal").
+    } else {
+      CORE:PART:GETMODULE("kOSProcessor"):DOEVENT("Close Terminal").
+    }
+  }.
+  
+  MakeButton(row, "Update", {
+    switch to 0.
+    run reinstall(bootname).
+  }).
+}
diff --git a/prog/aircraft.ks b/prog/aircraft.ks
new file mode 100644
index 0000000..a47f659
--- /dev/null
+++ b/prog/aircraft.ks
@@ -0,0 +1,91 @@
+
+RunOncePath("lib/flap_control").
+RunOncePath("lib/stabilize_aircraft").
+RunOncePath("lib/ui").
+
+clearguis().
+
+global done is false.
+on AG9 {
+  set done to true.
+  return true.
+}
+
+function landingMode {
+  print "Setting flaps to landing mode.".
+  SetFlapAngle(20).
+  when SHIP:VELOCITY:SURFACE:MAG < 0.1 then {
+    takeoffMode().
+  }
+}
+
+function takeoffMode {
+  print "Setting flaps to takeoff mode.".
+  SetFlapAngle(10).
+  on GEAR {
+    if GEAR = true {
+      return true.
+    }
+    landingMode().
+  }
+}
+
+if SHIP:STATUS = "LANDED" or SHIP:STATUS = "PRELAUNCH" {
+  takeoffMode().
+} else {
+  landingMode().
+}
+
+// Top-level elements.
+local iface is gui(250, 300).
+set iface:X to 200.
+set iface:Y to 700.
+local top is iface:AddVLayout().
+local rows is List(MakeRow(top), MakeRow(top)).
+local stk is iface:AddStack().
+
+AddStockButtons(rows[0], "aircraft").
+
+// rows[0]:AddLabel("FLAPS").
+// // Todo: make angles configurable...
+// set rows[1]:AddButton("Takeoff"):onClick to {
+//   SetFlapAngle(10).
+// }.
+
+// set rows[1]:AddButton("Land"):onClick to {
+//   SetFlapAngle(20).
+// }.
+
+// set rows[2]:AddButton("Extend"):onClick to {
+//   SetFlaps(true).
+// }.
+
+// set rows[2]:AddButton("Retract"):onClick to {
+//   SetFlaps(true).
+// }.
+
+// rows[3]:AddLabel("AUTO").
+
+MakeMenu(
+  stk,
+  MakeButton(rows[1], "Level"),
+  List(
+    List("Kp", "SCALAR", "0.02"),
+    List("Ki", "SCALAR", "0.01"),
+    List("Kd", "SCALAR", "1")
+  ),
+  "Execute",
+  {
+    parameter options.
+    HoldHorizon(
+      options["Kp"]:TEXT:ToNumber(),
+      options["Ki"]:TEXT:ToNumber(),
+      options["Kd"]:TEXT:ToNumber()
+    ).
+  }
+).
+
+
+iface:show().
+
+wait until false.
diff --git a/prog/rocket.ks b/prog/rocket.ks
index fe006d8..4bbc917 100644
--- a/prog/rocket.ks
+++ b/prog/rocket.ks
@@ -25,17 +25,7 @@ local rows is list(MakeRow(top), MakeRow(top), MakeRow(top)).
 local stk is iface:AddStack().
 
 // Buttons and menus.
-local btn is rows[0]:AddButton("TERM").
-set btn:TOGGLE to true.
-set btn:ONTOGGLE to {
-  parameter d.
-
-  if d {
-    CORE:PART:GETMODULE("kOSProcessor"):DOEVENT("Open Terminal").
-  } else {
-    CORE:PART:GETMODULE("kOSProcessor"):DOEVENT("Close Terminal").
-  }
-}.
+AddStockButtons(rows[0], "rocket").
 
 MakeMenu(
   stk,