KAP-140:

The ALT-ARM knob should only be arming the altitude interception when 
the AP is activated. When rotating the AP alt-selector knob with 
disabled AP, just the altitude alarm beep is configured.

From Benedikt Hallinger

Ticket-Id: https://sourceforge.net/p/flightgear/codetickets/2489/

Aircraft/Generic/kap140.nas

@@ -1327,6 +1327,12 @@ var armButton = func {
   #print("arm button");
 #  Disable button if too little power
   if (getprop(power) < minVoltageLimit) { return; }
+  
+  # Do nothing if the AP is not turned on
+  if (lockPitchMode.getValue() == pitchModes["OFF"]) {
+      lockPitchMode.setIntValue(pitchArmModes["OFF"]);
+      return;
+  }
 
   var pitchArm = lockPitchArm.getValue();