Speed updated from another script, but doesn't persist

I have two scripts, one the "PlaneController" which is attached to the airplane you control, the other a "TargetTrigger" which is attached to each segment of a level you fly through.

The PlaneController has a variable called speed which I want to be increased every time you fly through one of the triggers. The TargetTrigger script looks like this:

 function OnTriggerEnter(other : Collider) {
     if(other.gameObject.name == "Plane") {
         Debug.Log("Calling Level Generator");
         GameObject.Find("Generator").GetComponent(LevelGenerator).NewTunnelSection();
         
         if(other.gameObject.GetComponent(PlaneSwiftController) != null) {
             Debug.LogWarning("Speed up the plane from "+other.gameObject.GetComponent(PlaneSwiftController).speed+" to "+(other.gameObject.GetComponent(PlaneSwiftController).speed+50));
             other.gameObject.GetComponent(PlaneSwiftController).speed += 50;
             Debug.LogWarning("Speed is now "+other.gameObject.GetComponent(PlaneSwiftController).speed);
         }
     }
 }

The PlaneController only ever sets the speed in the declaration and only ever uses it to push the airplane forward. Literally these are the only two places it is used:

 public var speed : float = 200.0; // FIXED in swift mode (other values being kept in case we modify this)

 // unrelated code...

 function Update() {

     Debug.LogWarning("Speed: "+this.speed);

     // unrelated code...

     rigidbody.velocity = transform.forward * speed * Time.deltaTime;

     // unrelated code...
 }

The log output is where it gets curious: sometimes the speed still reads as 200, despite the trigger being hit and indicating the speed was indeed increased. Here is an example output:

Speed up the plane from 200 to 250

Speed is now 250

Speed: 250

Speed: 200

Speed: 250

Speed: 200

Speed: 250

Speed: 200

Speed: 250

Speed: 200

Speed up the plane from 250 to 300

Speed is now 300

Speed: 300

Speed: 200

Speed: 300

Speed: 200

Notice that it alternates and it does so with the original speed. I wonder if this is an issue of a second thread of the PlaneController being started in the TargetTrigger script, but I don't see how since each call would then have to create a new thread, and the log doesn't support that.

Any clue what's going on?

Thanks!