Make sure to call the method used to 'put' an int 'extra' in the Intent.

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class LaunchActivity extends Activity {

  public Button mLaunchButton;
  public int mFuelLevel;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_launch);

    // mFuelLevel is set elsewhere.
    // Code ommitted for brevity!

    mLaunchButton = (Button)findViewById(R.id.launchButton);
    mLaunchButton.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        // Add your code in here!
        Intent intent = new Intent(LaunchActivity.this, FlightActivity.class);
        startActivity(intent);
      }
    });
  }

  private void startActivity(String intent){
    Intent fuel = new Intent(this, FlightActivity.class);
    fuel.putExtra("FUEL_LEVEL", 100);
    startActivity(fuel);
  }
}