Easy to fly helicopter script

This is the script

using UnityEngine; using System.Collections;

public class csHellicopter : MonoBehaviour {

 public KeyCode UpKey;
 public KeyCode DownKey;
 public KeyCode FrontSpin;
 public KeyCode BackWardSpin;
 public KeyCode LeftTurn;
 public KeyCode RightTurn;
 public KeyCode LeftSpin;
 public KeyCode RightSpin;
 public KeyCode EngineOnOffKey;

 public GameObject[] MainMotor; //Helicoptor Big Propeller. Use Y Rotate Value.
 public GameObject[] SubMotor; //Helicoptor Small Propeller. User X RotateValue.
 public AudioSource _AudioSource; //Helicoptor Engine Sound.

 public float MainMotorRotateVelocity;
 public float SubMotorRotateVelocity;

 float UpDownVelocity = 0.0f; 
 float MainMotorRotation = 0.0f;
 float SubMotorRotation = 0.0f;
 
 public float UpDownValue;
 float UpDown;
 float yUpDown;

 //Rotate Value
 float Pitch;
 float UpDownTurn;
 float yUpDownTrun;

 float Yaw;
 float LeftRightTurn;
 float yLeftRightTurn;

 float Roll;
 float LeftRightSpin;
 float yLeftRightSpin;

 bool EngineTurnOnOff = false;
 float EngineValue = 0.0f;

 public enum ControlState
 {
     Mouse,
     KeyBoard
 };

 public ControlState ControlType = new ControlState();

 void FixedUpdate()
 {
     if (EngineTurnOnOff)
     {
         GetComponent<Rigidbody>().AddRelativeForce(Vector3.up * UpDownValue * UpDownVelocity);

         if (transform.position.y > 100)
             GetComponent<Rigidbody>().AddRelativeForce(-Vector3.up * UpDownValue * UpDownVelocity);

         UpDown = KeyValue(DownKey,UpKey , UpDown, yUpDown, 1.5f, 0.01f);

         if (ControlType == ControlState.KeyBoard)
         {
             UpDownTurn = KeyValue(BackWardSpin, FrontSpin, UpDownTurn, yUpDownTrun, 1.5f, 0.1f);
             LeftRightTurn = KeyValue(LeftTurn, RightTurn, LeftRightTurn, yLeftRightTurn, 1.5f, 0.1f);
             LeftRightSpin = KeyValue(LeftSpin, RightSpin, LeftRightSpin, yLeftRightSpin, 1, 0.1f);
         }

         else if (ControlType == ControlState.Mouse)
         {
             //Pitch Value
             UpDownTurn = Input.GetAxis("Mouse Y");
             Pitch -= UpDownTurn * Time.fixedDeltaTime *0.01f;
             Pitch = Mathf.Clamp(Pitch, -1.2f, 1.2f);

             //Yaw Value
             LeftRightTurn = Input.GetAxis("Mouse X");
             Yaw += LeftRightTurn * Time.fixedDeltaTime *0.01f;

             LeftRightSpin = KeyValue(LeftSpin, RightSpin, LeftRightSpin, yLeftRightSpin, 1, 0.1f);
         }

         //Pitch Value
         Pitch += UpDownTurn * Time.fixedDeltaTime;
         Pitch = Mathf.Clamp(Pitch, -1.2f, 1.2f);

         //Yaw Value
         Yaw += LeftRightTurn * Time.fixedDeltaTime;

         //Roll Value
         Roll += -LeftRightSpin * Time.fixedDeltaTime;
         Roll = Mathf.Clamp(Roll, -1.2f, 1.2f);

         transform.rotation = Quaternion.Slerp(transform.rotation, Quaternion.EulerRotation(Pitch, Yaw, Roll), Time.fixedDeltaTime * 1.5f);
     }
 }
 
 void Update ()
 {
     SoundControl(); // Check Helicoptor Engine Sound
     MotorRotateControl(); // Check Motor Rotate State
     MotorVelocityContorl(); // Check Helicoptor UpDown State
     EngineControl(); // Check Engine Turn On/Off
 }

 float KeyValue(KeyCode A,KeyCode B, float Value , float yValue , float _float , float SmoothTime)
 {
     if(Input.GetKey(A))
         Value -= Time.deltaTime * _float;
     else if (Input.GetKey(B))
         Value += Time.deltaTime * _float;
     else
         Value = Mathf.SmoothDamp(Value, 0, ref yValue, SmoothTime);

     Value = Mathf.Clamp(Value, -1, 1);
     return Value;
 }

 void MotorVelocityContorl()
 {
     if (EngineTurnOnOff)
     {
         float Hovering = (Mathf.Abs(GetComponent<Rigidbody>().mass * Physics.gravity.y) / UpDownValue); //for maintain altitude.

         if (UpDown != 0.0f)
             UpDownVelocity += UpDown * 0.1f; //if Input Up/Down Axes, Increace UpDownVelocity for Increace altitude.
         else
             UpDownVelocity = Mathf.Lerp(UpDownVelocity, Hovering, Time.deltaTime); //if not input Up/Down Axes, Hovering.
     }
    CheckUpDownVelocity();
 }

 void CheckUpDownVelocity()
 {
     if (UpDownVelocity > 1.0f)
         UpDownVelocity = 1.0f;
     else if (UpDownVelocity < 0.1f)
         UpDownVelocity = 0.1f;
 }

 void MotorRotateControl()
 {
     if (MainMotor.Length > 0)
     {
         for (int i = 0; i < MainMotor.Length; i++)
             MainMotor[i].transform.Rotate(0, MainMotorRotation, 0);
     }

     if (SubMotor.Length > 0)
     {
         for (int i = 0; i < SubMotor.Length; i++)
             SubMotor[i].transform.Rotate(0, 0, SubMotorRotation);
     }

     MainMotorRotation = MainMotorRotateVelocity * EngineValue * Time.deltaTime;
     SubMotorRotation = SubMotorRotateVelocity * EngineValue * Time.deltaTime;
 }

 void EngineControl()
 {
     if (Input.GetKeyDown(EngineOnOffKey))
     {
         if (EngineTurnOnOff)
             EngineTurnOnOff = false;
         else
             EngineTurnOnOff = true;
     }

     if (EngineTurnOnOff == true)
     {
         if (EngineValue < 1)
             EngineValue += Time.deltaTime;
         if (EngineValue >= 1)
             EngineValue = 1;
     }

     else if (EngineTurnOnOff == false)
     {
         if (EngineValue > 0)
             EngineValue -= Time.deltaTime / 2;
         if (EngineValue <= 0)
             EngineValue = 0;
     }
 }

 void SoundControl()
 {
     if (EngineValue > 0)
     {
         _AudioSource.mute = false;
         _AudioSource.pitch = EngineValue;
     }
     else
         _AudioSource.mute = true;
 }

}

      if (EngineTurnOnOff)
      {   

         if (Input.GetKey(UpKey)) {
          GetComponent<Rigidbody>().AddRelativeForce(Vector3.up * UpDownValue * UpDownVelocity);
         }

               if (transform.position.y > 5000 )
              GetComponent<Rigidbody>().AddRelativeForce(-Vector3.up * UpDownValue * UpDownVelocity);
          UpDown = KeyValue(DownKey,UpKey , UpDown, yUpDown, 1.5f, 0.01f);

          if (ControlType == ControlState.KeyBoard)
          {
              UpDownTurn = KeyValue(BackWardSpin, FrontSpin, UpDownTurn, yUpDownTrun, 1.5f, 0.1f);
              LeftRightTurn = KeyValue(LeftTurn, RightTurn, LeftRightTurn, yLeftRightTurn, 1.5f, 0.1f);
              LeftRightSpin = KeyValue(LeftSpin, RightSpin, LeftRightSpin, yLeftRightSpin, 1, 0.1f);
          }