Airplane smooth acceleration problem

Hello! I'm new to Unity. I make a airplane, using a rigid body, the script is working correctly. But I cannot do airplane is smoothly increased from the current speed to the maximum and achieve the same slowing down (time is now or instant acceleration or deceleration). I tried to do through Rigidbody.velocity, but it does not work. I beg you with a code or advice on how to achieve a gradual acceleration and deceleration.

Thank you in advance!

Here is my script:

 var Speed = 60;
 var boostSpeed : float = 10;
 var maxSpeed = 100;
 private var Amount : Vector3 = Vector3 (10, 0, 0);    
 
 function FixedUpdate () {
 
 //Roll
 if (Input.GetKey(KeyCode.LeftArrow)) {
 rigidbody.AddRelativeTorque(Vector3.forward * 7 + Amount * Time.deltaTime);
 }
 else if (Input.GetKey(KeyCode.RightArrow))
 rigidbody.AddRelativeTorque(Vector3.back * 7 + Amount * Time.deltaTime);
 
 //Forward current speed
 transform.Translate(Vector3.forward * Time.deltaTime * Speed);
 
 //Boost-Brake <--- PROBLEM
 if (Speed < maxSpeed && Input.GetKey(KeyCode.W)) {
 rigidbody.AddRelativeForce(Vector3.forward * 70 + Amount * Time.deltaTime);
 }
 else if (Speed <= maxSpeed && Input.GetKey(KeyCode.S))
 rigidbody.AddRelativeForce(Vector3.forward * -20 + Amount * Time.deltaTime);
 
 //Yaw
 if (Input.GetKey(KeyCode.D)) {
 rigidbody.AddRelativeTorque(Vector3.up * 3 + Amount * Time.deltaTime);
 }
 else if (Input.GetKey(KeyCode.A))
 rigidbody.AddRelativeTorque(Vector3.down * 3 + Amount * Time.deltaTime);
 
 //Pitch
 if (Input.GetKey(KeyCode.UpArrow)) {
 rigidbody.AddRelativeTorque(Vector3.right * 5 + Amount * Time.deltaTime);
 }
 else if (Input.GetKey(KeyCode.DownArrow))
 rigidbody.AddRelativeTorque(Vector3.left * 5 + Amount * Time.deltaTime);
 }