How can I accelerate my Object to a maximum speed

Currently I'm working on a project , in which you can fly an aircraft. I have got the basic movement of it set up , but it instantly goes to maximum Speed if I press "W". So how can I slowly accelerate it to maximum Speed and decelerate it if I no longer press "W". Thank you very much in advance for your answer.

Here is the script I already made:

using UnityEngine;

using System.Collections;

 public class RealisticPlane : MonoBehaviour {
     public float maxSpeed = 10;
     public float maxLift = 5;
     // Use this for initialization
     void Start () 
     {
     
     }
     
     // Update is called once per frame
     void Update () 
     {
         transform.Rotate ( Input.GetAxis ("Vertical") , Input.GetAxis("RollAxis") , -Input.GetAxis ("Horizontal"));
         if (Input.GetKey (KeyCode.W))
         {
               transform.Translate(Vector3.down * Time.deltaTime * maxSpeed);
         }
 
         float heightPosition = Terrain.activeTerrain.SampleHeight (transform.position);
         if (heightPosition > transform.position.y)
         {
             transform.position = new Vector3 (transform.position.x,
                                               heightPosition,
                                               transform.position.z);
         }
     }
 }