How to add ease-out effect to SVG <animateMotion>

Question

I am animating a SVG with the help of SMIL. All works as expected in chrome except that I want to add an "ease-out" effect to the animatemotion tag. Right now the plane moves along the path in a linear fashion. What I want is that the animation has a slow stop at the end just like how CSS3 ease-out effect.

Here is the SVG code.

<!-- Generator: Adobe Illustrator 19.0.0, SVG Export Plug-In . SVG Version: 6.00 Build 0)  -->
<svg xmlns:xlink="http://www.w3.org/1999/xlink" version="1.1" id="Layer_1" xmlns="http://www.w3.org/2000/svg" x="0px"
 y="0px"
 viewBox="0 0 500 700">
<style type="text/css">
    .st0{fill:none;stroke:#000000;stroke-miterlimit:10;}
</style>
<animateMotion
        id="1"
        xlink:href="#plane"
        dur="2s"
        path="M50,601c10.4-18.7,22.7-44.6,32-77c11.1-38.8,12.7-69,14-97c2.4-51.8-4.1-75.4-8-87c-6.4-19.3-12.4-36.4-29-43
c-15.1-6-36.6-2.4-49,13c-7.9,9.8-12.9,25.1-8,38c12.4,32.5,79.1,28.2,82,28c31.9-2.4,55.4-19.1,82-37c17.2-11.6,30.3-22.9,39-31"
        fill="freeze"
/>

<g>
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    c3.9,0,7.8,0,11.7,0c0.1-0.2,0.3-0.4,0.4-0.6c0,0,0.1,0,0.1,0c0.1-0.3,0.2-0.6,0.3-0.8C116.3,590.6,115.5,588.1,113.2,586.8z"/>
    <g id="plane" transform="scale(0.5) translate(-263.9, -167.6)">
        <g>
            <animateTransform attributeName="transform"
                              attributeType="XML"
                              type="rotate"
                              from="0 343.9 167.6"
                              to="-360 343.9 167.6"
                              dur="2s"
                              calcMode="spline"
                              keySplines="0.6 0.3 0.4 0.335"
                              keyTimes="0;1"


            />
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        </g>
    </g>
    <path d="M56.6,331.6c-3.4,0.1-6.1-2.5-5.2-5.4c1.1-3.9,2.9-7.6,4.7-11.2c1.1-2,4-2.3,6-1.2c2.1,1.2,3,3.7,2.1,6
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</g>
<path class="st0" d="M104,601c10.4-18.7,22.7-44.6,32-77c11.1-38.8,12.7-69,14-97c2.4-51.8-4.1-75.4-8-87c-6.4-19.3-12.4-36.4-29-43
c-15.1-6-36.6-2.4-49,13c-7.9,9.8-12.9,25.1-8,38c12.4,32.5,79.1,28.2,82,28c31.9-2.4,55.4-19.1,82-37c17.2-11.6,30.3-22.9,39-31"/>

Answer 1

Most of the question author's application code draws a path for an airplane in the form of a bunch of sausages.

This code can be as simple as possible by applying stroke-dasharray andstroke-linecap = "round"

<svg xmlns:xlink="http://www.w3.org/1999/xlink" version="1.1" id="Layer_1" xmlns="http://www.w3.org/2000/svg" x="0px"
 y="0px"
 width="100%" height="100%" viewBox="0 0 500 700">
 
<g id="path_G" transform="translate(100 -200)">
 <path  stroke="skyblue" stroke-width="10" stroke-dasharray="15 25" stroke-linecap="round" fill="none" d="M50,601c10.4-18.7,22.7-44.6,32-77c11.1-38.8,12.7-69,14-97c2.4-51.8-4.1-75.4-8-87c-6.4-19.3-12.4-36.4-29-43
c-15.1-6-36.6-2.4-49,13c-7.9,9.8-12.9,25.1-8,38c12.4,32.5,79.1,28.2,82,28c31.9-2.4,55.4-19.1,82-37c17.2-11.6,30.3-22.9,39-31" />  

<path id="path_GL" stroke="skyblue" stroke-width="1"   fill="none" d="M50,601c10.4-18.7,22.7-44.6,32-77c11.1-38.8,12.7-69,14-97c2.4-51.8-4.1-75.4-8-87c-6.4-19.3-12.4-36.4-29-43
c-15.1-6-36.6-2.4-49,13c-7.9,9.8-12.9,25.1-8,38c12.4,32.5,79.1,28.2,82,28c31.9-2.4,55.4-19.1,82-37c17.2-11.6,30.3-22.9,39-31" />  
</g>
</svg>

Instead of two animations, animateMotion andanimateTransform, we’ll use the same animation

  <animate
          attributeName="startOffset"
          dur="16s"
          repeatCount="indefinite"
          values="0%;120%"
          calcMode="spline"
          keySplines=" 0.185,0.675,0,0.995"/>

To make CSS3 ease-out effect we use a bezier curve generator

<svg xmlns:xlink="http://www.w3.org/1999/xlink" version="1.1" id="Layer_1" xmlns="http://www.w3.org/2000/svg" x="0px"
 y="0px"
 width="50%" height="50%" viewBox="0 0 500 700">
 
<g id="path_G" transform="translate(100 -250)">
 <path  stroke="skyblue" stroke-width="10" stroke-dasharray="15 25" stroke-linecap="round" fill="none" d="M50,601c10.4-18.7,22.7-44.6,32-77c11.1-38.8,12.7-69,14-97c2.4-51.8-4.1-75.4-8-87c-6.4-19.3-12.4-36.4-29-43
c-15.1-6-36.6-2.4-49,13c-7.9,9.8-12.9,25.1-8,38c12.4,32.5,79.1,28.2,82,28c31.9-2.4,55.4-19.1,82-37c17.2-11.6,30.3-22.9,39-31" />  

<path id="path_GL" stroke="skyblue" stroke-width="1"   fill="none" d="M50,601c10.4-18.7,22.7-44.6,32-77c11.1-38.8,12.7-69,14-97c2.4-51.8-4.1-75.4-8-87c-6.4-19.3-12.4-36.4-29-43
c-15.1-6-36.6-2.4-49,13c-7.9,9.8-12.9,25.1-8,38c12.4,32.5,79.1,28.2,82,28c31.9-2.4,55.4-19.1,82-37c17.2-11.6,30.3-22.9,39-31" />  

<text font-size="64"  font-family="Times New Roman" fill="dodgerblue" >
  <textPath id="result"    xlink:href="#path_GL">
    <tspan dx="0" > &#9992; </tspan> 
        <animate
		  attributeName="startOffset"
		  dur="20s"
		  repeatCount="indefinite"
		  values="0%;120%"
		  calcMode="spline"
		  keySplines=" 0.185,0.675,0,0.995"/> 
  </textPath>
</text>		
</g>
     
</svg>