Create the flame as a bitmapped strip that varies from one end to another. Either make it seamless via Photoshop > Image > Offset and smooth over the mismatched edges, or make it long enough to cover the required time. If creating a seamless image, set start and end points to create a seamless loop. Place it in a new timeline and create a mask object. Set the strip to scroll to provide the illusion of flickering.
In the main timeline align the object and set a timeline action to play the flame timeline. To create the change when landing, use the main timeline to adjust the width of the mask and the flame.