A plane is a mental symbol and is often used in dreams to represent a plan (which is of course involves mental activity). It can also suggest a highly intelligent way of thinking and also a spiritual one. As long as a plane flies, the plan appears to be workable. A crashing plane would imply a plan that is failing or has a failing in it. As your plane is going down, you appear to be perceiving a failing in a plan being used.

Your position in the plane (I was in the back) tends to suggest that you cannot see yourself functioning in an upfront manner – not using an honest or legitimate way of thinking where a plan is involved. Being near the bathroom would suggest that this impression associates with a point of view in which some feelings of guilt may be involved. When the idea that the plan has a failing in it sets in, (when the plane started going down )you may need to secure yourself in a position or attitude of resting at ease. (had to buckle my seat belt)

The two downward plunges would suggest that the idea of a failing existing in a plan is actually a negative idea. This seems confirmed here since the plane does not actually crash but merely lands roughly. The fact that the plane does land implies there are grounds of reasoning to justify the plan. {Note the Quantas jet experience serves to provide a reality factor to the idea that while there may be some glitches in a plan it is not a complete failure. It may simply be a bit difficult to bring a plan to a completion and be a bit hard on you emotionally.]