too tall or too heavy and you won't get out in time
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Not quite correct, If your seated height is too tall, then your head will be above the horns (canopy breakers) on the seat, which means your head hits the canopy of the PC9 before the seat. Not a good thing. The over weight is due to the seat mounting ponts on the floor of the PC9. For other aircraft in the RAAF inventory, these limits may not matter so much, but as the Training aircraft for RAAF and Navy are PC9, then those limits apply for selection.