Levels at a holding fix or visual holding location shall as far as practicable be assigned in a manner that will facilitate clearing each aircraft to approach in its proper priority. Normally, the first aircraft to arrive over a holding fix or visual holding location should be at the lowest level, with following aircraft at successively higher levels. When the level below is vacant, the ATCO will re-clear the pilot to the lower level. The pilot will acknowledge the clearance and immediately commence descent.

In a holding pattern, aircraft are ‘stacked’ up, one on top of another with the necessary vertical separation applied (1000 ft).

• Considering the above, we can conclude that aircraft A arriving first at the holding fix, will occupy the lowest available level. Aircraft B will then be stacked up, 1000 ft above aircraft A => FL170.