Negative Flight Levels

[Beagle-eye]

Hi all

To pick up on what Chevron says .... the field allows for a negative flight level so I am trying to determine what would cause a negative flight level.

As background .... we take the FL and correct it for local barometric pressure (QNH) to get a reasonable approximation of height. We then use this to direct other sensors at the aircraft.

Thanks

B-E