IFR quiz question?

Question

Just before engine start you are on the radio with very helpful Clearance Delivery requesting a quick IFR flight plan from Oklahoma City (KOKC) to Lubbock (KLBB) via GPS direct (Heading 234*), requesting 18,000, a KingAir 200 /G.

KOKC 700ovc 2 rain Calm 52 48 30.01

KHBR 100ovc 1/2 fog Calm 50 50 29.91

KLBB 25,000bkn 10 Calm 48 55 29.90

Clearance Delivery says to pick a different altitude. Why?

Answer 1

Since the barometric pressure at both Hobart and Lubbock is

less than 29.92 (inches of mercury) FL 180 is not usable.

You need a pressure of at least 29.92 for FL180 to have the

minimum required vertical separation from aircraft at 17,000

feet.  You need to request either 16,000 feet or FL 200 to be

correct for the direction of flight.

Answer 2

with the pressure being between 29.91 and 29.42 for a part of your leg, the lowest avaible flight level would be FL185- which they won't give you- so FL190 for all normal purposes... FL180 is not active.