Robert Lyons wrote:

- Why would people be using two different altimeter settings anyway?

Because the regulations require it. In the States, all aircraft flying at or
above 18,000' ASL are required to set their altimeters to 29.92. Aircraft flying
below 18,000' are expected to set their altimeters to the local barometric
pressure.

- Why would this problem affect just FL180 and FL200? Why not FL160, 220,
and every other altitude?

It only affects flight levels at which the actual altitude of a plane using the
29.92 setting would be low enough to conflict with aircraft using the actual
pressure setting. In this case, a plane flying at 18,000' using 29.92 would be
at 16950 ft indicated with the actual altimeter setting. If another aircraft
using the local barometric pressure is flying at 17,000', there is a possibility
of collision. One pilot "knows" he's at 18,000' and the other pilot "knows" he's
at 17,000', but they're actually at nearly the same altitude. Since altimeters
can be slightly inaccurate, they could easily be at exactly the same altitude.

Since ATC is tasked with ensuring a certain amount of separation between IFR
aircraft, ATC will simply not assign 18,000' as a flight altitude.

FL 200 and higher don't present problems here. FL 210 would be an altitude of
19,950', and there will be no aircraft at that altitude using the local setting.

George Patterson
You can dress a hog in a tuxedo, but he still wants to roll in the mud.