At my home airport a Precision Obstacle Free Zone (POFZ) was recently created at the arrival end of runway 27 right. The deadline for POFZ markings to be applied to all U.S airports was January 1, 2007. My understanding was that airports that didn't meet this deadline had to raise the ILS approach minima for any affected runways.
The POFZ area is demarcated using the same surface markings used for an ILS critical area: An area that is kept free of vehicles and aircraft to prevent interference with the broadcast signals from the localizer and glideslope. The motivation for the POFZ is actually to keep obstructions from penetrating the TERPS 34:1 obstruction plane that leads to the runway threshold. Depending on the runway and taxiway layout, the holding area may be much further from the entrance to the runway than a conventional ILS critical area though the surface markings are the same.
It's fascinating to see how many pilots react to the ILS critical area surface marking. Many pilots and instructors avoid taxiing over those markings, even in VFR conditions. These pilots don't seem to understand that the ground controller is required tell you when you shouldn't enter the area.
A POFZ is only protected when an aircraft is flying the ILS approach, is within 2 miles of the runway threshold, and the ceiling is being reported below 250 feet and/or the visibility is less that 3/4 statute miles or the RVR is less that 4000 feet.
An ILS critical area is protected when an aircraft is at or inside the Final Approach Fix, the ceiling is being reported as less than 800 feet and/or the visibility is less than 2 statute miles.
A POFZ is considered clear even if the wing of an aircraft holding on the taxiway penetrates the POFZ, but the tail or fuselage must not penetrate the POFZ.
If the POFZ is not clear, the minimum height above touchdown is 250 feet and the minimum visibility is 3/4 statute miles.
At my airport, crossing into the PFOZ is necessary for light aircraft to get into the best position to do their engine run-up before takeoff. Since there is no officially designated run-up area for runway 27R, turning around to do your pre-takeoff checks before the ILS critical area surface markings actually puts you in an area where the jet blast from a large aircraft taxiing on a perpendicular taxiway could really rock your world.
Jet bast encounters are not at all obvious to most pilots until they experience being in the wrong place at the wrong time. Once that has happened, it becomes easier to think ahead. And remember that if the conditions are VFR, you should be able to cross into an ILS critical area. If you have any doubts about entering an ILS critical area (or POFZ), you can always ask the ground controller for guidance.
