Skylane 182/RG Turbo Skylane/RG
Air Traffic Control Fully Certified
Other / Unknown
ATC Issue All Types
General None Reported / Taken
ATC Equipment / Nav Facility / Buildings; Airspace Structure; Procedure
A Cessna 182 was level at 140 on a flight plan from BCE to LAS via MMM.V394.LAS. There is an MIA polygon that is just on the ZLC46 ZLC44 ZLC boundary; RADAR sort box 727; which has an MIA of 125 with uncontrolled airspace within the polygon. The way that the polygon is labeled on the overhead charts as 125*148; meaning that the MIA is 125 and there is uncontrolled airspace. However on the scope when you bring up the MIAs the polygon simply shows 148. When the aircraft entered this polygon the MSAW was activated flashing 148; even though the actual polygon's MIA was 125. This is unsafe because the polygons displayed on the scope should be the lowest usable altitude. The polygons within ZLC airspace would show this particular polygon as 125*. This allows the Controller to both know the lowest possible altitude as well as to know that there is uncontrolled airspace within the polygon. The polygon that activated the MSAW on the 182 was in ZLA airspace. I don't understand why the display of the MIA polygon changes based on what Center's airspace it is in. I also called the ZLA07 Controller to confirm that they too were observing the MSAW alert and I was told they were and it was the result of the uncontrolled airspace. Thus; their MSAW alert is not set up correctly. MSAW is the Minimum Safe Altitude Warning; it is not to be used as some other alert such as uncontrolled airspace and in this particular case was a false alert; with the aircraft being level at 140 and the MIA being 125. All MIA polygons when displayed on the scope should show the lowest minimum altitude. This should be done the same way nationwide. The MSAW alert should only alert if the aircraft is below the MIA for that polygon; it should not be dependent on if the aircraft is below the uncontrolled airspace. The MSAW is the last effort to keep aircraft from flying into terrain. The [controllers] for ZLA should also receive refresher training as to what the MSAW actual means as well as what uncontrolled airspace actually is.
ZLC Controller voiced concern regarding variances in the MIA's between ZLC and ZLA; the reporter suggesting a national standard be adopted to prevent this type of confusion.