PA-31 Navajo/Chieftan/Mojave/T1040
Air Traffic Control Fully Certified
Other / Unknown; Situational Awareness
ATC Issue All Types
General None Reported / Taken
Procedure; ATC Equipment / Nav Facility / Buildings
The problem is that when aircraft are transiting through Sector 34 at or near the MIA we are receiving erroneous MSAW alerts. The closest and only logical altimeter setting to use is the The Dalles (DLS) altimeter which is not in the NAS equation for altimeter reporting stations. Over the DLS/LTJ area the only altimeter reporting stations used by NAS switch directly from the (PDX) altimeter straight to the (PDT) altimeter completely ignoring the (DLS) altimeter setting. An interesting fact to note is that the The Dalles (DLS) altimeter is actually the only weather reporting station that is within the lateral boundaries of Sector 34! PDX and PDT stations are both outside!!! At times such as today; the amount of difference between the The Dalles (DLS) altimeter and the (PDX)/(PDT) altimeters (which are the next closest altimeter reporting stations) is causing erroneous MSAW alerts for aircraft at or near the MIA as well as 'altitudes off by more than 300 FT' occurrences. The Cascade mountain range between PDX and DLS; the Columbia Gorge which connects the two geographical regions and high/low pressure differences between the two areas are mostly responsible for the difference in altimeters. Today the difference in altimeters was: PDX ALSTG 1553 29.54; DLS ALSTG 1600 29.70; a PA31 was at the MIA of FL070 on the (DLS) altimeter and was showing FL067 triggering a subsequent alert of 'MSAW 070'. For all practical purposes; it is not an unsafe situation and the MSAW is just a false alert because as long as the (DLS) altimeter is updating regularly and the pilot has it dialed in terrain separation is insured. When controllers issue an altimeter other than the (DLS) just to correct an altitude read out that is high or low; now that can put the aircraft closer to terrain or possibly other aircraft depending on which altimeter setting each aircraft is operating on. Some controllers have purposely issued altimeters other than the (DLS) just to prevent annoying MSAW alerts and displays of 'altitudes off by more than 300 FT' which trigger the subsequent query to the pilot about his altimeter setting and altitude. I know the problem is the The Dalles (DLS) altimeter is not in the equation; but newer controllers; who don't understand how the sort boxes are set up and how the NAS system works; end up just telling the pilots their Mode C is off by more than 300 FT and to 'stop altitude squawk'. The poor pilots think they have a Mode C problem requiring mechanical work when that is clearly not the case. Recommendation; include The Dalles (DLS) altimeter reporting station in the appropriate NAS sort boxes for the DLS/LTJ area to prevent unnecessary and inappropriate MSAW alerts and altitudes 'off by more than 300 FT' occurrences.
ZSE Controller voiced concern regarding false MSAW alarms and erroneous Mode-C altitude information reportedly due to lack of local altimeter settings.