ZSE controller described an apparent false MSAW alert event noting the eram software logic operates ...

Narrative:

Aircraft X was flying from portland approach's airspace to the sle airport. They were filed over ubg..ocaye..sle. Ocaye is a fix on an RNAV approach for the airport; but aircraft X wanted to just do a visual approach. When he was about 10 miles north; I cleared him for the visual. When he descended below 030 the MSAW alert started flashing. I called the eram smes over to observe and write up the situation. I pulled up a route line to see what it showed; thinking that eram might be basing the MSAW alert off of the aircraft's trajectory as opposed to the aircraft's location. The route was showing a position roughly at the ubg VOR; over 30ish miles north; then down over ocaye and finally into sle. At the sme's suggestion I re-routed the aircraft direct to the sle airport and the MSAW immediately quit flashing. It appears that the MSAW in eram is not based on the aircraft's actual location; but rather on where eram thinks the aircraft might be. The obvious problem with this is that I don't care if the aircraft's trajectory is going to hit a mountain; I care if the aircraft itself is going to hit a mountain. False alerts like this one are not quite as bad as no alert at all; but obviously false alerts have a safety factor themselves. To my recollection; host doesn't work like this. Eram needs to either update the trajectory to match what the aircraft is doing; presumably not issuing a false MSAW alert; or needs to base MSAW strictly on the aircraft itself; not on the trajectory.

Original NASA ASRS Text

Title: ZSE Controller described an apparent false MSAW Alert event noting the ERAM software logic operates differently from the Host; surmising the ERAM MSAW is not based upon an aircraft's position but rather its trajectory.

Narrative: Aircraft X was flying from Portland Approach's airspace to the SLE airport. They were filed over UBG..OCAYE..SLE. OCAYE is a fix on an RNAV approach for the airport; but Aircraft X wanted to just do a visual approach. When he was about 10 miles North; I cleared him for the visual. When he descended below 030 the MSAW Alert started flashing. I called the ERAM SMEs over to observe and write up the situation. I pulled up a route line to see what it showed; thinking that ERAM might be basing the MSAW alert off of the aircraft's trajectory as opposed to the aircraft's location. The route was showing a position roughly at the UBG VOR; over 30ish miles North; then down over OCAYE and finally into SLE. At the SME's suggestion I re-routed the aircraft direct to the SLE airport and the MSAW immediately quit flashing. It appears that the MSAW in ERAM is not based on the aircraft's actual location; but rather on where ERAM thinks the aircraft might be. The obvious problem with this is that I don't care if the aircraft's trajectory is going to hit a mountain; I care if the aircraft ITSELF is going to hit a mountain. False alerts like this one are not quite as bad as no alert at all; but obviously false alerts have a safety factor themselves. To my recollection; HOST doesn't work like this. ERAM needs to either update the trajectory to match what the aircraft is doing; presumably not issuing a false MSAW alert; or needs to base MSAW strictly on the aircraft itself; not on the trajectory.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	965462
Time
Date	201108
Local Time Of Day	0001-0600
Place
Locale Reference	ZSE.ARTCC
State Reference	WA
Aircraft 1
Make Model Name	PA-28 Cherokee/Archer/Dakota/Pillan/Warrior
Operating Under FAR Part	Part 91
Flight Phase	Final Approach
Route In Use	Visual Approach
Person 1
Function	Enroute
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types

ZSE controller described an apparent false MSAW alert event noting the eram software logic operates differently from the host; surmising the eram MSAW is not based upon an aircraft's position but rather its trajectory.