A320
Flight Crew Last 90 Days 210; Flight Crew Total 16000; Flight Crew Type 2800
Confusion; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Took Evasive Action; Flight Crew FLC Overrode Automation
Chart Or Publication; Environment - Non Weather Related; Procedure; Aircraft
There is a potential terrain conflict on the HHOOD2 RNAV Arrival to PDX. The Airbus FMGC; when executing the arrival; descends the aircraft below the published MEA. The segment in question is from HHOOD to BLRUN when landing on 28L/R. There is no charted crossing restriction at BLRUN; the next charted restriction is at SHAFR (at 240 knots and at or above 6;700). There is; however; the listed MEA of 7;200 (in small print) between HHOOD and BLRUN which is very easily missed. The Airbus FMCG saw the path beyond SHAFR as too steep for the aircraft to accomplish under normal circumstances; so it attempts to get the aircraft as low and slow as possible to get ready for the approach to 28L/R. When looking at the FMGC before the arrival; I saw that it ignored the 240 knots requirement at SHAFR (common for the Airbus); descending and slowing the aircraft to be at SHAFR exactly at 6;700; but at 223 knots. To do this; it descends at 250 knots from 10;000 to 6;700; and then starts slowing. Unfortunately; this would have descended the aircraft below the 7;200 feet MEA prior to BLRUN; the FMCG shows the aircraft at 7;090 at BLRUN; which puts it 110 below the published MEA. If we had followed SOP and set the lowest published restriction on the arrival when cleared to 'descend via'; we would have descended below the MEA and caused a terrain conflict by follow the VNAV path. We prevented this potential by manually inserting an at/above 7;200 feet at BLRUN.  It also became a non-factor this time because PDX Approach cleared us direct to OMNSI on initial contact passing HHOOD and stopped our descent at 7;200.  I'm guessing that this hasn't come up before because this is the normal PDX approach clearance on initial contact; then stepping you down to avoid terrain as you approach the airport. However; the safety potential is there; and needs to be addressed.
An A320 pilot noted the HHOOD 2 RNAV BLRUN waypoint has no crossing restriction but a 7;200 feet MEA. The FMGC will attempt to descend below the MEA in order to cross the SHAFR constraint at 6;700 feet so as to make subsequent descent constraints.