Date: 201505
Local time of day: 1201-1800
Location: AUS.Airport, TX
Flight conditions: VMC
Aircraft: EMB ERJ 170/175 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach; Initial Approach
Airspace: Class C AUS
ATC / advisory: Tower AUS
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Human-Machine Interface; Situational Awareness; Workload
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action; Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Often times online we make decisions about the safe operation of our aircraft with immediate feedback that the decision was correct.  These decisions are obviously correct because there were no adverse effects to the operation; so we immediately discount them to conserve brain power and focus on the next potential threat.  It's only after some time spent in our own beds at home and a home cooked meal that we start to reflect on these decisions.  After 10;000 hours in the left seat of the E170 I realize that anytime I am saying the words '...I don't know..' there are only three possibilities.  The first possibility is that I have had my head up my rear for years and should have known better; it's something that's not known; or something has changed.  In any case it's time for me to do some research and find out so that I'm not say 'I don't know' again the next time.  This is one of those decisions I am now reflecting on and this report is more of a request for someone to point out my obvious misunderstandings and not so much a concern over possible violation of SOP/FARs.  This is a question about receiving a GPWS displayed (no aural warnings) on the 17R LOC approach to AUS; glideslope (GS) out of service; in VFR conditions. Approaching AUS the NOTAMS and ATIS stated that the 17R GS was out of service.  It is a VFR day but I decide it's a great time to practice a non-precision approach.  I input and briefed the LOC 17R approach and after being cleared for the visual at 4;000 outside of CHADE I commence the approach; dialing 3;000 to BROMS; 2;100 to HOMAL; and 900 for the minimums.  However; as I try to maintain a mostly consistent Flight Path Angle (FPA) 3.0 degree descent I tried my best to ensure making the 1;040 at WUPGA before continuing down to 900 feet; which I believe I did.  As accurate as memory serves; it was only inside of WUPGA after leaving 900 feet that the PFD started flashing a GPWS; there were no aural warnings.  I was visual throughout the approach and landing.  I scanned the VASI which did show I was slightly low but in no danger of terrain; as I remember I just scanned more vigilantly; corrected slightly; and completed the seemingly normal landing; which was probably slightly low on the GS at points. However; I believe we were inside airport property; or very close.  With no aural warnings; nothing seemed out of the ordinary; and the First Officer noticed nothing irregular and had no response when I asked him why the GPWS was blinking on the PFD.   I know the obvious recommendation here is to stay on the GS; that's what it's for right; but as I recall I wasn't what seemed to be terribly low and I could see the runway perfectly.  We were certainly not in any danger of hitting the ground.  If I was too low why did the GPWS just blink and not give a 'TOO LOW TERRAIN' warning?  If this had been an actual stormy IMC approach at night I would have been quite concern about this.In hindsight and evaluating the approach I'm thinking about the WUPGA step down fix; there is obviously some reason for this step down.  Analyzing the approach chart I see there is a 574MSL tower of some sort (not seen on my visual approach) that is inside of WUPGA!  I would expect this tower to be outside/further away from the airport than WUPGA; it would make sense to have a step-down fix that 'steps you down' AFTER you clear the fix and NOT before.  In any case here are a few of my questions for clarification; I hope there is someone that can access this approach [in flight operations] and is able to give me a reality check between my perception and the facts; this was day four of our trip after all.  Does our GPWS have towers programmed into the database and how does this work?  Does the GPWS warn of terrain as in 'dirt only' or does it raise the 'terrain' floor everywhere for towers; if so does it do this only within some proximity to towers/obstacles?  What does just blinking 'TERRAIN GRND PROX' on the PFD actually mean al alert (getting close to the threshold; or actually losing the clearance margin) and what would be my required response to this in actual IMC where I can't see a clear path in front of the aircraft?  Was I too close to this 574 foot tower; and had I been closer would the GPWS given me an aural warning for this tower?  Was the aural warning active but silenced with the 'K' codes due to proximity to the runway?  We are trained to respond to the GPWS aural warnings and we don't really care whether it is actual dirt or a tower; but we are not trained on what a blinking only indication means.   If this is an indication because of deviation from the GS should this not be disabled (as should be the GS indicator in my opinion) when programming in and conducting a LOC approach?




Synopsis: An ERJ-170 Captain questions his knowledge about EGPWS capabilities and the blinking 'TERRAIN GRND PROX' alert.