Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human-Machine Interface; Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Flight Crew Became Reoriented; Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Procedure; Human Factors
We were on the PAATS3 STAR. It was not busy so ATC had us hold heading at ESSSO to avoid the rest of the arrival and shortcut to base leg Runway 9R. We had to expedite descent since we were not doing the full arrival. We used spoilers; flaps; and gear. ATC cleared us to 2;000 feet and cleared us for visual approach. The First Officer armed the approach and we noticed glideslope alive from above. It captured and we continued down. I failed to notice we hadn't crossed the FAF yet [and] were descending below FAF altitude. As I noticed we were low; ATC gave us a low altitude alert and the First Officer climbed back to FAF altitude (1;800 feet). The aircraft had captured a false Glideslope and we didn't catch it immediately. The airplane was stable at 1;000 feet and First Officer did a fine job with the rest of approach and landing.We were rushed to descend and were worried about being high not low. I should have been cross checking our position reference to our altitude. I will be cognizant of the possibility of false glideslope captures and be vigilant on monitoring altitude and position on approach.
We must have captured a false glideslope which overrode the 2;000 feet foot protection usually afforded by the altitude selector of the automation. We were very busy trying to go down and slow down at the same time while having the runway in sight at all times. This high work load initially made us miss that the plane was descending a bit too early. As soon as we recognized things were not what they should be; I disconnected the automation and hand flew the airplane back to the proper altitude. Lesson learned is to wait with 'approach mode armed' while on a vector until actually reestablished on a proper LNAV course; I should have just armed 'LOC only' until we had crossed KELEE before selecting Approach mode.
B757 flight crew reported receiving a low altitude alert from ATC on approach to Runway 9R PHL after descending on a false glideslope.