Date: 201308
Local time of day: 0601-1200
Location: GTF.Airport, MT
Flight conditions: VMC
Aircraft: Large Transport; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
ATC / advisory: TRACON GTF
Aircraft component: FMS/FMC
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion
Second reporter function: Pilot Flying; First Officer
Anomaly: Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action
Contributing factors: Human Factors; Aircraft
Primary problem: Ambiguous

Narrative: I suggested we fly the RNAV Z (RNP) to Runway 21 at GTF since it was now clear skies and the approach sets up just like a base leg visual approach for Runway 21. It also allows for practicing RNAV procedures. The First Officer briefed the approached and we verified the FMS. The only changes we made to the FMS were putting a slower speed at the FAF KUDEY of 125 KIAS and a hard altitude of 4;700. We asked for direct YOGOS from ZLC Center and were cleared the approach from YOGOS at or above 6;100 from GTF Approach. After passing YOGOS the First Officer performed his LAV procedures and set 3;700 in the altitude window and speed intervened to 170; I believe. We obtained VNAV Path between YOGOS and ZILEP. We were in VNAV Path for the remainder of the approach. I was clearing outside the majority of the approach and pointing out visual references since the First Officer had never been to GTF. As we approached KUDEY; I pointed out the tall towers on final that are near the turn to final. However; as we began to get closer it appeared to me that we were descending too low toward the towers. I checked inside and we were still VNAV Path. I told the First Officer we needed to climb. Shortly thereafter we received a GPWS call for 'Obstacle.' The First Officer disconnected the autopilot and initiated a level off or slight climb. We continued to get an 'Obstacle' call out. At that time I said we need to climb more. The First Officer initiated a more aggressive climb at this time. In the process we got one GPWS warning for an obstacle. Since we were VMC; daylight and visually could see the towers we did not perform an escape maneuver. After we got past the towers; we were reestablished on the approach and still in VNAV Path. However; I could see the PAPI's for the runway and they were showing 4 reds. I called this out and said we are showing low. Meanwhile the First Officer continued calling for configuration of the aircraft. By 800 AGL we were stable and on good PAPI indications and continued the approach to landing.During the debrief we could not figure out why we were on VNAV Path and would receive GPWS call outs on the approach. We can not think of anything that we did that was incorrect. As far as we can tell; we performed RNAV procedures correctly. The only thing we changed in the FMS was the speed at KUDEY (the FAF). I believe we put the correct altitude to correspond with KUDEY. An aircraft system of some type may have caused this; however; I did not make an entry in the logbook. Not sure how I would have written it up and what maintenance could check. I checked the RNAV Y approach for Runway 21. It has a crossing altitude of 5;300 FT over the towers. The RNAV Z is 4;700. The towers are listed at 4;117. I do not know when a GPWS call will be made for an obstacle but that is less than 600 FT clearance on the approach for the Z. I have flown the Z approach in VMC several times. Not sure if it is the exact approach or if it has changed recently. It does seem like a low clearance altitude over the towers on the Z approach. Good thing we can fly these approaches in VMC conditions only. That would not have been fun in night IMC.


Second narrative: During RNAV Z to Runway 21 at GTF in day VMC with autopilot and autothrottles connected; the crew noticed large towers along final which appeared very close. Crew verified we were in VNAV Path; dead on LNAV course; on speed and properly configured. Captain and First Officer agreed that we needed to manually fly around the towers and climb. After disconnecting the automation and beginning an immediate level off and then climb; we received an 'Obstacle' callout and continued to visually maneuver around the towers and back to proper glideslope --- all in day VMC. We continued to configure the aircraft; stabilized the approach fully by 800 AGL and landed safely. We are unsure of what caused this event. The airplane was fully automated; on VNAV path with all appropriate procedures completed. The programmed altitude over those towers on the RNAV Z approach is only 4;700; while the towers are listed at 4;117. This amount of clearance seems very small. The Captain told me he has flown this approach many times in VMC and never experienced this anomaly. We are unsure if it is an approach anomaly or was an aircraft systems anomaly. Verify that the altitude of 4;700 FT on this segment of the approach meets the appropriate TERRPS criteria AND is a prudent altitude to avoid unnecessary GPWS callouts when the aircraft is properly established on LNAV and VNAV path.


Synopsis: Air carrier flight crew in day VMC experiences A EGPWS terrain warning during the RNAV Z to Runway 21 at GTF with autopilot and autothrottles connected. The towers which caused the warning are visually avoided and when the PAPI is sighted; it also indicates low. A normal landing ensues.