Air carrier flight crew reported receiving a low altitude alert while descending on the BFL FASTO2 arrival. The crew expressed confusion over ATC's clearance verbiage; the lack of crossing restriction altitudes depicted other than MEA; and the operator's SOP for the conduct of the RNAV arrivals of this sort.
Synopsis
Air carrier flight crew reported receiving a low altitude alert while descending on the BFL FASTO2 arrival. The crew expressed confusion over ATC's clearance verbiage; the lack of crossing restriction altitudes depicted other than MEA; and the operator's SOP for the conduct of the RNAV arrivals of this sort.
Narrative
We were given the following clearance from ATC during a dark night time descent with the FASTO2 arrival on our route clearance: 'Cross PMD at 14;000... cleared approach into Bakersfield' with no approach name or approach type specified in the clearance; but we were previously told to expect the RNAV 30R approach which we were set up for and had briefed. I put an altitude constraint of 14;000 into the box at PMD. We had already verified all published constraints on the STAR and RNAV 30R approach (STAR has no altitude constraints). I set 2500 into the altitude window (FAF altitude on the RNAV 30R); pushed managed descent; and armed the approach. We crossed PMD at 14;000 and immediately after PMD we went into Final App [mode] on the FMA and I called 'Set Missed Approach Altitude.' Approach was continued in Final App and gradual speed reduction was initiated. At 8800 MSL we received from ATC 'low altitude alert; check altitude immediately' with the local altimeter setting. ATC then asked us to verify we are descending 'via the FASTO2 descent profile and 'maintain 10;000 to KELEN.' We were not yet at KELEN. I immediately set 10;000 into the altitude window and selected open climb. We leveled off at 10;000. I set 2500 into the window again; waited till KELEN then pushed managed descent again. The Prog page showed us 4300 feet high of the profile at this point so I called for gear down and used full speed brake to return to profile and we re-intercepted Final App. We were still 15 miles out so plenty of time to return to profile although from 10;000 to 3500 at FASTO is a very steep descent. We configured normally and landed on 30R with an uneventful landing. After parking at the gate we didn't understand the request to verify we were descending via the FASTO2 descent profile as there are no magenta altitude constraints on the STAR nor did we get a descend via clearance. There are only altitudes for NAV reception and minimum segment altitudes. We don't normally don't enter minimum segment altitudes into the box as constraints so we weren't sure what he was talking about. Then we looked at the terrain contours at the gate and clearly 8800 feet where we were is too low. We complied with the 14;000 at PMD; and also met all published constraints on the RNAV 30R approach. We were in Final App and followed it down. A few seconds before the low altitude alert; my FO (First Officer) spoke up and said he didn't like it but he wasn't sure why. Seconds later we received the low altitude alert and immediately climbed to 10;000. We never got a radar altimeter nor an EGPWS but believe we were on a descent path to get both. I find this arrival to be safety concern.Cause - I've never seen Final App take me into terrain before so I can only conclude 1 of 2 things needs to happen here. Either the Controller should have assigned us radar descent altitudes for the area we were in; or there should be magenta constraints on the STAR if the Controller is expecting us to descend via a descent profile. Neither of which existed. I was also under the impression that Final App protects us with regards to terrain clearance when all ATC issued altitude constraints and all published STAR and Approach constraints are in the box correctly. Everything looked correct to me and yet we ended up 1200 below a segment altitude with a low altitude alert. Suggestions - Looking at other STARs into other airports with terrain on the descent route (such as Sacramento as an example arriving from the east); there are usually magenta constraints to protect your terrain clearance. I don't understand why this arrival doesn't have at least 1. Obviously ATC issued descent altitudes would have solved this issue since you can't descend via on this arrival but yet the Controller was expecting us to comply with the minimum segment altitudes on the arrival on our own. I also thought that once in Final App those minimum segment altitudes were protected. Here they were not.
Second reporter narrative
Around XA10 PDT; cleared on FASTO 2 into BFL. Direct PMD at 14;000. Approach gave us cleared approach into Bakersfield". We crossed PMD at 14;000 - armed/activated the RNAV for 30R; verified Final App [mode] and then started down on the FMS path. Both arrival and approach verified as correctly loaded. I was PM. It seemed odd there were no altitude constraints on the FASTO; I felt a bit uncomfortable and asked the PF what was guaranteeing terrain clearance. Discussed; and since there were no further constraints the RNAV path should keep us safe with our clearance. Got an "altitude alert" from ATC and an additional clearance to climb and maintain 10000 until KELEN. Lowest I saw was around 9000. Climbed back up to 10000; descended at KELEN and reintercepted the RNAV; landed uneventfully.Cause - The FASTO doesn't have any altitude constraints - but it does have an MEA. I think the Controller expected us to descend via those altitudes (which matched what he gave us later). However; I've never heard of or been trained on loading anything other than published constraints. My best guess was it was a bad clearance; but even after talking about it for two days we couldn't precisely determine where the problem was. We never got particularly close to the terrain; but definitely could have on another day; would definitely like to learn what the issue was and help make sure doesn't happen to anyone else.Suggestions - Frankly; unsure. Possibly update the STAR with constraints (like LAX or the other west coast airports)."
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.