Air Carrier Line Check Airmen and flight crew reported using non standard procedures that lead to airspeed; course and altitude deviations during an RNAV approach. During the approach the flight crew was notified by ATC of a low altitude warning.
Synopsis
Air Carrier Line Check Airmen and flight crew reported using non standard procedures that lead to airspeed; course and altitude deviations during an RNAV approach. During the approach the flight crew was notified by ATC of a low altitude warning.
Narrative
I was in the Jumpseat for a Captains FAA Observation.Up until the descent; the observation was going well. With ZZZ landing East; the crew decided to brief for a RNAV (RNP) Y to Runway XXL. Descending through FL190 or so there was some CBs that had built up; no Flight Attendant notification of possible turbulence. There was some moderate turbulence associated with the CBs; I suspect this was the start of loss of some SA.The crew had been given direct ZZZZZ on the ZZZZZ1 and a descend via clearance; 6000 ft. was set in the MCP. New Captain trying to get the plane to start a descent (system I believe was still calculating the direct ZZZZZ and had not started down yet) pushed the Alt INTV which deleted the ZZZZZ's altitude restriction of 12000 to 9000 ft.; retaining the speed of 250 kts. He did verbalize this but did not reiterate that the restriction was 12000 to 9000 ft.; the LCA (Line Check Airman) only retyped in 12000. This started the high on Path situation. The plane achieved 12000 ft. some 5 miles prior to ZZZZZ. The SPD INTV was selected to 280 kts. 280 kts. is what they crossed ZZZZZ at; I verbalized to the right seater (the LCA in the right seat); 'check your speed; should be 250 kts'. New Captain used VS; then selected 250 kts. in the MCP but still was not descending for the next restriction ZZZZZ1 at 210/6000.VS was this Captain's go to mode; he tried several times to select VS but was pushing the round white 'light sensor' on the MCP; located above and right of the VS select button. Started down in VS (at this time some 2300 ft. above the Path); speed now 250 kts. I verbalized the restriction. The LCA received relief from ZZZZZ1's altitude of 6000 ft.; speed of 210 kts. was not mentioned. ATC stated; cross ZZZZZ2 at 3000 ft. cleared for the RNV Z XXL. The crew was also dealing with the fact that they had originally requested RNAV (RNP) Y; this is what they briefed. Approach stated later they could not issue the Y but could for the Z; the crew was working on a this approach change briefing in the middle of the altitude restriction issue adding to further loss is SA. With 3000 ft. in the MCP; VNAV would not reengage initially so VS was still in use. LCA stated to use LVL CHG as new Captain was struggling with speed management. At this point it looked like everything was coming back under control. Once speed was lowered to an acceptable value; VNAV was selected again and TDZE was selected. I started taking some notes; glancing up to see that they were out of VNAV descending now through 3000 ft. (still prior to ZZZZZ2) I started to say something but the LCA had already started to comment. It looks as if 2800 ft. was going to be the lowest they were going to get; with correction now being applied.I suspect but can't be for sure; a new mode was selected trying to gain control of the speed or VNAV just disengaged. Flaps were first called for at 250 kts. and a comment from the LCA that he was too fast.When trying to correct the low altitude situation with (I believe) a MCP mode change; LNAV was deselected as CWS R was now displayed in the FMA. Unfortunately this happened right at ZZZZZ2 where a left turn was required.There was a great number of distractions going on; it seems that the input from the LCA was not promptly executed; paired with a lagging SA; this was leading to additional distractions. ATC had now called 'low altitude warning' as the new Captain allowed (and the LCA along with myself didn't catch) the plane to sink again now to 2500 ft. The CWS R grabbing our attention and the LCA calling for a left turn; this off course situation was also being called out by ATC.Turning off the Autopilot to regain a desirable state was achieved and the remaining approach and landing was uneventful.
Second reporter narrative
A number of things and errors led to going 400 below altitude on ZZZZZ arrival; for RNAV approach. We were cleared to descend via the ZZZZZ arrival for RNAV/RNP Y XXL. Just past ZZZZZ1 we were cleared direct to ZZZZZ2; this put us high on the path by 2000+ ft. Captain selected alt intervene during this descent which deleted the altitude restriction at ZZZZZ2. He asked for 12;000 to be put back in. He was using vert speed trying to control aircraft. When realized we weren't going to make 6000 ft. restriction we asked for relief. The aircraft was getting fast approx 264 Kias below 10;000. New clearance was to comply with 3000 ft. restriction by ZZZZZ3. Vertical speed was being used to try and control descent. Taking the plane out of vertical path. Because we had been previously cleared the approach Final approach fix altitude was put in to altitude window. When the plane was put into vert speed mode. It no longer had projections and descended below 3000 ft.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.