2023-07 · NASA ASRS report 2014798
Air carrier crew reported an altitude deviation and ATC low altitude alert while on approach to a foreign airport over terrain in moderate rain conditions. The pilot flying incorrectly determined and set a lower altitude than assigned from the approach chart while the pilot monitoring was distracted; and ATC alerted the crew of the deviation. The flight crew quickly corrected the altitude deviation and completed the approach and landing safely.
The ZZZZZXX Arrival; RNAV Transition to ILS Runway XXR & the ILS XXR approach were programmed and briefed prior to TOD (top of descent) and weather at the field was deteriorating due to TSTMs (thunderstorms). On descent we received an amended release advising us that our destination alternate had changed to ZZZZ1 from ZZZZ2 due to TSTMS and deteriorating weather conditions there as well. We determined that we would only have one attempt at landing at our destination before we would have to proceed to our new alternate in Country due to the new increased fuel burn. While on the ZZZZZXX arrival; our radar was indicating cells with moderate to heavy precipitation along the arrival and approach corridors to the airfield. We began to receive multiple radar vectors and airspeed assignments from ATC as we heard the controller state to our preceding traffic that previous flights had landed successfully but that heavy winds were reported over the airfield. Although I had previously briefed that my preferred method of descent was using VNAV; I decided to use V/S mode to comply with the altitudes assigned by ATC to better position the aircraft for a successful approach and landing and avoid being too high from shortened vectors and rapidly changing weather conditions. This had happened to me on my previous approach some weeks before under similar; but somewhat worse weather conditions in to ZZZZ. While complying with multiple radar vectors and speed assignments; and approximately 20nm from the field; we were given an altitude assignment of 4;500 feet then switched to ZZZZ director. After another speed assignment and radar vector; we were cleared to proceed direct to ZZZZZ1 and descend via the RNAV Transition to ILS Runway XXR. Even though I had previously briefed the transition and approach; I was not certain of the final descent altitude on the RNAV transition chart although the FO (First Officer) and relief pilot confirmed that it was 1700 feet. I selected the altitude in the ALT window on the MCP (Mode Control Panel); but then toggled between the ILS approach plate and RNAV Transition plate on my EFK to verify that I was looking at the correct altitude myself which was not obvious to me since both plates shared two common waypoints on the ILS approach. As I looked back up; the aircraft was just about to enter one of the smaller cells on the arrival and I then realized that I was still in V/S mode. The FO pointed out almost simultaneously that I should be at 4500 ft. until ZZZZZ1 and I was at 4200 ft. I immediately began to correct the altitude and while climbing at approximately 4300 ft.; ZZZZ director issued a low altitude alert and advised us to check our altitude to which we replied correcting. Regrettably; I had allowed myself as PF (pilot flying) to become distracted by an unfamiliar approach and the deteriorating weather conditions just long enough to violate the altitude assignment. Had I more thoroughly reviewed and briefed the handling section of the somewhat unique RNAV Transition to ILS Runway XXR approach plate; I may have been better prepared for the clearance to descend via and may have avoided taking my eyes off the altitude until I was back in the VNAV mode to assure altitude compliance. The weather and internal imposed pressure of desiring to descend to the assigned altitude of 4500 ft. quickly to foster a more successful approach and landing should have been better managed by me. Even though I am aware of the many pitfalls of using the V/S mode in this situation; I will endeavor to be more vigilant of its use in the future and always reaffirm the vertical mode when changing the altitude in the altitude window.
We were conducting the ZZZZZ XX Arrival RNAV transition to ILS XXR. The Captain was PF (pilot flying) and I was PM (pilot monitoring). We were notified on initial decent of our alternate changing from ZZZZ2 to ZZZZ1; which would drive us having to proceed to our alternate if we went missed based on fuel for the further alternate. Prior to TD (Top of descent) we began getting vectors and speed changes taking us off the arrival. There was moderate to heavy rain along the arrival and approach corridor. Following vectors we were cleared direct ZZZZZ1 and descend via the RNAV to ILS XXR; so we put that in the FMS and 1;700 in the altitude window. PF then elected to join path using V/S. Then I looked at the weather ahead on the corridor and was checking to see if we might need to request alternate missed approach instructions due to potential weather in the missed corridor. I then noticed we were at 4;300 feet which was below the 4;500 foot altitude constraint at ZZZZZ1. I stated to PF we needed to be at 4;500 here and you need to climb back up now. During the transition back to climb I noticed a lowest altitude of 4;200 feet. Climbing back through 4;300 feet. ATC called us with a low altitude alert. We leveled at 4;500 feet and flew the rest of the arrival; approach and landing normally. Instead of the other tasks I was attending to as PM I should have been more actively making sure the PF was on the correct path earlier in the sequence; which would have trapped this earlier than I did.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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