Air carrier flight crew reported a CFIT event during approach. Flight crew regained aircraft control and continued approach.
Synopsis
Air carrier flight crew reported a CFIT event during approach. Flight crew regained aircraft control and continued approach.
Narrative
We realized that ZZZZ air traffic control had issued us an erroneous or unclear descent instruction and we ended up descending too low on the arrival into ZZZZ. It was my FO's first time flying here; and while I have been here dozens of times; it had been a while. We spent a good amount of time in cruise reviewing procedures in the manual and briefing the arrivals and approaches. While descending on the ZZZZZ X we heard instructions to cross 10 miles on this side of ZZZZZ at 11;000". Accent of controllers aside; we heard this pretty clearly and that was read back. The crossing restriction was loaded in the FMS and set in the altitude window. But as we continued descending on the arrival we were noticing cues that indicated there was a problem with this clearance as we had interpreted it. 11;000 was lower than the charted altitude on the arrival for the subsequent waypoint; causing the altitudes on the subsequent waypoint to display in inverse. The FMS displayed the scratchpad message for flight path angle too steep. Most importantly though; we were both using the terrain function on the Multi-function Flight Displays (MFDs). I zoomed in to the 5-10 mile range and saw terrain shading ahead of us and the highest terrain altitude was 11;100. As pilot flying I stopped descent with the ALT button at about 12;000 feet. Before we could query ATC they called us first requesting our altitude. We were instructed to climb back up to 14;000 which I did using VS mode. Visually we had exited a cloud layer about this time and we could see terrain ahead of us. Based on review of FlightAware data we stopped descent at about 12;000 feet about 5 miles north-northwest of a mountain. No GPWS warnings or cautions occurred. The flight continued and we were cleared to continue descending on the arrival and approach. Ultimately we were following procedures that let us recognize the terrain threat and act. The next day after continuing to self assess the event I think I realized what the primary error might have been. ATC instructions were to cross 10 miles "on this side" of ZZZZZ at 11;000. We are of course familiar with that phraseology; but ATC in the US doesn't say it like that. We read it back like that all the time; but US ATC always specifies N/E/S/W direction for crossing restrictions. I think when the controller said that; they meant it as on 'their' side not 'ours'; which would be south of ZZZZZ not north; hence the "on this side" instruction was unclear. This was very much an unforeseen threat to us regarding ATC. Suggestions: The biggest initial giveaway that we had programmed in an erroneous clearance was the inverse altitude for the subsequent waypoint on the arrival. Receiving instructions to descend lower than published altitudes sometimes happens due to traffic; but in mountainous terrain areas we should never accept this. We overlooked the terrain shading on the arrival chart; which shows terrain at about 11;000 feet around ZZZZZ; hence we can't descend to 11;000 north of this point."
Second reporter narrative
We noticed that ZZZZ Air Traffic Control had issued us a lower than expected decent instructions that did not coincide with the decent plate. We stopped the decent as soon as we noticed this and at the same time; they told us to climb back up to 14000. While descending on the ZZZZZ X we heard the instructions to cross ZZZZZ 10 miles on this side of ZZZZZ and descend to 11000. With the accent aside. We both confirmed this to be what the read back was. This was then loaded into the FMS and set in the Altitude window. As we descended down we noticed cues that this was a problem with the clearance. 11000 was lower than the charted altitude on the chart with the waypoint. The FMS also showed a message saying decent path angle to steep. Most critically we noticed we were both displaying terrain on the Multi-function Flight Display (MFD) and saw the highest altitude was 11000 on the 5-10 mile scale. We did not receive terrain avoidance and GPWS indications. With these cues noticed we executed leveling off and climbing back to charted altitude and we queried the ATC instructions and received reclearance for the ZZZZZ X arrival. We continued without incident. Suggestions: Clarify altitude constraints; Slower talk
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.