ASRS 1739630

Reporter 1

Flight plan was "MRSHH1.DDRTH.TELCU Pretty early, we got cleared direct SAA. We got the weather at the field and decided to do the RNAV 05. We got cleared direct ZUBUB(IF) and shortly after that we got cleared to descend and maintain 13000 ft. until established for the approach then cleared for the approach. I was the Pilot Flying, upon reaching 13000 ft. I decided to set the next altitude 10100 ft. (after receiving the clearance for the approach). For some unknown reason I began to descend to the set altitude 10100 ft. Around 11700 feet we received a "Caution terrain" I disconnected the autopilot and began to climb to 13000 ft. As soon as we received the warning I understood immediately my mistake. We climbed back to 13000 ft. and completed the approach and landing without any further issues. I have no idea why I decided to descend lower than 13000 feet even though its very clear on the approach plate and from the clearance. Was it because I had woken [early morning, two hours prior to] show time, that perhaps might have been a contributing factor since neither of us reacted when we saw the terrain color change. What ever the reason was have no excuse, this will never happen again. After landing my partner talked about it, it really bothered me because this event could have been so much worse and the fact that I made such a beginner's mistake. I am so thankful we had the alert system on the airplane.

Reporter 2

We were assigned the RNAV 5 for SAA. ATC cleared us direct to ZUBUB and we were told to maintain 13000 feet until established. We were heading approximately northwest. As we neared the smart turn towards YIDUR the Captain selected 10100 ft. for the next altitude and commenced the descent at around 2000 fpm. I was asked to activate vectors to final. This removed the smart turn and put us in heading mode. We had the MFD set to relative terrain. This mode started to display green terrain on our flight path. Which was unexpected for this portion of the approach. Then I saw the first yellow and that is when we received the GPWS Caution Terrain. The Captain immediately started a climb back to 13000 ft. and we continued the approach.