Gulfstream IV / G350 / G450
Flight Crew Air Transport Pilot (ATP)
Communication Breakdown; Fatigue; Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Human Factors; Weather
Descending prior to 18;000 feet; the altimeter setting of 29.61 was set in the standby altimeter per the ATIS. As we transitioned through 18;000 feet on our way to 11;000 feet we were given a local altimeter setting of what we believed to be 30.61. I was sitting in the right seat and for some reason; I changed what was in the standby altimeter to 30.61. All three altimeters were now changed to 30.61. At this point I do not recall if I read back 30.61 or 29.61. Either way; 30.61 was set in all three altimeters. We continued to 11;000 feet and received further descent clearance to 3200 feet and an updated altimeter setting of what was supposed to be 29.58. Instead; I read back 30.58 and we both set 30.58. Now all three altimeters read 30.58. We broke out at 7300 feet into good marginal VFR conditions with visibility at least 5 miles. The Descent and first part of the Before Landing Checklists were complete and altimeters cross checked but because all three altimeters were now set to the wrong setting; we still had the wrong altimeter set. We were given further descent clearance to 3200 feet. As we approached 3200 feet we received further descent clearance to 2200 feet. We continued a slow descent towards 2200 feet. We could see some tall smokestacks in the distance below our altitude and off to our right. The stacks still seemed closer to our route than would usually be seen. We were just beginning to question; amongst ourselves; our assigned altitude and vector when approach asked us to level at 2200 feet. At the same moment and roughly 2600 feet indicated; we got an obstacle warning. We arrested our gradual descent but subsequently got a pull up command moments later. We complied with the command with pitch and power as I informed approach that we were climbing for obstacle avoidance and were currently out of 3100 feet (2100 feet with correct altimeter). Approach told us to descend to 2200 feet. I told him we were complying with an obstacle avoidance command and then I asked for the altimeter setting. Approach answered with an altimeter setting of 29.58. I immediately changed all altimeters and we leveled at 2200 feet. We were cleared for the ILS where the rest of the approach and landing were uneventful. Because of the visual conditions; we could see we were never in a dangerous position and kept clear of the obstacles. The whole situation was a result of a combination of a few things. One was a lack of common sense on my part. I knew the ATIS called for an altimeter setting of 29.61 yet blindly changed the standby altimeter to something that was drastically different. The weather conditions should have been a good indication that a high pressure setting like 30.61 was unlikely. Another was not hearing what was being said; instead I heard what I expected. I expected to hear an updated altimeter setting near what was set (30.61). When 29.58 was given; I read back 30.58. Complacency played a part in this situation as well. Once we broke out into basically visual conditions; I didn't go back and compare what I had written down for the ATIS with what we had set. A simple ATIS review would have caught the mistake. In addition; the aircraft is loaded with tools to aid in altitude awareness. Some quick mental gymnastics once the radar altimeter was called alive would have caught our altitude discrepancy between what was being indicated on our altimeters and what the radar altimeter was showing. All that being said; more attention to detail would have mitigated this issue. Last but not least of all I believe that fatigue may have played a small role. I do not recall exactly how much sleep I had gotten the few nights prior to this day but I do remember that the quality of sleep was very poor. Combine this with the mental exhaustion of keeping up with the constant changes on a daily basis and I feel like this contributed to the issue.
Looking back we had a couple of clues that we had set the incorrect altimeter. First was we were flying in to an area we knew was deteriorating conditions in advance of a strong cold front; and an altimeter setting of 30.58 was unlikely. Secondly we called 'radar altimeter alive' somewhere around an indicated altimeter of around 4000ft. With a field elevation of approximately 700ft we should have been alerted to the error.Thirdly; looking out the windshield in hindsight it did look as the vector was taking us close to the smokestacks. Additionally I had the HUD down and the EVS on in preparation for the approach. The EVS picture was distracting as I was looking at the EVS image of the smokestacks and that interrupted my scan where the radar altimeter could have alerted us to the situation earlier. [Enhanced Vision System].The final factor was rest. The night before we arrived at the hotel after 1 am and didn't get to bed until closer to 2 am and woke around 7. Although I only got 5 hours of real sleep I thought I felt fine. However when reaching the hotel after another flight that day; I was exhausted and fell asleep at 8pm.
G450 Flight Crew reports setting an incorrect altimeter setting (30.58 vs 29.58) during descent and approach. This error is not discovered until ATC issues a low altitude alert at the same time the crew receives a GPWS warning. Fatigue was cited as a factor by both pilots.