Flight Crew Air Transport Pilot (ATP)
Confusion; Fatigue; Communication Breakdown
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Human Factors
On descent into Frankfurt cleared to descend to 6;000 feet. Transition level was FL070. The controller advised the crew that the altimeter was 998. 29.98 was read back to the controller at least three times without correction by the controller. Once passing through FL070 both the Pilot Flying and Pilot Monitoring (PM) set 29.98 inches in their altimeters. The controller noticed the aircraft was descending below 6;000 feet and advised the crew to set 29.47 inches. The aircraft descended approximately 350 feet below it's assigned altitude. The crew on this flight had experienced a maintenance delay [on departure] and had extended their duty day in order to complete this flight. Two factors contributed to this altitude deviation. First the crew was fatigued due to the delay and secondly the controller not specifying the altimeter setting in millibars or correcting the improper altimeter read back by the PM.
During the descent into Frankfurt and while being vectored; ATC directed a continued descent from FL080 to 6;000 feet. The transition altitude was 7;000 feet and the controller stated the altimeter was 998. The Pilot Monitoring (PM) read back 2998 and the controller repeated? Altimeter 998? The PM again read back 2998. Both pilots set 2998 and the Pilot Flying (PF) continued the descent to 6;000 feet on autopilot. Approximately 30 seconds after the aircraft leveled at 6;000 feet; the controller asked if we had set the correct altimeter of 998. The PM read back 2998 for the third time. The controller stated? Altimeter 998 or 2947.? At this time; both pilots realized the error and adjusted the altimeter to a millibars setting of 998. The PM read back 998 or 2947. The aircraft was approximately 400 feet below the assigned altitude. The PF disengaged the autopilot and immediately climbed to the correct altitude then reengaged the autopilot. Contributing factors included a very congested radio frequency; heavily accented English by the German controller; and the altimeter was not stated on the Frankfurt ATIS.
An air crew descended below assigned altitude after passing the transition altitude. They confused the altimeter setting of 998 millibars with 29.98 inches.