Flight Crew Last 90 Days 150; Flight Crew Total 12500; Flight Crew Type 3800
Communication Breakdown; Human-Machine Interface; Situational Awareness
Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance; Flight Crew Overcame Equipment Problem
Weather; Human Factors; Aircraft
Descending to BOS; First Officer (FO) pilot flying; cleared to descend via QUABN 3 RNAV. WX indicated low CAT 2 mins; so a CAT 3 autoland was briefed by the Captain and transfer of control from the FO was decided to occur once the next ATIS was updated. Through transition altitude; Approach descent checklist was completed and altimeters were set to 29.96 (in error but read back by both pilots. The altimeter seemed correct at the time).Final ATIS indicated VFR at the field and the FO briefed the normal CAT 1 approach as a backup to the visual and never turned control over to the Capt. Level offs were made via the descent at 11000; 7000 and 3000 without any reports of deviation from ATC. A further descent to 2000 was issued and after descending (out of 3000); we were contacted by ATC with a low altitude alert and the correct altimeter setting of 29.28. An immediate climb was made to return to 2000 once the proper altimeter indicated we had actually descended to approximately 1600 feet. Clear night visual was maintained with good visibility of shore lights and no GPWS warnings occurred.A long duty day preceded the event and after debrief of the incident; we were surprised that there was no previous indication of a deviation at our level off altitudes even though the incorrect altimeter was set at those times. We also commented during the initial descent out of 3000 that the altitude seems a little low but after checking that the glide slope intercept was 1500 feet; it seemed that the 2000 was not out of order.Further approach to landing was normal.
The GPWS and Enhanced GPWS did not signal a terrain conflict. After reaching the assigned altitude of 2000 ft.; we resumed the approach. The weather conditions at the time were VFR night. The surface was a well lit suburban/city environment making the terrain visible and distinguishable.During our Threat/Error debrief; both the First Officer and Captain noted how easily the chain of events lead to the low altitude alert issued by the controller. We were also surprised at how easily we bypassed several checks and observation which could have quickly lead to our discovering the incorrect altimeter setting. Also a contributing factor was the length of our duty day. At the time of this event; we had been on duty for approximately 10 hours and some 11 hours 30 minutes awake for the Captain.
An A320 flight crew descending for a planned CATIII autoland at BOS incorrectly set their altimeters to 29.96 when the actual pressure was 29.28. A low altitude alert was received from ATC as they descended to 2000 MSL (or 1320 MSL with the correct pressure setting) at which point their altimeters were corrected; they climbed back to 2000 and completed a visual approach consistent with the now improved weather.