On 1 August 1997, an Air Malta B737, descending for an approach into Manchester UK in poor weather, descended significantly below the cleared and correctly acknowledged altitude, below Minimum Sector Altitude.
The following is an extract from the UK AAIB report:
"…Before descent, the crew obtained the airport weather information and the commander briefed for the expected approach to Runway 24 at Manchester. Information 'X', timed at 0850 hrs, confirmed the landing runway as 24 and described the weather as follows: "Surface wind 220°/06 kt, visibility 2,500 metres in drizzle, cloud broken at 100 feet and overcast at 300 feet,temperature and dew point 16°C, QNH 1011 mb and a wet runway." During the descent, the crew were instructed to enter the 'Hold' at 'Dayne' and, while in the 'Hold', were transferred to Manchester Director on frequency 121.350 MHz at 0936 hrs. On initial contact,the crew confirmed their cleared level as FL 70 and this was acknowledged by the controller together with the message that they would shortly be leaving the 'Hold'. Then, at 0937 hrs, the controller passed three separate messages to AMC 202: the first was to fly at a speed of 180 kt, the second was to descend to FL50 and the third was to cancel the 'Hold' and to turn right onto 010°. All these messages were passed clearly and acknowledged correctly by the crew. At 0940 hrs,the crew reported level at FL50. Shortly afterwards, the controller passed two further messages to AMC 202. The first was: "AMC202 reduce to 160 kt and maintain until at four DME"; this was acknowledged as: "160 kt till four DME AMC 202". Then, the controller transmitted: "202 correct descend to altitude four thousand feet QNH 1011"; the crew replied: "Four thousand feet on QNH 1011 AMC 202". Within the cockpit, the clearance to an altitude was the prompt for the crew to action their 'Approach checks'. At 0942 hrs, the controller turned the aircraft left onto a heading of 335°M and this was acknowledged correctly. Then, at 0943, the controller noted from his radar display that AMC 202 was indicating at a altitude lower than cleared and immediately asked the crew to confirm their altitude. When AMC 202 stated that they were: "Passing two thousand four hundred feet", the controller promptly replied: "AMC 202 climb to three thousand feet immediately your cleared level was four thousand feet". The crew responded immediately with an application of power coincident with their radio acknowledgement. Thereafter, the controller advised them that the minimum safe altitude in their immediate area was three thousand feet and gave them radar vectors to intercept the ILS. The aircraft landed uneventfully shortly afterwards."
"…the crew were sure that they had been…cleared to 2,000 feet"
A contributory factor was deemed to be pilot workload and the company subsequently reduced the number of actions required during the "approach checks".