Stobart AT72 at Dublin on Sep 2nd 2016, descended below assigned altitude and minimum safe altitude
Last Update: January 15, 2021 / 16:46:23 GMT/Zulu time
The Irish AAIU notified the aircraft descended through its assigned altitude of 2000 feet and was still descending through 1100 feet MSL when the controller received a minimum safe altitude warning and queried the crew. Flaps and gear were still up at that point, during the go-around however it appeared the flaps had been extended. The conditions were VMC with few clouds at 2200 feet. The occurrence has been rated a serious incident and is being investigated by the AAIU.
On Jan 15th 2021 the AAIB released their final synoptic report concluding the probable causes of the serious incident were:
While conducting an ILS approach, the aircraft was descended unintentionally to an altitude that triggered the ATC Minimum Safe Altitude Warning.
- The Flight Crew’s lack of experience on the ATR 72 ‘-600’ aircraft.
- The Commander became distracted by the manual operation of the TLU to the extent that the flight path was not adequately monitored.
- There were inappropriate pitch inputs made to the Commander’s control column that caused the autopilot to disengage.
- The level of cockpit communication and co-ordination between the Flight Crew was sub-optimal.
The AAIU summarized the sequence of events:
The Co-pilot was the Pilot Flying (PF) and the Aircraft Commander was the Pilot Monitoring (PM) on what was the second flight of their duty period. The aircraft had departed EGPH at 15.47 hrs. It entered the Dublin Control Area (CTA) 23 nautical miles (NM) east of Dundalk at 16.29 hrs and was cleared by Dublin ATC to descend to 3,000 ft and to route direct to the position LAPMO9. The aircraft was told to contact ‘Dublin Director’ for further ATC instructions. At 10 NM from LAPMO, ATC cleared the aircraft to route direct to the position MAXEV10 (Figure No. 1) for an ILS (Instrument Landing System) approach to Runway (RWY) 28, and directed the Flight Crew to report when established on the ILS. The aircraft was also requested to descend to 2,000 ft. At 16.32 hrs, the aircraft was given a speed restriction of 190 kts in order to maintain separation from another item of traffic that was 7.5 NM ahead in the approach sequence. One minute later, the ATCO advised the aircraft to further reduce speed to 160 kts upon reaching MAXEV.
The aircraft reached the ATC altitude restriction of 2,000 ft at 16.38 hrs, approximately 2.5 NM before MAXEV, at a rate of descent of 2,000 ft per minute and an indicated airspeed (IAS) of 185 kts. At 16.39:07 hrs, the aircraft, still descending, passed MAXEV at an altitude of 1,670 ft and an IAS of 181 kts, while commencing a right turn onto a magnetic heading of 284 degrees. Thereafter, the aircraft maintained a rate of descent of 1,200 feet per minute.
At 16.39:26 hrs, as the aircraft passed an altitude of 1,200 ft, an MSAW alert was triggered on the controller’s ATC radar screen, causing the ATCO to request the aircraft to “confirm established on the localiser11 runway 28?” The Aircraft Commander responded “Em ye yes confirm we are now eh going eh going around eh [callsign]”. The ATCO requested clarification of the intentions of the Aircraft Commander by asking “eh [callsign] just confirm you are going around?” to which the Commander replied “Affirm”. The ATCO acknowledged this transmission with “Ok [callsign] roger”. During this interaction, the aircraft continued descending until reaching an altitude of 1,082 ft and an IAS of 168 kts at a point 1.75 NM west of MAXEV. The altitude of an aircraft at this point on the ILS glideslope for RWY 28 should have been 1,975 ft.
During the first approach the aircraft flaps remained at zero degrees and the undercarriage remained in the ‘UP’ position. Following the MSAW alert and the controller’s intervention, the aircraft executed a go-around and climbed to an assigned altitude of 3,000 ft.
During the go-around, at 1,390 ft and 177 kts while climbing, the flaps were selected from zero to 15 degrees. The IAS gradually reduced and reached 147 kts at 2,800 ft as the aircraft began to level at 3,000 ft. The flaps remained at 15 degrees for a further three minutes until selected to zero. The aircraft was provided with radar guidance by the ATCO for a subsequent ILS approach and landing, which was completed without incident. The aircraft taxied to the parking stand and the passengers disembarked.
The AAIU analysed the crew actions:
The aircraft was conducting a scheduled passenger flight from EGPH to EIDW. The Co-pilot was PF and the Commander was PM. The failure of the TLU (Travel Limiter Unit limiting the rudder travel) automatic mode was recorded in the Technical Log as a deferred item for the flight in accordance with the aircraft MEL. This deferral required that the TLU be operated manually during the flight. The Commander, who was operating the TLU, stated that the operation of this system in the ‘-600’ version of the ATR 72 was not familiar to him. In particular, he was concerned that the aircraft would exceed its maximum permitted operating speed (VTLU) when the switch was cycled to LO SPD.
The Investigation established that the TLU deferral criteria and in-flight operational procedures in the MEL for both the ‘-500’ and ‘-600’ versions of the ATR 72 are the same, although the Commander’s assessment of the cause of the occurrence was ‘MEL of the TLU is different from ATR 600 and ATR 500.’ The Commander told the Investigation that he set the TLU to LO SPD at 3,500 ft and 185 kts. According to the FDM data, the aircraft passed 3,500 ft at 16.37:22 hrs while the aircraft was at an IAS of 202 kts. This speed is above the VTLU indication on the Commander’s cockpit speed tape, and would cause the ‘Barber’s Pole (of the ‘-600’ version) to move to 185 kts, indicating an overspeed situation to the Commander.
He said that he then told the PF to disconnect the autopilot, that he continued to cycle the TLU between LO SPD and HI SPD, and next told the PF to go-around. The Commander did not indicate to the Investigation that any other actions were completed within the intervening period, including monitoring the progress of the ILS approach, or conducting the approach and before landing checklist. This indicates that the Commander was focused on the effects of TLU selection from 16.37:22 hrs until 16.39:45 hrs (2 minutes 23 seconds) to the exclusion of other activities. This would suggest that the Commander, who was PM, had become distracted from monitoring the position and configuration of the aircraft.
The Co-pilot stated that at an estimated 185 kts, the Commander changed the TLU switch to LO SPD, the autopilot disconnected and the speed reduced towards 170 kts. The Co-pilot stated that he did not disconnect the autopilot and said that as a result of the disconnection, he presumed that the Commander had disconnected it. The Aircraft Manufacturer identified that there was a force of -39daN applied to the Commanders control column at this time, which was enough to cause the disconnection of the autopilot. It is probable that the Commander applied this force inappropriately to his control column due to his concern the aircraft was about to exceed the VTLU. When the autopilot disconnected, the Co-pilot, who was the PF, asked the Commander if he had taken control of the aircraft. This was the phase of flight during which the approach checklist, including extending flap and landing gear, and the checklist confirming that the aircraft had captured the ILS should have been completed.
During this period of confusion in the cockpit, the aircraft continued to descend below the ILS glideslope without the Flight Crew being aware of it, to an altitude that triggered the MSAW in the Control Tower. Furthermore, between 16.39:36 hrs and 16.39:55 hrs, both Pilots were operating their respective control column pitch control simultaneously.
The Commander emphasised in his AAIU Pilot Report Form that he was satisfied that the aircraft was at all times operating in VMC conditions with the ground in sight. However, the Investigation considers that the Commander’s distraction due to the TLU, combined with the Co-pilot’s uncertainty as to whether or not the Commander had taken control of the aircraft, were factors in the aircraft deviating from the ILS/MSA/permitted approach profile. This militated against the likelihood that the Flight Crew would have detected the presence of other traffic operating under VFR to the East of Dublin Airport.
During the go-around, the FDM indicates that the flaps were extended to 15 degrees during the initial climb through 1,400 ft at an IAS of 177 kts, resulting in a linear reduction of IAS to 145 kts at 2,600 ft. Although the Commander took control of the aircraft during the goaround, he returned control to the Co-pilot prior to completion of the procedure. The Copilot stated that the Commander configured the aircraft during the go-around but could not recall who extended the flaps. The Commander was unaware that the flaps had been extended.
While the CVR was not retained following the flight, it is evident from the other available sources that there was a breakdown in cockpit communication and Flight Crew co-ordination during this phase of the flight. The CVR was not isolated. This resulted in the loss of critical information which would have been invaluable to the Investigation, such as verbal interactions between the Flight Crew including transfer of control of the aircraft in accordance with the Operator’s Operations Manual Part A, Section 12.
The CVR would also retain the various aural warnings associated with the disconnecting of the autopilot, and how, and by whom, the multiple mode changes of the AFCS during the approach and go-around were initiated. The FDR data was similarly not isolated. This resulted in the loss of a number of additional parameters that were not included in the FDM data that was retained by the Operator.
The incident report submitted by the Commander through the Operator’s internal safety reporting system did not reflect the serious nature of the occurrence. This limited the opportunity for staff from the Operator’s Safety Office to retain the FDR data prior to it being over-written. Consequently, the following Safety Recommendation is made to the Operator in this regard.
Furthermore, based on the information provided to the Investigation, it appears that during the ILS approach between 16.38:36 hrs (autopilot disconnect) and 16.39:56 hrs (go-around), there were periods when the aircraft was flying without the certainty that it was being directly controlled by either of the Pilots, or by the autopilot. Thereafter, and for the duration of the go-around, both Pilots were manipulating the aircraft pitch control simultaneously.
EIDW 021800Z 25010KT 9999 FEW022 SCT050 18/09 Q1014 NOSIG
EIDW 021730Z 25009KT 230V290 9999 FEW022 SCT050 18/10 Q1014 NOSIG
EIDW 021700Z 27008KT 9999 FEW022 SCT050 18/11 Q1014 NOSIG
EIDW 021630Z 30010KT 9999 FEW022 SCT050 18/11 Q1014 NOSIG
EIDW 021600Z 25010KT 9999 FEW022 BKN040 18/10 Q1014 NOSIG
EIDW 021530Z 25011KT 9999 FEW022 BKN040 18/09 Q1014 NOSIG
EIDW 021500Z 25012KT 9999 FEW022 BKN040 19/10 Q1014 NOSIG
EIDW 021430Z 27009KT 9999 -SHRA FEW022 BKN040 18/11 Q1015 NOSIG
EIDW 021400Z 25010KT 9999 FEW022 BKN040 17/10 Q1014 NOSIG
Aircraft Registration Data
This article is published under license from Avherald.com. © of text by Avherald.com.
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