Batik A320 at Surabaya on Jan 6th 2018, descent below cleared altitude causes TCAS RAs

Last Update: October 12, 2018 / 15:04:11 GMT/Zulu time

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Incident Facts

Date of incident
Jan 6, 2018

Classification
Incident

Airline
British Airways

Flight number
ID-6137

Departure
Makassar, Indonesia

Destination
Surabaya, Indonesia

Aircraft Registration
PK-LUI

Aircraft Type
Airbus A320

ICAO Type Designator
A320

A Batik Air Airbus A320-200, registration PK-LUI performing flight ID-6137 from Makassar to Surabaya (Indonesia), was descending towards Surabaya when the crew was cleared to descend to 11,000 feet MSL but descended below 11,000 feet.

A Lion Air Boeing 737-900, registration PK-LPF performing flight JT-3795 from Jayapura to Surabaya (Indonesia), was holding at 10,000 feet about 40nm northwest of Surabaya, when TCAS Traffic Advisories followed by Resolution Advisories activated in the flight decks of both aircraft.

As result of the TCAS RAs the A320 climbed back to 11,000 feet. Both aircraft continued to Surabaya for safe landings.

Indonesia's KNKT rated the occurrence a serious incident and opened an investigation.

On Oct 12th 2018 the KNKT released their final report concluding the probable cause of the serious incident was:

- The vertical separation reduced to below the requirement due to the target altitude of BTK aircraft was set to 10,000 feet, which made the aircraft passed the assigned altitude.

- The unrecovered fatigue and family issue reduced the pilot awareness during less cockpit activity, resulted in reduced the ability to monitor other traffic and misunderstanding of assigned altitude.

The KNKT reported the closest vertical separation was 400 feet at 2.7nm lateral separation and the closest horizontal separation was 1.9nm at 600 feet vertical.

The KNKT described the sequence of events:

At 0316 UTC (1016 LT6), the controller instructed the LNI pilot to descend to altitude of 10,000 feet and then set the target altitude on the controller radar system to 10,000 feet for the LNI flight. One minute later when the controller aircraft situational display (radar display) showed the LNI passed altitude of 10,300 feet, the controller instructed the BTK pilot to descend to altitude of 11,000 feet and then set the target altitude on the controller radar system to 11,000 feet for the BTK flight.

All controller instructions were read back properly by both pilots.

The BTK pilots noticed on the Traffic Collision Avoidance System (TCAS) display that there was LNI flight below and did not pay attention of its altitude. The BTK PM was aware that the aircraft was under radar surveillance control and believed that all aircraft would be controlled and separated by the controller.

The BTK PM advised the PF that the descend clearance was to altitude of 10,000 feet. The BTK pilots stated that during the descent, the cockpit condition was not busy as the flight was on direct flight to make holding over waypoint TIRUS using autopilot.

At 10:18:13 LT, when the BTK altitude passed 13,200 feet, the Flight Data Recorder (FDR) recorded that the target altitude selector on the Flight Control Unit (FCU) was set to 11,000 and one second later changed to 10,000 feet.

At 10:20:15 LT, the BTK passed altitude of 11,000 feet while the LNI was maintaining on altitude 10,000 feet and then the Short-Term Conflict Alert (STCA) of the controller radar system for LNI and BTK activated with aural and visual alert.

At 10:20:21 LT, the BTK pilot noticed TCAS Traffic Advisory (TA) on the aircraft navigation display with intruder below and maintained at altitude of 10,000 feet.

At 10:20:24 LT, the LNI pilot confirmed to the controller whether any traffic above. The controller then instructed the BTK pilot to maintain altitude of 11,000 feet with no respond.

At 10:20:35 LT, the LNI pilot noticed the TCAS Resolution Advisory (RA) warning followed with instruction “DO NOT CLIMB”, and one second later the BTK pilots received TCAS RA instruction “DO NOT DESCEND”. At about the same time, the controller instructed the BTK pilot to climb to altitude of 11,000 feet with no respond.

The LNI pilot descended the aircraft while the BTK pilot stopped the descent and maintained the aircraft at altitude of 10,500 feet.

At 10:20:40 LT, the LNI pilot advised the controller that the pilot received TCAS RA warning.

The closest distance of both aircraft laterally was about 1.9 Nm and vertically was about 600 feet at 10:20:57 LT.

At 10:20:53 LT, the BTK pilot confirmed to the controller that the instruction was to descend to altitude of 10,000 feet and the pilot had been read back the controller instruction. The TMA East controller responded that the instruction was to descend to altitude of 11,000 feet.

At 10:21:06 LT, the BTK pilot convinced to the controller that the read back was to descend to altitude of 10,000 feet and asked why the controller did not correct the read back if it was wrong. The controller advised the BTK pilot that the instruction to descend to altitude of 10,000 feet was for LNI pilot.

At 10:21:13 LT, the controller re-instructed the BTK pilot to maintain altitude of 11,000 feet and acknowledged.

There was no further significant event on both flights, the LNI landed at 1039 LT while the BTK at 1043 LT. The aircraft was undamaged and no one injured in this occurrence.

The KNKT reported the Batik Captain (48, ATPL, 15,869 hours total, 6,750 hours on type) was Malaysian, his family continued to live in Malaysia when he joined Batik Air in 2016. A medical emergency arose in his family in Malaysia prior to the occurrence, the captain had travelled to Malaysia between Dec 29th and Dec 31st 2017 to deal with the emergency. The captain was pilot monitoring on this flight. The first officer (44, ATPL, 2,812 hours total, 2,662 hours on type) was pilot flying.

The KNKT analysed the Batik pilot attention:

On 4 January 2018, two days prior the occurrence, the BTK pilots had overnight flight from Jakarta to Manokwari. The pilots signed on for duty at about 2200 LT and signed off at about 0640 LT the following day. The night duty during the window of circadian low might increase the potential of fatigue and required more time to recover.

On 5 January 2018, the pilots rested in a hotel in Manokwari. During the rest in Manokwari, there was no significant physical activity for the pilots. Both pilots did not recall the duration of sleep they took that day. On 6 January, the BTK pilots signed on the duty at 0605 LT and were scheduled for two flights with route Manokwari – Makassar – Surabaya. Considering the time for the pilot to prepare, having breakfast and transport from the hotel to the airport, the pilots might had woken up early. The Manokwari time was two hours earlier than Jakarta time and the pilot biological clock might still match with Jakarta time. Signed on at 0605 LT was considered an early morning schedule for the pilots.

All flights including the occurrence, the BTK PIC acted as PM and the BTK SIC acted as PF. The PM mentioned that his family medical issue was still in mind, especially when not many activities to be done.

The PF stated that during the occurrence flight he felt sleepy that might be caused by the overnight flight which increasing of potential fatigue and not fully recovered. The condition of the pilots woke up in early morning might reduce the time for recovery sleep.

The BTK pilots noticed that there was LNI below, however did not pay attention of its altitude. The PM was aware that the aircraft was under radar surveillance control and believed that all aircraft would be controlled and separated by the controller. The condition of relying on the controller reduced the PM awareness to monitor the other traffic communication and made the altitude clearance for LNI was not monitored.

During the descent, the cockpit condition was not busy as the flight was on direct flight to make holding over waypoint TIRUS and using autopilot. At 1018 LT, the BTK pilot was instructed to descend to altitude of 11,000 feet when the aircraft was on descend passing altitude of 13,200 feet. The previous communication made by the BTK pilot was at 1011 LT. Less activity in the cockpit within seven minutes might have made the PF felt sleepy and as for the PM this period of time reminded him to the family medical issue. These conditions reduced both pilots awareness.

The PM readback to the controller the descend clearance was to altitude of 11,000 feet, however, the advised to the PF of the clearance was to descend to altitude 10,000 feet. After clear of conflict, the PM was sure that the readback was to descend to altitude of 10,000 feet and asked why the controller did not correct the read back if it was wrong.

The cognitive or mental symptoms of fatigue can include short term memory problems, poor concentration and reduced ability to pay attention.

The slip of altitude clearance between the pilot mind and the readback was indication of decreasing human performance which most likely due to fatigue.

The PF did not make correction when the PM stated incorrect assigned altitude and set the target altitude selector on the Flight Control Unit (FCU) of 10,000 feet. This might be caused that the PF also did not listen the correct assigned altitude as the communication to the controller was not the main task of PF. The decrement of ability to pay attention was a cognitive symptom of fatigue.

The unrecovered fatigue and family issue reduced the pilot awareness during less cockpit activity, resulted in misunderstanding of assigned altitude and reduced the ability to monitor other traffic.