AT75 / B739, Medan Indonesia, 2017
AT75 / B739, Medan Indonesia, 2017
On 3 August 2017, a Boeing 737-900ER landing at Medan was in wing-to-wing collision as it touched down with an ATR 72-500 which had entered the same runway to depart at an intermediate point. Substantial damage was caused but both aircraft could be taxied clear. The Investigation concluded that the ATR 72 had entered the runway at an opposite direction without clearance after its incomplete readback had gone unchallenged by ATC. Controllers appeared not to have realised that a collision had occurred despite warnings of runway debris and the runway was not closed until other aircraft also reported debris.
On 3 August 2017, a Boeing 737-900ER (PK-LJZ) being operated by Lion Air on a scheduled domestic passenger flight from Banda Aceh to Medan as JT197 touched down on runway 23 at destination in normal day visibility, and shortly after, collided with an ATR 72-500 being operated by Wings Airlines on a scheduled domestic passenger flight from Medan to Meulaboh as IW1252. Both aircraft sustained substantial damage but none of the 223 occupants were injured and both aircraft were taxied clear of the runway. ATC were advised of resultant debris on the runway but allowed the runway to remain in use until other aircraft subsequently using it had also reported the presence of debris after which it was closed.
An Investigation was carried out by the Indonesian National Transportation Safety Committee (NTSC) - the Komite Nasional Keselamatan Transportasi (KNKT). Data from the FDR and 2 hour CVRs of both aircraft were successfully downloaded and used to support the Investigation. ATC recorded data including from the TWR cab CCTV were also available.
The 45 year-old Captain of the 737, who had been PF at the time of the accident, was recorded by the Investigation as having 8,200 hours total flying experience which included 2,296 on type. The 23 year-old First Officer on the 737 was recorded as having 500 hours total flying experience, all of which were on type. The 59 year-old Captain of the ATR72, who had been PF at the time of the accident, was recorded by the Investigation as having 13,006 hours total flying experience of which 624 hours were on type. The 23 year-old First Officer of the ATR 72 was recorded as having 263 hours total flying experience of which 109 hours were on type. A third "observer pilot" was occupying the supernumerary seat in the ATR 72 flight deck.
Three controllers were in position at Medan Aerodrome Control at the time of the collision, a 34 year-old Supervisor employed there since 2014, a 24 year-old TWR controller employed there since 2017 and a 23 year-old GND controller employed there since 2016.
The departure of the ATR 72 was slightly delayed because of a problem with passenger baggage handling. On request, the GND controller issued a clearance to taxi from the apron via taxiways 'U', 'T', 'B' and 'C' to the full length holding point for runway 23 on taxiway 'C' (see the aerodrome layout illustration below). Whilst the aircraft was still with GND, the 737 had called TWR to report on the ILS LOC for runway 23 and had been cleared to land. Almost immediately after this exchange had been completed, the GND controller transferred the ATR 72 to TWR with the instruction to continue and hold short of runway 23. On checking in with TWR, the ATR 72 crew requested takeoff from the taxiway D intersection, which was an RET for runway exits in the opposite 05 direction, in order to expedite their departure following the delay at the gate and this was approved as permitted by ATC procedures.
Whilst the ATR 72 crew were actioning their ‘Before Takeoff’ Checklist, its Captain advised the TWR controller that they were approaching runway 23 but when the controller responded by asking if they were ready for an immediate departure, no reply was received. He asked the same question again and this time the Captain confirmed that they were - although at this point, the pre-takeoff ‘cabin secure’ report had not been received from the cabin crew.
TWR then issued a conditional clearance to line up behind the Lion Air on short final once it had passed and also advised an amendment to the Departure Clearance to route direct to destination after takeoff. Only the amendment to the Departure Clearance was read back - by the First Officer - and this significantly incomplete read back was acknowledged without comment by the TWR controller just as the 737 was passing 500 feet agl on final. Another aircraft then reported on final approach to runway 23 and was instructed to continue with the advice that there was traffic to depart ahead but no mention that they were No 2 to land. CVR data showed that the ATR72 Captain was becoming concerned at the absence of a ‘cabin secure’ report and after monitoring the Cabin Crew PA until completion, the First Officer instructed the cabin crew to take their seats for takeoff without further reference to the Captain. At this time, the 737 was passing 200 feet.
As the 737 passed 40 feet agl, the 737 First Officer alerted the Captain to the presence of an aircraft close to the runway and after no response repeated this at 10 feet agl. Two seconds after main gear touchdown, the First Officer advised the Captain that the aircraft previously seen was now entering the runway which was acknowledged. Four seconds later, the sound of impact was recorded on the CVR of both aircraft.
The first transmissions after the collision came from the 737 Captain who was unable to get any meaningful response from the TWR and after declaring a PAN without any detail, was instructed to vacate the runway via RET taxiway ’G’ towards the end of the runway. The TWR controller then instructed the next aircraft on approach to discontinue it and approved the request from its crew to orbit left. This was followed by an instruction to the 737 to contact GND where the controller instructed the aircraft to taxi to Parking Bay 31. Whilst taxiing in, the 737 Captain and First Officer confirmed that they had both heard the ATR72 being told to line up after they had landed. They also advised ATC that debris hazardous to other aircraft was likely to be on the runway and repeated this two minutes later after suspecting that the controller who acknowledged the first call had taken no action.
The ATR 72 crew, having mutually confirmed their belief that they had been cleared for an immediate takeoff and observing the significant damage to their aircraft, then contacted TWR and were told to ‘Standby’ whilst the Tower Supervisor, who had taken over the TWR controller position, instructed the 737 to contact GND. They were then given a further amendment to their Departure Clearance to which they responded they were “unable to depart” and were told to “hold on the runway”. One minute and 20 seconds after this, the Tower Supervisor issued a takeoff clearance to the ATR 72 to which its crew again stated that they were unable to depart and added that they needed to return to the apron for damage to be inspected. This time clearance to exit the runway via taxiway ‘E’ and then contact GND was given.
The ATC Supervisor who had taken over the TWR position appeared not to have understood that a collision had occurred despite being told of debris on the runway and having been able to observe two damaged aircraft taxing to the apron not far from the TWR cabin. Another aircraft waiting for a full length runway 23 departure on taxiway ‘C’ advised its position and that it would remain clear of the runway to TWR but after initially being told to standby, it was then instructed to enter and line up. Several aircraft were subsequently permitted to land on the debris-contaminated runway before a runway inspection had been performed which was not until 30 minutes after the collision had occurred. Recorded events immediately following the collision do not make any mention of the airport RFFS being advised of or otherwise aware of it having occurred or attending either the scene or the two aircraft during or after their taxi to the apron after the collision.
It was noted that the weather conditions at the time of the collision were good and the only relevant feature was that the variable easterly wind may have created a small tailwind component for traffic using runway 23. Although the 737 was carrying out an ILS approach, the lowest cloud over the aerodrome throughout the period was recorded as 1800 feet.
Damage to the Aircraft
The damage to the two aircraft (see the illustrations below) was indicative of the outer left wing of the 737 having cut off the outer right wing of the ATR 72 and part of the detached wing had then penetrated the forward right fuselage of the ATR in the vicinity of the flight deck. Approximately 3.4 metres of the outer left wing leading edge of the 737 was damaged as was approximately 3.4 x 0.4 metres of slat number 1 on this wing, with a piece of this of approximate size 65 cm × 40 cm becoming detached and falling onto the runway. Approximately 2.8 metres of the outer right wing of the ATR 72 was detached from the aircraft with most of it falling onto the runway but a small part was swept forward and penetrated the forward right fuselage below and just forward of the flight deck.
Most of the significant detached debris from the two aircraft was found on the right hand side of the runway 23 centreline. It was noted that the 3750 metre-long runway was 60 metres wide and that the combined wingspan of the 737 (35.79 metres) and the ATR72 (27.05 metres) was slightly greater than the width of the runway. However, there was no evidence that a runway excursion had occurred.
Findings arising from the assembled evidence
A number of issues were identified by the Investigation in a review of the assembled evidence. These included, in summary, the following:
- ATR 72 Crew Traffic Awareness
The ATR 72 crew were unaware that the 737 had been cleared to land because they were still on the GND frequency when this clearance was given. The acceptance by ATC of the ATR 72 request to enter the departure runway at taxiway ‘D’, an RET for aircraft exiting runway 05, meant that the crew could not easily check that the final approach was clear visually before entering the runway. The Investigation could also not find any procedure requiring ATC to issue a caution to aircraft entering a departure runway using an RET primarily intended for traffic exiting in the opposite direction, especially when a departing aircraft is subject to a conditional clearance to enter the runway after a landing aircraft.
- The effects of the delay to the ATR 72
As well as requesting departure from the ‘D’ RET, the Captain of the ATR 72 also advised TWR that his aircraft was ready for departure whilst it was still taxiing out even though the cabin crew were still some way from completing their pre takeoff duties. Both actions might be taken as an indication that he wished to expedite the departure because of delay to the flight leaving the gate.
- 737 Crew Traffic Awareness
The 737 crew heard the ATR 72 crew confirm that they could accept an immediate departure and on hearing this, the Captain had asked the First Officer to remind TWR that they were on short final. However, he cancelled this request when they heard that the clearance for the ATR 72 was to enter runway after they had landed
- Misunderstanding of the conditional clearance by the ATR crew
The conditional clearance to the ATR 72 to line up behind the landing aircraft was combined with an amended departure clearance and was delivered at a rate of speech faster than the recommendations given in the ICAO Manual of Radiotelephony Doc 9432. It was also issued without confirmation as to whether the crew were aware of the 737 on short final to land which was not in accordance with State regulatory requirements. The ATR 72 First Officer, who had very little experience as a pilot, was unable to receive all the information of the clearance and therefore read back only the its last sentence about the change to the departure clearance. This significantly incomplete read back was not challenged by the controller who “assumed that the pilot had acknowledged the clearance properly and considered that immediate action to correct the discrepancies in the pilot read back was unnecessary”. It was also noted that the ATR 72 crew would have heard the TWR controller advising another aircraft behind the 737 on short final that there was an aircraft (the ATR 72) to depart ahead of their landing. This call was intended to make the other arriving aircraft crew aware that there would be a departure between the landing 737 and their own subsequent landing, but it was considered that it might also have encouraged the ATR 72 crew to believe that they were number one in the sequence since - as noted - they were unaware that the 737 was ahead of the other aircraft and had been cleared to land.
- Use of Conditional Clearances
It was noted that the procedure for issue of conditional clearances at Medan did not describe in detail the content of the corresponding State regulatory requirements and referred only generally to ICAO PANS-ATM Doc 4444 for examples of the phraseology and detailed the requirements for conditional clearances. Although the TWR controller on duty had been trained in the use of conditional clearances when studying to become a controller in Aviation College in 2015, there was no evidence that the use of conditional clearances had been the subject of any training since this time.
- Monitoring of Traffic by ATC
The applicable requirements for aerodrome/tower controllers include maintaining a continuous watch on all flight operations on and in the vicinity of an aerodrome. The movement of the ATR 72 contrary to its clearance was not monitored by the controller due to his other controlling activities and his assumption that the ATR 72 had satisfactorily acknowledged the clearance to line up only after 737 had landed.
- Actions open to the 737 crew on observing the incursion
The 737 was passing 40 feet agl when the First Officer warned the Captain that the ATR 72 had crossed the Holding Point. The distance between the two aircraft at this time was estimated as just under 650 metres. Taking into account an expected height loss shortly after a go-around is initiated, a go around commenced at this time would most likely not have avoided the collision and indeed may well have resulted in more severe circumstances. Instead, the 737 Captain responded to the incursion by turning his aircraft 2° to the right three seconds after touchdown and whilst a wing-to-wing collision was unavoidable, this action to deviate to the right of the runway centreline had clearly reduced the severity of the collision.
- ATC response to the collision
The location of the collision was in the 11 o’clock relative direction from the TWR building and the controller may have been unable to clearly observe what happened because the impact point was on the far side of the ATR 72. However, ATC were told that there was debris on runway by the 737 crew and by another departure aircraft crew and the crew of an aircraft that was subsequently permitted to land on the runway. Applicable ATC procedures required that if there was any doubt about the safety of any part of the movement area, the Airport Runway and Accessibility Unit must be asked to inspect that area and ATC must suspend use of the area until the inspection result indicates that the relevant part of the movement area is safe for operation especially if Foreign Object Debris (FOD) is suspected as a result of “unusual aircraft operation”.
- Controller Training Issues
The last simulation of the handling of “unusual aircraft operations” for the Tower supervisor on duty was conducted in 2005 and for the Tower controller on duty it was conducted during their Aviation College studies to become a controller in 2015. The last performance check at the Medan Unit did not discuss unusual conditions or conditional clearance requirements. It was considered that information stored in the memory for long periods without use or rehearsal was very likely to be forgotten. It was also noted that serial inappropriate use conditional clearances over time without correction might well have led the controller to believe that his procedure was correct.
The Contributory Factors which facilitated the collision were formally documented as “misunderstanding of the communication of a conditional clearance to enter runway by the ATR 72 pilots who were not aware that the 737 had received a landing clearance and the fact that ATC did not notice the ATR 72 entering the runway”.
At the completion of the Investigation, no significant Safety Action beyond that taken by the parties involved in the immediate aftermath of the accident and recorded in the Preliminary Report was documented.
The five initial Safety Recommendations included in the Preliminary Report were not also included in the Final Report and are reproduced below:
that AirNav Indonesia ensure that air traffic controllers comply with the requirements of DGCA AC170-02 and ICAO Doc 9432 if a conditional clearance is used.[04.A-2017-25.1]
that AirNav Indonesia ensure that air traffic controllers listen to (all) read-backs in order to ascertain that the clearance or instruction (given) has been correctly acknowledged by the pilot and must take immediate action to correct any discrepancies revealed by the read back. [04.A-2017-25.2]
that AirNav Indonesia ensure that its air traffic controllers properly mitigate any hazard on the runway without delay (and specifically do so) prior to the issue of any takeoff or landing clearance. [04.A-2017-25.3]
that Wings Airlines ensure that all its pilots read back or acknowledge clearances or instructions, including conditional clearances, in such a manner that clearly indicates that they have been understood and will be complied with.[04.O-2016-25.4]
that Wings Airlines ensure that all its pilots understand and comply with air traffic controller instructions or clearances and that they must seek clarification when there is any uncertainty in respect of an instruction or clearance. [04.O-2016-25.5]
The Final Report was released on 19 April 2018 and three additional Safety Recommendations followed on 26 April 2018 as follows:
that AirNav Indonesia and the Halim Airport Operator inform aircraft operators to initiate takeoff from the displaced threshold of runway 24. [04.A-2016-57.1]
that the Directorate General of Civil Aviation review a requirement for all aircraft movement on the manoeuvring area including the movement of aircraft under tow to communicate with the air traffic controller on the same frequency. [04.R-2016-55.1]
that the Directorate General of Civil Aviation develop a procedure for vehicles and aircraft on the manoeuvring area which requires them to be equipped with serviceable lights visible to the air traffic controller. [04.A-2016-56.1]
- Runway Incursion
- Conditional Clearance Runway Incursions
- Conditional Clearance
- Visual Scanning Technique
- Controller Detection of Manoeuvring Area Conflicts
- Cross-checking Process
- Crew Resource Management
- Guidelines for Dealing with Unusual/Emergency Situations in ATC
- Foreign Object Debris (FOD)
- Post-Incident Airport Operations
- ATCO Training Overview