B739, vicinity Atlanta GA USA, 2017
B739, vicinity Atlanta GA USA, 2017
On 29 November 2017, a Boeing 737-900 on an ILS approach at Atlanta became unstable after the autothrottle and autopilot were both disconnected and was erroneously aligned with an occupied taxiway parallel to the intended landing runway. A go-around was not commenced until the aircraft was 50 feet above the ground after which it passed low over another aircraft on the taxiway. The Investigation found that the Captain had not called for a go around until well below the Decision Altitude and had then failed to promptly take control when the First Officer was slow to begin climbing the aircraft.
On 29 November 2017, a Boeing 737-900 (N852DN) being operated by Delta Air Lines on a scheduled domestic passenger flight from Indianapolis to Atlanta was making an ILS approach to destination in day IMC when it became unstable and after deviating left of track then continued below Decision Altitude and having aligned with the landing runway’s parallel taxiway, the go around was only commenced from 50 feet agl causing proximity to another aircraft on the taxiway.
An Investigation was carried out by the NTSB supported by downloaded FDR data and relevant recorded ATC data.
It was noted that the 57 year-old Captain had recorded a total of 9,440 hours flying experience which included 1,780 hours on type and 7,777 hours in command on all types. The 43 year-old First Officer had a total of 3,000 hours flying experience which included 593 hours on type.
The First Officer was acting as PF and the crew were aware by ACARS message that the final stages of the destination approach were unlikely to be made with visual reference available until near to the instrument approach decision altitude. Relatively poor surface visibility - 200 metres in mist with fog patches and an overcast with a 300 feet base were being reported.
Once in contact with Atlanta TRACON, the flight was cleared for an ILS approach to runway 09R. FDR data showed that the AP was disconnected with about 3½ miles to go and passing about 1,230 feet agl in VMC which the First Officer subsequently stated had been because “he wanted to practice hand flying the approach”. It was noted that this was actively encouraged by the airline “in appropriate conditions” which were specified.
Soon after this, IMC was entered and the aircraft began to drift to the right of the LOC. At 500 feet agl, the First Officer then also disconnected the A/T which the Captain subsequently stated had not been previously briefed and, as with the earlier AP disconnection, had been a surprise to him. He added that when the A/T was disconnected, “things got squirrely.”
At 300 feet agl, the First Officer began to correct the LOC deviation but having returned to the LOC, this corrective track was continued and when the aircraft reached the centreline at 300 feet agl, the track was maintained and the deviation left of the LOC which followed took the aircraft towards the extended centreline of parallel taxiway ‘N’ which was about 200 metres north of the landing runway centreline.
The descent was continued below the procedure DA with visual reference available but the drift to the left of the landing runway extended centreline was not corrected. At 120 feet agl, by which time the aircraft had almost reached the taxiway ‘N’ extended centreline and was about 400 metres from the end of it, the Captain ordered a go around. FDR data showed that the takeoff/go-around switch on the thrust levers was pressed and the takeoff/go-around engage parameter transitioned to takeoff/go-around. Three seconds later, the TWR controller instructed the aircraft to go around which the Captain responded with “Delta 2196 is on the go” and a few seconds later, the controller added “it looks like you’re over the taxiway”. The aircraft had continued to descend to about 50 feet above the western end of taxiway ‘N’ before it began to climb and was calculated to have been beyond full scale ILS LOC deflection at the time the go around was commenced.
Surface radar data showed that another Delta Air Lines aircraft, an MD-88, had been moving westwards on taxiway ‘N’ when the go around occurred and the two aircraft came within 87 metres horizontally and 257 feet vertically. The TWR controller provided heading instructions to the aircraft on go around before transferring it back to radar where vectors to an ILS approach to runway 10 were provided and the rest of the flight was without further event.
Why It Happened
According to the aircraft operator’s FOM procedures, the ILS approach initially flown became unstabilised when the First Officer failed to track the ILS LOC whilst controlling the aircraft manually. These procedures stated that “if a stabilised approach could not be established and maintained, pilots were to initiate a go-around and not attempt to land from an unstable approach”. These procedures also stated that an approach should only continue below the applicable DA if “the aircraft is in a position from which a normal approach to the runway of intended landing can be made” which was not the case. The delay in the Captain’s call for a go around was also a breach of procedures since the circumstances required one at that time and the lack of the required immediate response to this call was then followed by a delay in the Captain taking control which led to the proximity to the taxiing aircraft being much reduced as the go around was commenced.
An examination of the rostering and prior rest/sleep patterns of both pilots found no evidence that their performance might have been affected by fatigue and no other explanation for their inadequate performance was found. FOM content on pilot responsibilities regarding a go around were clear and “stated that the pilot flying and the pilot monitoring are responsible for monitoring the approach and that, if either pilot observes flight parameters outside the stabilised approach criteria, a go-around must be called and honoured”.
It was also found that the aircraft operator’s encouragement for pilots to maintain their manual approach flying skills was qualified in the FOM by the requirement that “manual flight (for the primary purpose of maintaining proficiency) should normally be exercised under suitable environmental and low workload conditions” and in the 737 OM by the requirement that “the AP should be used for ILS category 1 approaches when the RVR is below (the equivalent of) 1220 metres or the visibility was below (the equivalent of) 1200 metres”. The weather report received by the crew a few minutes before commencing the approach included RVR and visibility figures which were both low enough to require the AP to be used for a Cat 1 ILS approach.
The Investigation determined that the Probable Cause of the investigated event was "the flight crew members’ failure to properly monitor the aircraft flightpath, which caused the approach to become unstabilised and resulted in the aircraft descending below the decision altitude while misaligned with the localiser course".
Two Contributory Factors to the event both of which caused the aircraft to come within about 50 feet vertically of an occupied taxiway were identified as:
- The First Officer’s delay in setting go-around thrust after the Captain called for the go-around.
- The Captain’s failure to take control of the airplane after go-around thrust was not immediately set.
The Final Report was published on 10 May 2022. No Safety Recommendations were made.