B737, Burbank CA USA, 2018
B737, Burbank CA USA, 2018
On 6 December 2018, a Boeing 737-700 overran the 1,770 metre-long landing runway at destination by 45 metres after entering the EMAS. Normal visibility prevailed but heavy rain was falling and a 10 knot tailwind component existed. The event was attributed to the pilots’ continuation bias in the face of deteriorating conditions and a late touchdown on the relatively short runway. A lack of guidance from the operator on the need for pilots to re-assess the validity of landing data routinely obtained at the top of descent was identified.
On 6 December 2018, a Boeing 737-700 (N752SW) being operated by Southwest Airlines on a scheduled domestic passenger flight from Oakland to Burbank as SWA 278 overran the 1,770 metre-long landing runway 08 at destination by 45 metres after entering the EMAS and decelerating rapidly. Normal visibility prevailed but heavy rain began falling shortly before landing and touchdown occurred well past the point assumed by the non-current weather data obtained at the top of descent which was not subsequently reviewed. Damage to the aircraft was categorised as minor and there were no injuries to the 117 occupants who evacuated onto the EMAS surface using the airstairs.
An Investigation was carried out by the NTSB. Relevant data was downloaded from both the CVR and FDR and recorded ATC data were also accessed.
It was noted that the 58 year-old Captain had a recorded total of 15,410 hours total experience which included 11,350 hours on type and 4,995 hour in command on all types. The 53 year-old First Officer had a recorded total of 10,855 hours flying experience which included 5,000 hours on type. Both pilots stated that they had regularly operated into Burbank.
Twelve minutes after departing Oakland for the 50 minute flight to Burbank with the Captain acting as PF, the crew obtained the latest ATIS for their destination by ACARS. This gave the wind as from 280° at 5 knots, a visibility equivalent to 2000 metres in heavy rain and mist with BKN (broken) cloud at 1,100 feet agl. Two minutes later, they received, again via ACARS, the usual ‘Landing Data Report’ generated by the operator’s Performance Weight and Balance (PWB) system which was based on the most recent METAR and reported runway condition, the latter being reported as “good”. This report “indicated that maximum autobrake should be used for a landing on runway 08” and gave a “stopping margin” (defined in the PWB system as the distance between where the aircraft should stop plus a 15% “safety factor” and the end of the runway) equivalent to 75 metres. This stopping margin calculation was noted to be based on assumptions which included a touchdown at the equivalent of 455 metres past the runway threshold. Following the receipt of this Report, CVR data showed that the pilots “were concerned about the stopping distance given the wet runway, the tailwind conditions and the runway length” and included the remark that because of the runway conditions, the braking would be “pretty abrupt”. The PWB notification included a VREF reference landing speed of 126 KIAS and a “target speed” of 131 KIAS.
Twenty four minutes later, a Los Angeles ARTCC (Air Route Traffic Control Center) controller advised the flight of “moderate to heavy to extreme” precipitation between their current position and their destination before transferring it to the Southern California TRACON which then instructed the flight to descend 8,000 feet and to expect an ILS approach to runway 08 at Burbank. Corresponding radar vectoring and clearance establish on that approach was subsequently given with the advice that “about 10 minutes earlier, the pilot of King Air had reported a 15-knot loss of airspeed on final approach and that a corporate jet had just conducted a go-around”. About two minutes later, the controller added that the reason for the go around was the wind and after a further two minutes, the aircraft captured the ILS LOC and GS and was then transferred to Burbank TWR.
On checking in with TWR, the controller advised of “moderate to heavy precipitation” between the flight’s position and the airport, stated that the wind was from 260° at 9 knots and issued landing clearance. The controller then added that ten minutes earlier, another Boeing 737 flight had reported braking action as “good” but followed this with advice that “heavy precipitation was directly over the airport”. One minute later, the controller stated that the wind was from 270° at 10 knots and then a minute later gave it as was from 270° at 11 knots. The First Officer asked if the Captain wanted “to call it (11 knots) good” and received an affirmative response.
During his subsequent interview, the Captain stated that cloud was exited at “about 400 feet agl” and that at the procedure DA (which was equivalent to 280 feet aal), he had disconnected the A/T and AP and continued to descend by reference to the PAPI. At about this time the CVR recorded the Captain stating, “we need some kind of lights” followed by “I got some lights”. In interview, the Captain stated that at about 5 feet agl in the subsequent landing flare “the rain intensity picked up and visibility decreased a little bit”.
Analysis of the recorded flight data found that the runway threshold was crossed at 54 feet agl at 137 KIAS, 11 knots above the applicable VREF of 137 knots (the ILS procedure gave a non standard threshold crossing height of 60 feet when following the 3° GS). Ten seconds later, the aircraft touched down at 126 KIAS and at a groundspeed of 151 knots at a position equivalent to 763 metres past the runway threshold. This position was consistent with the TWR controller’s observation that touchdown had occurred “near the taxiway D7 intersection with runway 08” which was about 2,600 feet from the runway threshold.
FDR data showed the aircraft autobrakes, speedbrakes and ground spoilers were activated at touchdown and the thrust reversers were fully deployed three seconds later. Brake pressure reached its maximum of 3,000 psi in about 6 seconds. Sounds of impact began to be recorded on the CVR 21 seconds after touchdown. The Captain stated that having judged that deceleration with reverse thrust and maximum autobrakes was insufficient, he had begun “blending in manual brakes”. However, FDR data showed that manual braking had been applied to override the autobrakes immediately after touchdown although it was subsequently calculated that this non standard action had not affected the achievement of maximum braking.
FDR data showed that for the first 17 seconds of the landing roll, a rate of deceleration was in the range 0.3/0.4 g and the groundspeed decreased from 151 knots to 50 knots. During the next 5 seconds it decreased to between 0.15 and 0.20 g as the aircraft “crossed over the left edge of the grooved runway onto smooth pavement“ by which time, the groundspeed had reduced to about 24 knots after which the deceleration rate increased to about 0.6 g when the EMAS was entered.
It was noted that whilst the tailwind component reported to the crew in flight had been up to 11 knots, the one calculated and consistent with the aerodrome ASOS which shortly after the accident aircraft touchdown had recorded a 2 minute mean wind from 279°at 13-18 knots.
Why It Happened
Although both pilots were used to landings on the short runway at Burbank, it was found that expectations for the intended landing had not taken adequate account of the environmental challenges presented by the deterioration in weather conditions and the relevance of these to the challenges of landing on a short runway.
Detailed analysis of the available data found that the higher-than-expected tailwind, the later than normal touchdown and the faster-than-nominal approach speed had “all contributed to the overrun, with the late touchdown being the most significant contributor”.
It was considered that the decision to continue the approach to a landing “was consistent with a psychological concept referred to as plan continuation bias”. Despite hearing a surface wind report which meant an 11 knot tailwind component, the approach was continued on the basis that it would be “right on the edge” of the operator’s 10-knot limit. A go around to allow time to reassess the situation, which it was noted would have been consistent with operator guidance which instructed pilots to commence a go around if an intended landing “appeared unsafe” was not considered. The intentional decision to continue and land was considered “inappropriate”.
The Investigation formally recorded that the Probable Cause of the accident was:
- The flight crew members’ decision, due to plan continuation bias, to continue the approach despite indications of windshear and a higher-than-expected tailwind.
- the flight crew’s misperception of the aircraft touchdown point, which was farther down the runway than the crew assumed because of the faster-than-expected groundspeed.
A Contributory Factor to the accident was identified as “Southwest Airlines’ lack of guidance to prompt flight crews to reassess operator-provided landing data when arrival weather conditions differ from those used in the original landing data calculation”.
Safety Action taken by Southwest Airlines as a result of the accident and known to the Investigation included, but was not limited to, the issue of a ‘Safety Bulletin’ on relevant matters almost a year later. This included the requirements that:
- the “touchdown zone” must not exceed the first one third of the runway length
- a go-around must be executed if the aircraft touches down beyond the most restrictive of the following distances:
- the first one-third of the available runway length.
- the first 3,000 feet (equivalent to 914 metres) of the available runway length.
- 1,500 feet (equivalent to 457 metres) plus the planned PWB System-computed stopping margin.
This Bulletin was subsequently incorporated into an OM revision which took effect in March 2020. The 2020 pilot recurrent training both in ground school and through “line-oriented evaluation briefings” was enhanced to improve pilots’ appreciation of the issues relevant to landing safely.
The Final Report was published on 20 July 2022. No Safety Recommendations were made.