While Lithuanian investigators have yet to reach conclusions on last November’s fatal Boeing 737-400 freighter crash near Vilnius, they are likely to consider a possible switch mix-up that has previously occurred on other 737 flights.
Preliminary indications point to the Swiftair jet’s losing height and descending beneath the glideslope after its flaps failed to deploy during the approach.
Hydraulic pump switches for the ‘B’ circuit, which drives flap extension, were found in the ‘off’ position in the cockpit wreckage. Flight-recorder evidence suggests these switches were turned off during preparation for descent.
The inquiry has still to confirm – and explain – this apparent de-activation of the hydraulic pumps.
But it states that the crew, during preparation for descent, discussed the expectation of icing conditions.
The captain, who was the monitoring pilot, subsequently referred to the effect of anti-ice on the descent profile, and the crew discussed activating anti-ice a couple of minutes later.
Just as the Vilnius area controller was instructing the crew to contact Vilnius approach control, an “audible double click” was captured on the cockpit-voice recorder.
The inquiry has not specified the nature of the double click, but it states that both the engine-driven pump and the electric pump for the hydraulic ‘B’ circuit turned off a couple of seconds later, and autopilot ‘B’ also disengaged.
US aeronautical agency NASA hosts a database of incidents which are voluntarily contributed by pilots and other personnel involved.
Although these contributions are anonymised, the database contains at least half-a-dozen occurrences in which hydraulic pumps on 737s have been erroneously turned off after their switches were mistaken for the engine anti-ice switches.
These two sets of switches – located on the overhead panel – are situated adjacent to one another.
One narrative from May 2024, from a 737 captain, says they called for a co-around after noticing “no flaps had extended” after configuring for final approach.
“I was in the process of looking for the appropriate flap non-normal procedure when I happened to pull up the hydraulic system page and noticed that we had lost the ‘B’ hydraulic system,” it states.
The first officer stated that, during the descent, they had reached up to “[where] I believe the engine anti-ice switches were” and turned them off. The crew then noticed a number of issues with certain systems, including disengagement of autopilot ‘B’.
After using the alternate flap system to land, the crew was subsequently advised by maintenance personnel that there was no technical problem with the hydraulic system.
“However, one possibility that could have caused the loss of hydraulic ‘B’, was the accidental placing of both hydraulic B pump [switches] to the off position,” the narrative states.
It adds that the “proximity and similarities” of the engine anti-ice and hydraulic ‘B’ switches “could have [led] to mistakenly turning off the hydraulic ‘B’ pumps”, particularly given the distraction of weather conditions.
Another incident entry from January 2018 refers to a departure in “extremely busy” circumstances, with rapidly-changing weather.
As the aircraft climbed above icing conditions, according to the first officer: “I verbalised, ‘engine anti-ice coming off’ and placed my hand on what I thought were the [engine anti-ice] switches…I turned two switches off.”
The first officer acknowledges that they did not “verify and monitor” the action, and the crew received hydraulic system warnings a short time later. “We soon realised I had turned off both [hydraulic system ‘B’] pumps instead of the [engine anti-ice],” the narrative states.
Similar errors, in various phases of flight, are listed in other 737 incident entries in the database.
While the database narratives mainly feature occurrences in which the hydraulic system was turned off instead of engine anti-ice, at least one instance involved an intention to turn anti-ice on.
The Lithuanian crash inquiry states that – at the time of the hydraulic system de-activation – the captain had been attempting to reach Vilnius approach control, but experiencing difficulty because he was using the wrong radio frequency.
After successfully contacting approach control, the inquiry says, the captain noted that the autopilot had disengaged at about the same time as he had tried to switch on engine anti-ice.
“The crew did not discuss the matter further,” it states, and points out that the anti-ice switches were not turned on.
As the 737 neared touchdown, with its flaps still retracted, it began sinking below the glideslope and, despite a late attempt to conduct a go-around, the 737 crashed less than 1,600m from the threshold of Vilnius’s runway 19.
Two pilots and two other personnel were on board; the captain did not survive.
Although the aircraft took off from Leipzig at 03:08 on 25 November, and the accident occurred at 05:28, the inquiry has yet to determine the extent to which fatigue might have contributed to the crew’s decisions and actions.
