Murphy’s Law—anything that can go wrong, will—appeared to rule on the night of Sept. 3, 2014, when a Jet2 Boeing 737-300 was descending into the East Midlands Airport in England after a flight from Ibiza, Spain.
The aircraft, with 152 passengers and five crewmembers, experienced a chain of events—electrical problems, communications failures, smoke in the cabin, unease about the landing gear and a disabled evacuation slide—that were initiated by the lack of a formal checklist for the electrical problem and perpetuated by a series of otherwise innocuous failures that, together, created an incident that could have turned tragic, but did not.
The U.K. Air Accident Investigations Branch (AAIB), which recently released a final report on the incident, did not make any recommendation to regulators, Boeing or the low-fare carrier following the event, but Jet2 opted to deploy new procedures.
The precipitating incident involved a “loss of continuity” across a battery bus relay during the descent. The pilots noticed that the public address system (PA) had failed, and then “seemingly unconnected” electrical failures emerged, which caused equipment cooling fan, radio, weather radar, autobrake system and “terrain, reference speeds, engine fuel flow and N1 [engine speed]” icon indicators to disappear in succession from the pilots’ displays.
Similar relay issues had occurred on other airlines’ aircraft. As the result of a 1997 incident with a 737-500, the Danish Air Accident Investigation Board issued two recommendations to address the problem, one of which called for Boeing to develop a generic procedure for the loss of the battery bus bar. However, Boeing said it was unable to provide such a procedure because of the many different electrical configurations across the 737 fleet.
The crew of the incident aircraft diagnosed the problem and correctly identified the core issue as emanating from a battery bus bar, but “commented that there were no non-normal checklists in the Quick Reaction Handbook to help them,” says the AAIB. With the aircraft as configured, the pilots knew the aircraft battery on its own would continue providing power to systems for approximately 30 min.
What was not apparent under those circumstances was how simple the problem would have been to fix, given that both engine generators continued to provide power. A new battery bus-failure checklist—added to Jet2’s checklists after this event—instructs pilots to select the “Bat” (battery) setting for the Standby Power switch, and explains which “consequential” systems will be lost if the Bat setting does not fix the problem.
The Jet2 pilots ultimately declared an emergency because they were unsure if the batteries were discharging and whether other systems might become inoperative.
Meanwhile, failures began to dog the flight attendants who when attempting to brief the passengers found that the PA system was inoperable. An attendant at the front of the cabin then tried using a battery-powered megaphone, but discovered that the volume had been set to the lowest level and the volume knob was missing. “She walked through the cabin briefing the passengers a few rows at a time,” says the AAIB.
As the aircraft descended to land, the first officer moved the landing gear lever to the “down” position, but no confirmation lights verified the “down and locked” status. The pilots then discontinued the approach, asking the tower to verify the landing gear status as they flew over at 1,000 ft. Controllers could see that the nose gear was down, but said it was too dark for them to see the main gear. The pilots climbed back to 3,000 ft., where the first officer used “observation ports in the floor of the main cabin and the flight deck” to visually confirm the landing gear had extended properly, says the AAIB.
The subsequent landing was uneventful, but problems were not over for the crew or passengers. As the 737 approached its parking stand, flight attendants in the forward area of the aircraft smelled and saw smoke in the cabin, and the pilots smelled a “strong acrid smell which they ‘felt in the throat,’” the AAIB says. The pilots could not directly communicate with the flight attendants given the electrical failure.
The smoke in the cockpit and cabin was later attributed to the burning of the relay and to residue from oil and dust in the environmental system duct work that had heated up when fans failed in the cooling system.
Soon after, the captain declared a “mayday” and evacuation began via over-wing exits and emergency slides. At the right front door however, a slide twisted upon deployment and could not be used. At the left rear door, the flight attendant was at first unaware of the evacuation order and assumed there would be a normal exit onto portable stairs until she noticed passengers sliding off the back of the left wing. She then deployed the slide.
Investigators later determined that the twisting of the slide at the forward exit was not related to equipment failure but potentially to the door not being fully open when the slide deployed, causing a corkscrew motion.