German investigators have determined that a complex interaction of hydraulic, mechanical and icing problems resulted in a DHL Air Boeing 757-200 freighter’s cargo door opening as it climbed out of Leipzig.

Several separate illuminated status lights on the door control panel normally extinguish in turn as the door is closed, then latched, then locked.

Ahead of the 13 February 2021 departure, the aircraft’s captain, who was closing the door, testified that all the lights on the control panel had turned off, and the door was flush with the fuselage. The main cargo door warning light in the cockpit also extinguished, according to the crew.

But as the aircraft took off for Frankfurt, the cargo door started opening at a height of around 5,100ft. The crew returned to Leipzig and landed safely, with the door fully open.

Each part of the door-closing process is driven by a staged increase in hydraulic pressure.

Hydraulic actuators initially lower the door into place. At 2,000psi the eight door latches on the bottom of the door engage with spools in the cabin floor. Once pressure reaches 2,400psi, eight lock-pins are inserted into the latches. Finally, at 2,800psi, the vent doors close and the process is complete.

But German investigation authority BFU believes air in the hydraulic system caused a sequencing problem.

“This fault could lead to the lock-pins being moved into the locked position during closing of the main cargo door even though the latches were not yet in the latched position,” it states.

The inquiry points to scratch damage to latches, caused by the lock-pins, as evidence.

“A test was used to show that, with this kind of hydraulic sequencing problem, the lock-pins block the latches in a position which allow a complete opening of the main cargo door in flight,” it adds.

The aircraft had already experienced a hydraulic sequencing problem on the door three months earlier, in November 2020, and underwent maintenance including bleeding of the system.

BFU says that, between 2016 and 2021, the operator received reports of 14 cases of sequencing problems, all of which occurred in the cold season from November to March. Nine of the 26 aircraft in the 757-200 converted freighter fleet were affected.

Inspection of the operator’s 757s after the Leipzig incident discovered six aircraft had similar scratch damage to the door latches – five of these aircraft had previously experienced hydraulic sequencing issues.

The cold weather added a further complication. Prior to loading, the aircraft had been parked in overnight temperatures of minus 13°C to minus 16°C with freezing fog.

BFU believes ice build-up on micro-switches for the door status lights resulted in their indicating “permanently” that the door was both latched and locked, regardless of the actual positions of the latches and locks.

The inquiry says an ice layer of just 0.8mm would have been sufficient for this.

With the main door apparently closed and flush, and the micro-switches incorrectly indicating it was latched and locked, only the vent door status remained as a safety barrier.

BFU says the interlock bars of the vent doors should have prevented them closing if the lock-pins were not in the correct position.

But a deformation test on the interlock bars “proved it was possible to completely close the vent doors in this configuration”, says the inquiry, and cause the vent door status lights to extinguish.

“Even from the outside, no gaps between the vent doors and the main cargo door were visible,” it states. “The vent doors were flush with the main cargo door. This indicates that the interlock bars are too soft and too elastic.”

The combination of circumstances meant all the door status lights were off and the crew had no indication that the cargo door was not firmly closed, latched and locked.

BFU highlights strong similarities with a previous incident in Magadan, Russia, involving a Yakutia 757 freighter which had been parked in temperatures of minus 20°C in December 2014.

Although the status lights extinguished, the aircraft’s cargo door opened as it climbed away during departure, prompting the crew to return to the airport.

DHL Air subsequently adopted procedures for checking the latches through viewing ports, and instructed crews to check that warning lights were illuminated prior to door closure. The European Union Aviation Safety Agency published a bulletin describing inspections for ice contamination on the door and visual checks on the door locks.

BFU is also recommending modification of the system to avoid malfunctions from air in the hydraulic circuits, the door switches and sensors, and the vent door interlock bars.