Amazon, the multi-trillion dollar e-commerce monolith, seemingly cheaped out on on a key feature installed on its six-propeller delivery drones. Predictably, this backfired almost immediately.

On a (lightly) rainy December day at the company’s testing range in Oregon, not one but two Prime Air drones suddenly stopped spinning their propellers mid-flight and plummeted some 200 feet to the ground. The crashes, which destroyed both aircraft, happened within minutes of each other. 

And now, Bloomberg reports, we know why. According to documents from the National Transportation Safety Board, bad readings from the drones’ onboard lidar sensors led the drones to believe they had already landed. Their software, thinking it was on solid ground, cluelessly cut off power to the propellers.

But that’s not all. After Amazon decided to remove them, the drones no longer had backup sensors that were equipped on older versions. And these probably would’ve prevented the drones from shutting down, per Bloomberg‘s sources.

Amazon denies this line of thinking.

“Bloomberg’s reporting is misleading,” an Amazon spokesperson told the newspaper. “Statements that assume that replacing one system with another would have prevented an accident in the past is irresponsible.” 

Per the NTSB documents, a botched software update made the lidar sensors more susceptible to being thrown off by rain. Lidar is short for light detection and ranging, a form of technology that uses lasers to scan surroundings in a similar manner to radar.

A glitch may have been the main reason, but it definitely sounds like it could’ve been easily avoidable had Amazon kept the redundant system from its previous drone, the MK27. 

The backup sensors came in the form of two metal prongs on the bottom of the drone called squat switches. When the drone lands, the switches are depressed, providing confirmation that it’s on solid ground. A source told Bloomberg that the MK27 drone’s software was originally designed to confirm a landing only when two of its three sensors agreed.

This was removed with the MK30, and it’s unclear why. Reducing costs could be one reason, and it’s not uncommon to remove redundant systems to streamline a device.

It could also be because of a shift in how Amazon intends to deliver packages with its drones, according to a Bloomberg source. The MK27 was designed to make deliveries by landing in a customer’s yard, with enclosed propellers to make them safer. The MK30 moved away from this and drops packages from around a dozen feet in the air.

This is far from the only setback Amazon has faced during its drone development, which it first announced back in 2013. In 2021, a drone crash sparked an acres-wide blaze in Oregon, and outside of incidents like that, many residents living where the drones are being trialed simply find them annoying. After the latest December SNAFU, Amazon halted future experiments for months — though it maintained that the crashes weren’t the “primary reason” — and only recently lifted the pause.

Overall, development has been sluggish and the project remains years behind schedule. Currently, Amazon is only carrying out drone deliveries in College Station, Texas, and Tolleson, Arizona.

More on Amazon: The NYPD Is Sending Drones to the Sites of 9-1-1 Calls