A pair of Amazon.com Inc. package delivery drones were flying through a light rain in mid-December when, within minutes of one another, they both committed robot suicide.

Shortly before 10 a.m. on Dec. 16, the first Prime Air drone dropped its package at a dummy residence at the company’s rural Oregon testing range, then flew back to an asphalt pad to begin its landing sequence. Instead, some 217 feet (66 meters) in the air, the aircraft cut power to its six propellers, fell to the ground and was destroyed. Four minutes later and 183 feet over the taxiway, a second Prime Air drone did the same thing. 

Not long after the incidents, Amazon paused its experimental drone flights to tweak the aircraft software but said the crashes weren’t the “primary reason” for halting the program.

Now, five months after the twin crashes, a more detailed explanation of what happened is starting to emerge. Faulty readings from lidar sensors made the drones think they had landed, prompting the software to shut down the propellers, according to National Transportation Safety Board documents reviewed by Bloomberg. The sensors failed after a software update made them more susceptible to being confused by rain, the NTSB said. 

Amazon’s latest drone also lacks a backup sensor present on the previous iteration that might have prevented the incidents, according to three people familiar with the situation.

In a statement, Amazon took issue with this interpretation of events. “Bloomberg’s reporting is misleading,” spokesperson Kate Kudrna said. “Statements that assume that replacing one system with another would have prevented an accident in the past is irresponsible.” She said Amazon had found ways to replicate the removed sensors and that the drone is safer and more reliable than its predecessors. The design also meets Federal Aviation Administration requirements and has been reviewed and validated by regulators, she said. 

In 2013, Jeff Bezos, then Amazon’s chief executive officer, unveiled plans to deploy swarms of delivery drones. When asked how long that might take, he said they could be delivering packages to customers in five or so years.

That didn’t happen, and the effort has since been plagued by technical challenges and crashes, including one in 2021 that set a field ablaze at the company’s testing facility in Pendleton, Oregon. (Amazon says trials at its closed test site are designed to push the aircraft to its limits.) Federal regulators, meanwhile, took longer than drone industry boosters had hoped to determine how the autonomous aircraft might safely share crowded skies.  

Amazon opted to develop the drones in-house rather than farming the work out, a bet that its engineers could do a better job creating machines for its delivery operation than an aerospace company. While the e-commerce giant has hit major milestones, including winning permission from the FAA to fly beyond operators’ visual line of sight, the program remains a work in progress. Deliveries are currently limited to College Station, Texas, and greater Phoenix, with plans to expand to Kansas City, Missouri, the Dallas area and San Antonio, as well as the UK and Italy.

Starting with a craft that looked like a hobbyist drone — and was vulnerable to even modest gusts of wind — Amazon went through dozens of designs to toughen the vehicle and ultimately make it capable of carting about 5 pounds, giving it the capability to transport items typically ordered from its warehouses. Engineers settled on a six-propeller design that takes off vertically before cruising like a plane. The first model to make regular customer deliveries, the MK27, was succeeded last year by the MK30, which flies at about 67 miles an hour and can deliver packages up to 7.5 miles from its launch point. The craft takes off, flies and lands autonomously. 

The previous version — the MK27 — used a lidar sensor, which pings the ground repeatedly to estimate altitude. But the drone also had backup sensors: two metal prongs that protruded from the bottom of the machine. Called squat switches and commonly used in landing gear, they push in and provide physical confirmation when the vehicle touches down. The drone’s navigation system was designed to confirm that it had landed safely if two of its three sensors agreed, said one of the people, who requested anonymity to discuss confidential matters. 

With the MK30 redesign, engineers removed the squat switches. FAA documents describe the aircraft as having “a camera and avionics system that has redundancy for critical systems.” It’s unclear why the company did away with the sensors. Engineers often work to simplify products as they mature from research projects to manufacturing, removing parts deemed redundant to cut costs and weight. 

The tech industry also has fallen hard for computer vision software capable of analyzing data harvested by cameras and other sensors, betting that powerful algorithms can produce results more reliably and cheaply than existing methods. Amazon has spent years working to perfect systems capable of determining what shoppers at Amazon Go stores pull off the shelves, or tracking inventory in its warehouses. 

Removal of the squat switches could also reflect a change in how Amazon plans to deliver goods by air, one of the people said. The MK27 was designed to be capable of landing in shoppers’ backyards and featured enclosed propellers to limit the chances of hurting people or pets who got too close. Amazon had backed away from that goal when it designed the MK30, which has more exposed propellers and drops packages from about 13 feet up.

When the drone was out of service earlier this year, Amazon published a blog post saying that dust at its Phoenix-area operation could interfere with sensor readings. In “extremely rare cases” that could lead to inaccurate altitude data, the company said. The FAA approved Amazon’s updates on March 12, an agency spokesperson said, including tweaks to the drone’s altitude-sensing systems. 

Kudrna, the company spokesperson, declined to describe how those systems work but said the company had used multiple sensor inputs to eliminate the possibility that an errant lidar reading could cause accidents. Amazon has resumed testing and delivery flights.