(Update: With the investigation concluded and recommended improvements underway, the Matternet -Swiss Post partnership will take flight again in January 2020.)

A serious crash by a delivery drone in Switzerland has grounded the fleet and put a partnership on ice. Within a stone’s throw of some kids, the incident raised grim possibilities for the possibilities of catastrophic failure of payload-bearing autonomous aerial vehicles.

The drones were operated by Matternet as part of a partnership with the Swiss Post (i.e. the postal service), which was using the craft to dispatch lab samples from one medical center for priority cases. As far as potential applications of drone delivery, it’s a home run — but twice now the craft have crashed, first with a soft landing and the second time a very hard one.

The first incident, in January, was the result of a GPS hardware error; the drone entered a planned failback state and deployed its emergency parachute, falling slowly to the ground. Measures were taken to improve the GPS systems.

The second failure in May, however, led to the drone attempting to deploy its parachute again, only to sever the line somehow and plummet to earth, crashing into the ground some 150 feet from a bunch of kindergartners. No one was hit, but this narrowly avoided being a worst-case scenario for the service: not just a craft failing, but the emergency systems failing as well, and immediately over a bunch of children. The incident was documented last month but not widely reported.

Falling from a few hundred feet, the 12-kilogram (about 26 pounds) drone and payload could easily have seriously injured or even killed someone — this is why there are very strict regulations about flying over populated areas and crowds. Matternet noted that the area the drone came down in is not populated or a school, but rather a forested area of Zurich more appropriate for flying over.

Obviously they grounded the fleet following this incident and will not spin up again until Matternet addresses the various issues involved. How was it even possible, for instance, that the parachute line was capable of being cut by something on the drone?

IEEE Spectrum first noted the news stateside. The company issued the following statement on the matter:

This is the first time ever that our vehicle parachute system has failed. As stated in the report, the flight termination system was triggered nominally per the drone’s specification, but the parachute cord was severed during the parachute deployment. At Matternet, we take the safety of our technology and operations extremely seriously. A failure of the parachute safety mechanism system is unacceptable and we are taking all the appropriate measures to address it. Swiss Post and Matternet reacted to the incident immediately by grounding all the operations involving this vehicle type. Our experts analyzed the incident and proposed the appropriate mitigations which are being evaluated by FOCA. We will restart operations once Matternet and Swiss Post, FOCA and our hospital customers in Switzerland are satisfied that the appropriate mitigations have been applied.

Drone delivery is a promising field, but situations like this don’t do it any favors when regulators take a look. Despite sunny predictions from the industry, there is a huge amount of work yet to be done in terms of flight proving the technology, and although two failures out of some 3,000 may not sound like a lot, if one of those failures is an uncontrolled fall that nearly takes out some kids, that could set back the entire industry.

(This story has been slightly updated to accommodate a new statement from Matternet.)