The maiden voyage of Facebook’s internet-beaming drone prototype, Aquila, ended with a 20-foot section of the wing snapping off as it came in for a landing. Facebook says the structural failure happened after an unexpectedly powerful gust of wind that hit Aquila on its approach. But aviation experts interviewed by The Verge said Facebook’s engineers were reckless in flying the plane longer than planned and in attempting to land after wind conditions had grown unsafe.
A National Transportation Safety Board review of the incident found that the wing failed because a gust of wind knocked Aquila off its flight path, and the autopilot’s attempt to correct course resulted in the drone going faster than intended. The high speed and position of the elevons, which control the pitch and roll of the aircraft, caused a section of the ring wing to deform and snap off.
“Why were we flying on a day that that was possible, where we were gonna be so close to the limit?”
Facebook’s prototype was built to withstand a maximum wind speed of 7 knots, or about 8 mph. During the initial portion of the June 28th flight, skies were calm. But after a successful launch shortly after 6AM, Facebook engineers elected to fly the plane for 90 minutes — three times longer than planned.
In the intervening time, the Yuma desert heated up. Heat often exacerbates wind conditions, as the hot ground surface pushes air upward toward a lower-pressure part of the atmosphere. The longer that Aquila flew, in other words, the higher the risk that Facebook would encounter wind conditions that the drone could not withstand.
At 7:37AM, when Facebook initiated the landing procedure for Aquila, winds on the ground were measuring up to 10 knots. Wind is often more intense at altitude than it is on the ground. That means Facebook initiated landing when it knew conditions were higher than Aquila was designed to withstand. And indeed, the drone ultimately was hit with a gust measured at between 12 knots (13.8 mph) and 18 knots (20.7 mph).
Shawn Pruchnicki, who teaches air safety and accident investigation at the Center for Aviation Studies at Ohio State University, reviewed the NTSB report at the request of The Verge. “A couple things come to mind,” he said. “Why were we flying on a day that that was possible, where we were gonna be so close to the limit?”
In an interview, Yael Maguire, who oversees software development for Aquila, said that in 1,000 computer simulations, wind conditions had never exceeded the conditions in Yuma that day. “We were simulating outside the range of what we thought was possible,” he said. “This was higher.”
“You don’t want to test your actual design at your maximum limit.”
But Francesca Favaro, an assistant professor in the aviation department at San Jose State University, said planes should generally not attempt to fly in conditions outside, or on the edge of, what experts call “the safe flight envelope” — unless that is the express purpose of the flight test. “You don’t want to test your actual design at your maximum limit,” she said. Facebook has said the purpose of Aquila’s first flight was to test its takeoff procedure and collect data about its autopilot, motors, batteries, radios, and other systems. Considering the conditions at 7:37AM that morning, Favaro, who studies system safety and accident investigation, says, “I would not attempt the landing.”
An alternative would have been to “initiate a go-around” — aborting the landing attempt and trying again in the hopes that wind conditions improved, Favaro said.
So, why did Facebook decide to force a landing during dangerous conditions? Facebook says it always assumed Aquila would be damaged upon landing, and never intended to fly this particular prototype again. “Landing, while mandatory, is not something we’re optimizing for,” Maguire told me last week. “Ultimately, if you think about where we’re trying to go with this, we want this to be an aircraft that has a 90-day mission, where almost none of that time is spent focused on landing. We have to focus on flying at altitude and things like that. So that’s what we’re optimizing for.”
The company also acknowledges that Aquila is not yet ready to fly over populated areas: “Safety is a top priority for our team as we build Aquila,” a spokeswoman said. “We have a lot to figure out before we have to solve specific operational challenges, as this was only the first of many functional check flights and Aquila is not yet ready to fly in commercial airspace. But of course this is something we are thinking about and that our team and our testing will address.”
“The last thing you want is that external pressure.”
The company says that Martin Gomez, its director of aeronautical platforms, has been appointed to the FAA’s Drone Advisory Committee and will work on regulation and policy issues surrounding experimental aircraft.
But experts consulted by The Verge suggest Facebook is still struggling to master the fundamentals of flight protocol. In that light, Aquila’s structural failure upon landing is of less concern than the idea that Facebook ignored basic safety guidelines in landing the drone. And one expert suggested the team’s cavalier attitude may have been motivated by something other than scientific inquiry: an effort to impress their boss.
Pruchnicki criticized Facebook’s decision to have CEO Mark Zuckerberg present for the maiden voyage, saying it likely put added pressure on engineers to fly despite the wind conditions. “I never would have approved flying in those conditions — and certainly not having the CEO coming in to witness it,” he said. “Maybe on the fifth or sixth flight. [When] we’re still trying to figure out the basics, the last thing you want is that external pressure.”