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Facebook's internet drone crash-landed after wing 'deformed' in flight
Autopilot fail led to too-fast descent that bent wing beyond tolerances
Facebook's plans to beam internet access from drones has crashed, literally, after the company revealed the first flight of its “Aquila” craft ended with a bang.
The United States's National Transportation Safety Board (NTSB) late last week published a report about the incident that says “On June 28, 2016, at 0743 mountain standard time, the Facebook Aquila unmanned aircraft, N565AQ, experienced an inflight structural failure on final approach near Yuma, Arizona. The aircraft was substantially damaged.”
Here's the report's narrative of the incident:
At 0737, the crew commanded a landing to the designated landing site. During the final approach, the aircraft encountered an increasing amount of turbulence and wind speeds of up to 10 knots at the surface and 12 to 18 knots, as measured by the aircraft at flight altitude. The operators post-flight telemetry analysis showed that the aircraft experienced significant deviations in pitch, roll, and airspeed, consistent with turbulence during the final approach.
At 0743, while on final approach at 20 feet above the ground, the right outboard wing experienced a structural failure with a downward deflection. Four seconds later, the aircraft impacted the ground at a groundspeed of 25 knots in an approximately wings-level attitude. The aircraft sustained substantial damage as a result of the impact and wing failure. As a result of the aircrafts design (skid landing gear, low-slung engines and propellers), the operator expected some damage during normal landings.
Facebook published a report on the flight and says it held back from disclosing the test's failure because it had to wait for the NTSB to conclude its assessment.
The Social Network™'s version of events is as follows:
Our data show that the aircraft flew into higher-than-expected wind conditions as it descended in the final seconds of the flight. About five seconds before the structural failure, those conditions lofted the aircraft above the glidepath. The autopilot responded by lowering the nose of the aircraft to correct the trajectory, which increased the airspeed above the normal 25 miles per hour. As the aircraft descended back onto the glidepath, the autopilot started to deflect the elevons upwards. The combination of high airspeed and up elevon caused more bending and torsion than the structure could tolerate, resulting in a downward deformation and failure of the right wing.
Facebook's at pains to point out that Aquila was “ less than 20 feet above the ground and moving less than 30 miles per hour” at the time of the incident. So scratch the “Facebook Internet Plane In Death Plunge Towards Poorly-Connected Worthies” headline. For now.
Both Facebook and the NTSB agree that the autopilot software could not cope with the turbulence experienced on the landing approach, which lead to a too-steep descent.
Or as the NTSB report says, using Facebook's analysis of the incident:
The operators analysis of available data indicates that the structural failure was likely initiated by a wind gust that lofted the aircraft above the glidepath about 5 seconds prior to failure. The autopilot responded to this gust by lowering the nose of the aircraft to reestablish itself on the glidepath. The airspeed then increased to 28 KIAS from the normal 24 KIAS. As the aircraft descended back onto the glidepath, the autopilot started to deflect the elevons upwards.
The operator determined that the combination of high airspeed, up elevon, and low angle of attack, resulted in increased downward lift (and torsion) on the outer wing panels. This loading exceeded its structural limit and resulted in a downward deformation and failure of the right wing. At the time of the last gust (5 seconds prior to touchdown) the aircraft was near idle power and the inboard propellers were commanded to the windmilling state - the highest drag configuration available to the autopilot.
Facebook's therefore, predictably, re-designing Aquila and its software.
“Future Aquila airframes will incorporate a drag device such as a spoiler or airbrake that the autopilot can use to steepen the descent without increasing airspeed,” write The Social Network™'s Yael Maguire and Martin Luis Gomez. “In future flights, we will command the autopilot to give priority to keeping the airspeed under the limit, sacrificing altitude tracking if required. This could mean a less accurate landing, or a go around if the airplane deviates too far above the glidepath,” the pair add.
Designing a new aircraft is decidedly non-trivial, so problems are to be expected. Which is why there aren't many firms in the world that can do it. Facebook says its ambitions remain un-dimmed and that it will continue the Aquila program. Presumably because it cannot tolerate the idea of anyone's social lives existing beyond its ad-slinging reach. ®