A chance download of auxiliary data from three ground-based receivers in crowd-sourced surveillance provider Flightradar24’s European network yielded what could be the defining moment in the final minutes of Germanwings Flight 9525 on March 24, and it is potentially a new source of forensics data available for the accident investigators.

Cloaked in reams of stored data that Flightradar technicians downloaded 90 min. after the crash and deciphered two days later appears to be a change in the aircraft’s autopilot-commanded altitude from cruise flight at Flight Level (FL) 380 (approximately 38,000 ft.) down to 100 ft., the minimum the system will accept.

Commanded altitude is one of many so-called Downlink Aircraft Parameters (DAP) that can be included as auxiliary information from Automatic Dependent Surveillance-Broadcast (ADS-B) “Out” avionics, but the data today is not considered part of the primary 1090-MHz Mode S extended squitter surveillance stream that is rebroadcast and recorded. The primary data, meant to mimic radar coverage, includes aircraft identification, position and speed and vertical speed broadcasted every 0.5 sec. by an aircraft.

Mikael Robertsson, Flightradar24 co-founder, says primary ADS-B data makes up only 5% of the information it captures every 4-5 sec. from its more than 6,200 receivers in a global network. “It is a lot of extra information we normally don’t upload,” he says of the other 95%, which includes DAP data. “We save it in the memory of the receiver, just in case something happens.”

Along with commanded altitude, there are a growing number of auxiliary ADS-B parameters—commanded speed and vertical speed, roll angle and magnetic heading among them—being eyed for a variety of advanced safety and efficiency applications. European air traffic controllers are among the first to begin experimenting with the use of the auxiliary data for enhanced safety.

A change in Flight 9525’s commanded altitude could be a key forensic finding when coupled with a French criminal prosecutor’s assertion, based in part on the recovered cockpit voice recorder, that the aircraft’s co-pilot purposefully locked the captain out of the cockpit and descended the aircraft into the ground. If true, it would appear that the co-pilot used the altitude input to command the Airbus A320’s autopilot to initiate the dive. Flightradar24’s analysis of the DAP data shows that seconds after the altitude command was input, the A320’s continuous descent began.

There has been no official confirmation of the suspected command input by French accident investigation agency, BEA, but neither has there been a request for Flightradar24 to retract its assertion. “The BEA contacted us 45 min. after the accident and requested this data,” says Flightradar24 co-founder Mikael Robertsson. “We sent them the data 2 hr. later.”

Europe is testing DAP parameters including barometric pressure (QNH) and commanded altitude that pilots set in the cockpit. With QNH in hand, controllers can compare it with the actual barometric pressure in cruise or at a landing site to ensure the pilots set the instrumentation correctly. For commanded altitude, controllers can compare the value the pilots set against the assigned altitude to catch errors or misunderstandings about clearances. It is not clear in the case of Flight 9525 if DAP data was collected and stored by the European radar sites that capture the primary ADS-B data. In the U.S., the auxiliary data is currently not used by controllers nor is it saved.

For Flightradar24, capturing the auxiliary data was a race against the clock, as a receiver’s memory fills up in 4-5 hr. and is overwritten with fresh data. Built by Germany’s Gunter Kollner Embedded Development, the receivers have an SD (secure digital) card that can store weeks or months of data; however, Flightradar24 had to disable the memory storage option several years ago as the SD card’s life limits were too short. That leaves only the receiver’s internal memory for storage, a capability the company is attempting to increase by a factor of 2-3 through compression techniques.

Robertsson says a technician was able to connect to the three receivers near the French Alps impact site approximately 90 min. after the crash and download the data of interest. The capture represented the first successful download of the auxiliary data after an accident. Two earlier tries did not pan out. In March 2014, when Robertsson learned that Malaysia Airlines Flight 370 was missing, 6 hr. had passed since the aircraft’s diversion from its course, and a receiver in the vicinity had overwritten data from that flight. The company also made an attempt to download stored data on a receiver in the Ukraine in July 2014 following the shootdown of Malaysia Airlines Flight 17, but the receiver “crashed before we managed to download the data,” says Robertsson. “[The] Germanwings [crash] was the first time we really managed to get the data in time,” he says.

Using ADS-B documentation and signal specifications, Flightradar24 technicians decoded the stored data, focusing on the QNH value of 1006 mbar, which was the barometric pressure at Barcelona, Spain, when the aircraft departed. Robertsson says QNH is always paired with commanded altitude in the data stream, and commanded altitude could be compared against tracking data from earlier in the flight. Robertsson was not yet sure why the QNH setting remained the same during the flight, as pilots would be expected to enter a standard pressure of 1013 mbar during cruise at FL380. He says two A320 pilots who viewed the data believe the QNH was set for the departure airport and not updated.  

Editor's note: this article was published before the BEA confirmed the co-pilot of Flight 9525 modified the aircraft's speed after programming it to fly to the ground.