Inside the Black Box

With all the news recently surrounding the discovery of theso-called "black box" flight data recorder (FDR) from the Air France flightthat crashed into the Atlantic en route to Paris from Rio de Janeiro, it seemeda good time to explore the inner workings an FDR.

Modern FDRs are, like most current data acquisition devices,largely electronic. In the U.S., the Federal Aviation Administration requiresthe recording of no fewer than 11 to 29 parameters, depending on aircraft size.Magnetic tape systems can be used to record up to some 100 parameters; solid-statesystems can handle 5-10 times that amount or more. On July 17, 1997, the FAAissued a Code of Federal Regulations that requires the recording of at least 88parameters on aircraft manufactured after August 19, 2002.

Parameters recorded by FDRs include time, pressure (altitude),airspeed, vertical acceleration, magnetic heading, rudder-pedal position,control-wheel position, horizontal stabilizer, control surface positions, fuelflow, etc.

For the Embedded Systems Conference in 2009, David Carey of UBMTechInsights (UBM is the parent company of DesignNews) performed a teardown of an older FDR to get a look at how theyactually work.  The model he used was anFA-542 made by Sundstrand.

Older FDR models, such as the FA-542, were developed before theuse of magnetic tape or solid state recording become common. Thought these FDRsdon't feature the latest electronic advances, they serve as interesting examplesof the combination of mechanical and electronic engineering. And they're notcompletely obsolete. Some aircraft still in service have these types of FDRsinstalled.

In the FA-542, four transducers (two pneumatic and twoelectronic) were used to drive foil recording arms. Airspeed and altitude datacame in via plumbed lines directly to the FDR to pneumatic actuators positionedalongside motor drives for actuation of recording of pitch/bank.

Four scribingarms were positioned width-wise across an Iconel foil roll to record fourflight parameters. The electronics chassis in the FDR housed powerconditioning, signal conditioning and transport devices.

With so few flight parameters recorded by these older FDRs,Carey says that much would be left to speculation about the causes of a crashthrough a combination of FDR data and information obtained from the cockpitvoice recorder.

 

Though FDRs quickly evolved from foil recording to magnetictape and solid state over the past several years, the newer models are not publiclyavailable for teardown and inspection. Carey actually found this older foilrecorder on eBay.

Despite the internal advances made in FDRs over the years,Carey contends that enclosures for FDRs have remained fairly static due to thefundamentals of mechanical survivability in the event of a crash.