How is data transmitted to CVR and FDR?

Question

A recent news item mentioned that "black boxes" are typically stored in the tail section of an aircraft to maximize their odds of surviving a catastrophic incident.

If they're stored in the rear of the aircraft, presumably flight data and cockpit voice data is transmitted via wires which run the length of the aircraft.

Aren't these susceptible to damage during an incident? There have been incidents of explosions and fires which damage flight control cables - aren't the FDR and CVR susceptible in the same way?

Answer 1

You are very correct that this can happen- If i'm not wrong, what happened during the last 6 minutes of Swissair Flight 111 will never be known for this reason.

In general however, by the time the fire would spread to this extent the CVR and/or FDR would probably have picked up some hint of fire and the reason. To increase survivability, the FDR and CVR are connected off separate power buses ('boards'), so I'd imagine the cables for each not exactly next to each other on the fuselage, and if one trips, the other one will hopefully still run for a bit longer.

The cockpit voice recorder operates a simple microphone I believe, so I'd expect that it wouldn't be much more complicated than your computer microphone. The data would probably be converted from analog to digital before being transmitted backwards.

As for the 88 parameters that the Flight Data recorder currently requires, I'd imagine that there would be a serial connection from each relevant part- each flight computer, engine etc. all leading to the back of the aircraft.

Source: FDR Handbook- NTSB

Answer 2

Most airplanes implement data buses such as ARINC 429 which describes the physical interface and a standard protocol allowing different devices to communicate and exchange data with eachother. The A380 and 787 use ethernet as the phyiscal layer but still use an ARINC defined protocol for communications over ethernet.

Just about all of the electronic parts of an airplane communicate over these buses as there are many sensors and the different devices need data from a different subset of those sensors. What the flight data recorder, as well as FOQA (flight operations quality assurance) boxes do is tap into the data bus and record what is happening at regular intervals.

You are correct that the placement of these devices necessitate that the wiring for the data bus extends to the rear of the airplane. This may not be the only reason though as in the rear of some airplanes you have engines, halon bottles, hydraulic actuators, smoke detectors, door sensors, pressurization controls (outflow valves). The FDR doesn't need all of this data, but FOQA might use it and the EICAS system needs all of it, so the data bus will always extend the length of the airplane. This does make it susceptible to damage, but so are the flight control cables, hydraulic lines, bleed air lines, fuel lines, etc that may also be routed to the rear of the airplane.