The Black Box: FDR, CVR, ULB and ELT

They have been discussing the merits of the aviation flight recorder for a few weeks on the TV news so I thought I'd explain exactly what that is. It's often called a black box and not in the sense of the black box in technical flow charts. Ironically they are never actually black. They are usually bright orange or yellow container with reflective stripes. On aircraft the box contains two separate aviation recorders.

1. Flight Data Recorder (FDR)
2. Cockpit Voice Recorder (CVR)

Contemporary models contain both an FDR and a CVR in a "fireproof" and "shock-proof" container. These digital recorders once used old school magnetic tape had had to be housed separately.  Come companies make a combined Cockpit Voice & Data Recorder referred to in the literature as a CVDR.  Regardless the device is not kept in the cockpit where space its at a premium but instead is located in the tail of the aircraft. I put "fireproof" and "shock-proof"in quotes because there are limits to devices that might smash into a mountain at 900 mph. They are rated to survive an impact of 3,400g and temperatures over 1,800 °F.

The first functional models were made in France by Francois Hussenot and Paul Beaudouin in 1939. Their HB recorder used 88 millimeter film, and recorded both altitude and speed. WWI units were more durable but recorded with a stylus. Australian engineer David Warren devised the first modern FDR in the 1950s.  He built his "ARL Flight Memory Unit" in 1957. It was a combination CVR/FDR unit we'd recognize today complete with the bright orange color scheme. More here.

But you'll note that the FDR and CVR do not transmit. That job is done by yet a third device, the radio beacon in this case a Emergency Locator Transmitter (ELT). Those transmit one one of two frequencies 121.5 MHz or 406 MHz. The FAA began regulating those in the Federal Aviation Act of 1958, then requiring them via an amendment in 1970. The FCC authorizes the devices of course. More here.

On commercial flights yet another device is required: an underwater locator beacon (ULB.)  Most models of the ULB emit an ultrasonic 10 millisecond pulse (ultrasonic vibration) every second at about 37.5 kHz.  It's designed to activate automatically when immersed in water. These devices are powered by lithium batteries and the signal can be detectable up to two miles away. The battery is supposed to last a month.  More here.