System saves flight data to voice recorder

Simi Valley, CA--When Commerce Secretary Ron Brown's plane crashed in Bosnia in the spring of 1996, investigators had to try and reconstruct the accident using only a cockpit voice recorder (CVR). For reasons of cost, weight, and size, the other "black box"--the digital flight data recorder (DFDR)--is neither required nor installed on most military aircraft.

An invention by engineers at Patriot Sensors & Controls Corporation (PSCC) may soon change that. Called SIAAM, the Solid State CVR Sensor Interface And Acceleration Module, it exploits the excess capacity provided by most CVRs. Cockpit voice recorders have four channels--pilot, copilot, flight engineer, and area mike--but the one for flight engineer is rarely used. So instead of adding a complete recording system (i.e. DFDR), SIAAM captures sensor inputs and saves them to the voice recorder's "extra" channel.

This hybrid approach yields a system that costs 25%-50% less, weighs 1/10th as much, and occupies 1/8th the space of a stand-alone DFDR. And while the system can't process the 88 parameters required for current commercial airliners, its 29-parameter capability works well for those classes of aircraft--military, business, and general aviation--that currently carry no flight data recorder at all.

The brainchild of electronics engineer Lev Freidin, SIAAM currently handles 17 input parameters, but 29 are possible. Seven built-in sensors capture airspeed, altitude, three axes of acceleration, and ambient temperature. Other inputs, such as rudder pedal position or flap settings, are taken from existing aircraft sensors and discrete switches. Freidin uses the temperature sensor in conjunction with pre-calculated calibration curves saved in an EEPROM to correct the drift of the accelerometers. This approach eliminates the heaters typically used, which increase cost, weight, and power consumption.

Because the already-approved CVR stores the data, SIAAM needs no crash-hardening--a substantial cost savings. Further, it incorporates all the necessary input, processing, and output electronics into one housing, and includes several of the company's sensors, which are already specified on aircraft worldwide. A normal DFDR requires both a separate data acquisition system and sensors. "The sensors alone for a flight data recorder cost half as much as SIAAM itself," says Abrar Tirmizi, PSCC engineering manager.

Input signals are tracked by SIAAM's internal clock and sampled at the appropriate intervals. Each sample enters the system's Motorola 68HC811E2 microcontroller via the required digital converter. A 100% software-based 2,400-baud modem modulates the output to the CVR; its only hardware consists of a resistive transformer to scale the microcontroller output to 0.5 Vrms.

In another example of shrewd programming, Freidin applied a combination of software and inexpensive components to eliminate a special $500 synchro-to-digital converter chip.

  All of SIAAM's code fits and executes in the controller's 2 Kbytes EEPROM and 0.25-byte RAM. "This memory is enough for everything," says Freidin, "it holds the program, correction tables and control structure." The unit's total power consumption falls below two watts, enabling it to easily pass a requirement to run for 30 minutes without cooling.

Some of Freidin's biggest challenges stemmed from the limits of a CVR. For instance, how can all the digital data be stuffed onto the narrow bandwidth (0.2 to 3.6 kHz) of the voice recorder? And, how can the error rate be reduced to less than one-in-a-million, a DFDR requirement?       

SIAAM addresses these issues with data-correction and data-compression algorithms. A complete sensor-measurement cycle repeats every second and contains a record number and an error-correction check word. Next, the data stream passes to an asynchronous parallel interface port, where a parity bit is added. The stream itself is segmented and written to the tape into 1/16th of a second subframes.

Combining the parity bit with the check word forms a basic error-correction scheme. "It's very simple matrix code because of the short frames," says Freidin. "I could not use the kind of correction found in a CD, for instance; there was no room for this." The correction provides for an error rate of less than 10^[-6]. Uniquely, it corrects bad data input from the sensors as well as a bad recording. Says Freidin, "Errors are so rare that I had to run tests overnight just to find one."

In a world where size and mass reign supreme, the onboard portion of SIAAM measures just 81 × 108 × 183 mm, and weighs less than 1 kg. "A standard flight data recorder is ten times the weight and eight times the size," says Tirmizi.

Overall, SIAAM's design exhibits an economy typical of a Russian-trained engineer, such as Freidin, who is used to doing more with less. "We kept adding parameters; it started at seven and went up to 29," says Luis Reyes, sales and marketing manager. "Lev was the one who had to make it work."

Additional details…Contact Luis Reyes, Sales & Marketing Mgr., Patriot Sensors & Controls, 650 Easy St., Simi Valley, CA 93065-1808, (805) 581-3985, or Circle No. 503.