Issued 12/08/08

Click to Download AC 20-141A

This advisory circular (AC) provides Federal Aviation Administration (FAA) information on [ad]
certification (design and installation) and continued airworthiness of digital flight data recorder systems (DFDRS). DFDRS provide information for an investigative authority—the National Transportation Safety Board (NTSB) in the United States—to conduct more thorough investigations of accidents and incidents. The data recorded is also available to industry to enable the prediction of trends that may be useful in determining modifications needed to avoid accidents and incidents.

Subjects included in this Advisory Circular:

• Related CFR’s

• DFDRS Purpose

• Type Certification

• Instructions for Continued Airworthiness (ICA)

• Continued Airworthiness Programs

• Combination Cockpit Voice Recorder (CVR)/DFDR Units

Definitions included in this Advisory Circular:

a. Applicant. An individual or organization that is seeking Federal Aviation Administration (FAA) approval for a digital flight data recorder (DFDR) installation. The approval may be a type certificate (TC), amended TC, or Supplemental Type Certificate (STC). The approval may apply to a single aircraft or to multiple aircraft of a single type design.

b. Correlation. Describes the relationship between two variables. In this case, the two variables are the raw data stored in the DFDR and its conversion into engineering units to the device being measured or the source that this raw data represents in engineering units.

c. Correlation Coefficient. A number that describes the degree of relation between the data being sampled and the data derived from the recorder. The correlation coefficient used here is the Pearson product-moment correlation coefficient, however the coefficient of determination R2 or the sum of least squares methods are acceptable. Its value may vary from minus one to plus one. A value of positive one (1.0) indicates a perfect direct correlation. A value of zero indicates that there is no correlation, or that any predictive capability between the derived data (using the equation) and the raw data is purely coincidental. A value of negative one (-1.0) indicates a perfect inverse relationship between the derived value and the raw data. The absolute value of the correlation coefficient must be equal to or greater than 0.99 over the entire operating range of each mandatory parameter to accurately establish the conversion of recorded values to engineering units (EU). See Appendix 1 for one method, but not the only method, to determine the correlation coefficient.

d. Correlation Data. The documentation that is required by Title 14 of the Code of Federal Regulations (14 CFR) part 121, §§ 121.344(j)(3), 121.344a(d)(3), 125.226(j)(3), and 135.152(f)(2)(iii). This documentation must show how each parameter was correlated and be maintained by the operating certificate holder.

e. Date Manufactured. The point in time at which the aircraft inspection acceptance records reflect that the aircraft is complete and meets the FAA-approved type design.

f. Dedicated Sources. Any digital system interface, analog discrete interface, or analog sensor interface installed solely to produce an input signal to the DFDRS. For example, if it is necessary to install a horizontally mounted accelerometer to sense lateral acceleration, then this accelerometer is considered a dedicated sensor. Conversely, the vertical accelerometer may exist on the aircraft for another reason (vertical flight control, for instance). If the DFDR takes vertical acceleration data from such an existing accelerometer, then the vertical accelerometer is not part of the DFDRS. However, the accelerometer must meet the accuracy requirements listed in the appropriate regulatory appendix.

g. Digital Flight Data Acquisition Unit (DFDAU). A DFDAU is an electronic device that collects, samples, reformats, and digitizes analog and digital signals representing aircraft flight data. See ARINC Characteristic 717. The equivalent of this function may be incorporated in other hardware which may not meet the classical definition of an ARINC 717 DFDAU.

h. Digital Flight Data Acquisition Function (DFDAF). A DFDAF is software that performs the function of a DFDAU in a larger avionics system.

i. DFDR. A recording device that uses a digital method to record and store data onto a storage medium and to retrieve that data from the medium. A DFDR may be the storage device in a recording system that includes a DFDAF, DFDAU, or an FDAU. Or, it may be a stand-alone device using an internal data collection system to convert aircraft analog and discrete signals to digital form.

j. Digital Flight Data Recorder System (DFDRS). The DFDRS is the software, equipment, sensors, wiring, equipment racks, and other items installed in the aircraft to record flight data. It includes the following equipment items: DFDR, dedicated sensors, underwater locating device (ULD), and a DFDAF, DFDAU, or FDAU.

k. Dynamic Condition. The parameter is experiencing change at the maximum rate available, including the maximum rate of reversal.

l. Flight Data Acquisition Unit (FDAU). The FDAU is the predecessor to the DFDAU, defined by ARINC 573. It is primarily designed to accept analog and analog discrete input data, and in some cases, it will accept digital data on predefined inputs.

m. Flight Data Recorder (FDR). An FDR is a recording device that directly receives analog signals representing various aircraft functions (e.g., vertical acceleration, heading, altitude, or airspeed) and records those signals in digital or analog format. Older FDRs recorded the signals by scratch with a stylus on a moving oscillographic medium, typically a foil formed from steel or steel alloy. These older analog FDR installations typically conformed to ARINC Characteristic 542. The FAA now requires that DFDRs be used in the U.S. commercial fleet.

n. Functional Check. A check to determine if one or more functions of a DFDRS perform within specified limits. The following identifies and defines three types of DFDRS functional checks.