Another function often associated with a DAU is the blending of information. This involves the use of often complex logic circuits that are often described using formulas based on Boolean logic. Caution and warning functions on many aircraft are the result of the DAU sensing the real need of bringing attention to a certain condition. In the “good old days” an engine “low oil pressure” light would be illuminated anytime oil pressure was below a critical low value. Which means the entire time the aircraft is in for maintenance and has electrical power applied the warning circuit is active. With the ability to blend data, a DAU sensing the low oil pressure switch can also observe the engine cutoff switch. This means that when the engine is not operating the oil pressure warning circuit is not active.
Avionics systems that use an internal maintenance computer can often provide maintenance technicians with a valuable tool for problem recognition as well as direction for resolution. Memories dedicated to storing flight faults can help associate a fault with a specific circumstance and may help bring to light anomalies that could be the gremlin in that ever elusive flight fault. Some diagnostic systems provide a means of observing or even interacting with numerous systems while all the time being seated in the flight deck. Being able to see the condition of many components during operation can be a tremendous asset in solving problems.
An important fact in dealing with a DAU is its place in the aircraft with regard to certification. U.S. Federal Air Regulation (FAR) 25.1431 provides guidance to manufacturers regarding interaction of electronic components and required operating power supplies. One excerpt states that “radio and electronic equipment, controls and wiring must be installed such that it does not cause essential loads to become inoperative as a result of electrical power supply transients.” This statement has been the cause of many a “Gotcha”. As a result of this design constraint some manufacturers install backup power sources to data acquisition devices and often they are not clearly identified. In some cases while performing operational or functional checks, lack of knowledge about redundant power supplies may be the difference between an “on-time departure” or a delayed and possibly cancelled flight.
Many digital flight data acquisition units (DFDAU) have become required in early generation aircraft to allow compliance with regulatory agencies’ mandates for flight data recorders. Often these devices can supply secondary benefits such as provide limited amounts of trend monitoring for airframe and engines and in some cases the ability to generate reports that can assist with problem solving.
Simply by the nature of what they do, DAUs can be a true friend to the aircraft technician. By their nature they supply a common point where many systems come together and provide a point of access for troubleshooting.
The best approach to understanding the ins and outs of the DAU is knowledge. Manufacturers’ educational programs are often the best way to begin the learning process and visits from knowledgeable field technical representatives can also be very enlightening, especially when they show up at the beginning of the lunch hour equipped with their company credit card.
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