Introduction to Avionics Testing
As often required by federal regulations, the first time an avionics system is usually tested is during the manufacturing process. However, it is not possible to produce such equipment without operational tests regardless of legal requirements. Throughout manufacture, checks are performed during the assembly of the product, concluding with final test.
Test conducted during assembly do not represent every possible measurement that can be performed. For instance, equipment is not usually tested under temperature variations or relative humidity; these were performed during development and design. To induce early failures ("burn in"), temperature testing may be performed on all units in the factory. Testing for military avionics frequently includes temperature cycling in addition to burn-in. In fact, testing of military avionics is responsible for a significant part of equipment cost.
The next test an avionics system or component will usually go through is after installation aboard the aircraft, this is absolutely required by federal regulations. It is not unusual for individual components to correctly function alone, but not operate properly in a system.
Additional reason for testing is when a pilot registers complaints on a squawk sheet. There are noticeable failures such as "DME inoperative", but there are other squawks such as "DME erratic" or "DME distance is off". Given that "off" are not quantitative terms, the avionics removes the DME and operates it with known signal levels and distances to see what "erratic" and "off" might mean.
The equipment is calibrated and returned to service if the technician finds no problem. Not being able to replicate the squawk is called "no fault found", or NFF, and represents a large expense to the avionics industry. It is likely, that the pilot experienced difficulty with the DME but the real cause of the problem could be a high level of air traffic or ground station fault that caused poor reply efficiency. For the reason that the DME interfaces with other avionics, the real culprit may still be in the aircraft, if only the DME was removed a second time. Another word for "no fault found" is "unverified removal" --- the technician cannot replicate the problem on the workbench
No Fault - Stress Testing
At times when a complaint results is a No Fault Found (NFF), the technician might force the fault by stressing the equipment. This could involve applying vibration, heating or varying power supply voltage. Such procedures lift the cost of the NFF by increasing technician time and tying up test sets, however, this could be significantly less than a subsequent removal and check for the same indefinable problem.
Airline companies find it really expensive to remove a black box, ship it to the shop, do a bench test, and then return it to service with no fault found. The process also increase the number of costly spares needed on the shelf to keep fleets in the air.
In some instances, system will be tested even though there have been no squawks. These are periodic checks required by FAA regulations. Habitual testing of any electronic system is cautious because some equipment can bit by bit degrade without being noticed. All pilots are taught in their first flying lesson to check magnetos prior to takeoff and not to wait until someone writes them up on the squawk sheet.
There are various avionics systems aboard an aircraft that testing all can be time consuming. Like magnetos, most systems are redundant or failure of one does not leave the aircraft with a loss, not to mention, of navigation or communication. However, many systems are complex and the possibility of failure could be dangerously significant.
In many cases, periodic testing is prudent, and required by law. But how often, and to what performance criteria are system tested? First and foremost, the requirements of the Federal Aviation Regulations (FARs) must be met. These requirements differ, depending which rules apply. In fact, few avionics system are required to be tested from time to time and the level of performance mandated by FAR is usually less than the equipments capacity.
What are the performance criteria for avionics system? By the exception of FAR-mandated performance limits, most particularly transponders, regulations regularly make such statements as "capable of communicating with", "provide necessary navigation performance" and so on. Usually, the FARs does not give exact numbers but indicate that equipment should perform its intended function. The performance standards in these instances are the manufacturers specification. However, more stringent regulations apply to transport performing Cat. II (and higher) landings and for altimeters used in reduced vertical separation flight.
The manufacturers specification is frequently a minimum level and almost every avionics system performs better than the specification when new. It is also not idealistic to expect equipment to keep performance better than the manufacturers specification of its entire service life. About the test interval, this depends on the type of flying. Commercial services and air transport call for a short test cycle while aircraft used for primarily VFR flight may have a long cycle.
Technical Standard Order
One common specification is the TSO, or Technical Standard Order. Naturally, the TSO for an avionics system references a performances specification written by the Radio Technical Commission for Aeronautics, (renamed RTCA). Aircraft flown under certain sections of the FARs call for certain avionics systems installed in the aircraft to conform to TSOs. Small general aviation aircraft can fly with virtually no TSOed electronics; the transponder being a notable exemption. GPS receivers also must comply with TSOs to quality for instrument flight.
A complete set of specifications (or "characteristics") are written by the airline-operated ARINC organization. They cover each major item of avionics to assure "form, fit, and function". This means that a line replaceable unit (LRU) can be substituted by any manufacturer who meets the ARINC spec and have it work exactly the same.