Archive for the month of July 1997

The Dedicated Off-equipment Work Center (DOW) is composed of two of the original F-16 Avionics Intermediate Shops, plus additional equipment to gain MLU avionics testing capabilities.

The name of the DOW can easily be understood by understanding the term off-equipment: in this work center all F-16 avionics off-equipment maintenance is performed. It is still often referred to as AIS.

Though still part of the Intermediate level maintenance, for the Organizational level the differences between Intermediate and Depot level have faded.

Tasks

The Dedicated Off-equipment Work center of the Electronic Materiel Depot — operating at Intermediate level — has several tasks, amongst which:

• Testing of Line Replaceable Units for correct operation (Quality Check).
• Fault isolation up to the level of Shop Replaceable Units.
• Removal/replacement of defective Shop Replaceable Units.
• Removal/replacement of components of Shop Replaceable Units — depending on authorization.
• Calibration of Line Replaceable Units.
• Loading and/or verifying of Operational Flight Programs in Line Replaceable Units (no longer once MLU is completed).
• Performing hardware and software modifications of avionics sub-systems.
• Performing scheduled and corrective maintenance, repair, and modifications of own test equipment.

Changes after the F-16 Mid-Life Update

Some of the tasks traditionally performed at Intermediate level (AIS or DOW) will become obsolete once the Mid-Life Update for the F-16 has been completed. The Modular Mission Computer for instance — introduced with the MLU — can be repaired to a certain extend at Organizational level by removal/replacement of printed circuit boards. These removed items are then sent directly to the Depot level for further testing, therefore skipping the Intermediate level.

Another task that has shifted from I to O level is loading and verifying Operational Flight Programs (OFPs) of various Line Replaceable Units. With the MLU the single point OFP loading has been introduced: a single point for loading all software for all avionics subsystems as opposed to removing, loading and re-installing all appropriate avionics Line Replaceable Units separately. In this particular instance the impact on the DOW is low since most LRUs use EEROMs — enabling the loading process to be an integrated part of regular performance testing.

Test equipment

The Dedicated Off-equipment Work center has several test facilities at its disposal:

F-16A/B LRU test equipment (GDE Systems)

• 2x Radio Frequency test station (RF)
• 2x Computer/Inertial test station (C/I) including rate tables
• 2x Processor Pneumatic test station (P/P)
• 2x Displays/Indicators test station (D/I) including optical stations

F-16A/B MLU LRU test equipment

• 2x Improved Avionics Intermediate Shop (IAIS) including optical stations - (GDE Systems)
• 2x SUREtest 7111 Intermediate level Test Set (ITS) - (Northrop Grumman)

Other:

• Conventional test equipment, such as measuring equipment, additional power supplies, calibration standards, etc.

Test capability

F-16A/B test stations

When the F-16 entered service the Intermediate level maintenance was performed by the Avionics Intermediate Shop. The first test stations were delivered to the US Air Force in 1978 — the last test stations were delivered by GDE in 1994 (world-wide).

The Radio Frequency (RF) test station is used for testing the various parts of the Fire Control Radar, radio sets, and IFF equipment. The Computer/Inertial (C/I) test station is used for testing the Flight Control System, the Fire Control System, and several other LRUs. The name of this test station is somewhat outdated, since the inertial nav unit is no longer being tested at Intermediate level after the introduction of the newer Ring Laser Gyro unit in the early '90s. The Processor/Pneumatic (P/P) test station is used for testing the air data related LRUs, radar-related LRUs, and Fire Control-related LRUs. The Display/Indicators (D/I) test station is typically used for testing the aircraft's 40 kVA Generator Control Unit, as well as several instruments.

Once the Mid-Life Update has been completed, the workload of these test stations will be reduced drastically.

The D/I test station had a secondary task. With the aid of a set of special test adapters, additional test station hardware and special software this test station was also used in the repair of the printed circuit boards from the F-16A/B test stations. The majority of the cards repaired using these additions were conventional analog circuits that required accurate calibrations and adjustments. Most of the software was developped or rewritten by own personnel.

Improved Avionics Intermediate Shop

MLU specific LRUs as well as most F-16A/B LRUs can be tested using the Improved Avionics Intermediate Shop (IAIS). This modular IAIS test station basically is a combination of all four abovementioned F-16A/B test stations, depending on the configuration. It consists of 14-16 small and two-man portable transit cases which are easier and faster set up when compared to the AIS A/B stations. It was developed by GDE Systems to reduce airlift and personnel and to be independent from available power — it will use anything between 47-440 Hz. The first systems for the US Air Force were delivered in April 1993.

Photo: Based on GDE image.
IAIS test station.

Intermediate-Level Test Set

The Intermediate level Test Set is an advanced test station for the F-16 MLU radar system, in which the Radar Antenna, Radar Transmitter, Signal Data Processor, and modified Low Power Radio Frequency unit can be tested simultaneously as one complete radar system.

Photo: © 1998 A.L.P.
SUREtest 7111 ITS

Testing of Line Replaceable Units

All above mentioned test stations are computer-controlled, running the high order programming language F-16 ATLAS (Abbreviated Test Language for All Systems [Wikipedia]). The ATLAS language has been developed in the 60s by the USAF and several manufacturers. The aim was to create a uniform programming language that would enable manufacturing of uniform test equipment, which was more cost efficient. The F-16 ATLAS version used in the F-16 Avionics Intermediate Shop is a slightly altered version of the original ATLAS language.

For each specific Line Replaceable Unit a dedicated software program is started. It is required to hook up the LRU to the test equipment using a dedicated set of cables and a dedicated hardware interface. Each of the functions of the LRU will be tested in a beforehand determined sequence. The software will assist for a great deal in fault isolation and trouble shooting. Stimulus signals are generated by the test station and measurements and analysis of the measured values are made, indicating a so called Probable Cause Of Failure (PCOF) to the avionics technician in case the measured values are out of specifications. In some cases this may work perfectly fine, in many other cases it comes down to the skills of the technician to isolate the fault. Typically, repair of the LRU can be accomplished by adjusting or removing/replacing Shop Replaceable Units (i.e. Printed Circuit Boards, Power Supplies, etc.) and retesting the failed part of the software.

Shop environment v. aircraft

In theory the test software will always indicate which item in a Line Replaceable Unit is at fault. However, in many cases the fault will not be isolated after the suggested replacement. In other cases, the test software is not able to determine the faulty SRU. In such cases, the test software does not indicate any problem, while the LRU does cause a failure when installed in an aircraft.

Certain aspects contribute to this:

• The test software is not based on 100% fault detection.
• Flaws in the test software. Not all possible combinations of faulty SRUs and reported discrepancies have been incorporated or have been poorly incorporated.
• The test software does not take into consideration any errors made in the SRU replacement process (human errors). Basically this is the task of the Self Test/Built-In Test (ST/BIT) tests as performed by technicians on the aircraft.
• Circumstances under which LRUs are tested in the shop environment are different than in the aircraft. Both the environmental factors (temperature, moisture, vibration, G-forces) as well as the absence of other equipment connected to the LRU directly or indirectly. This can result in a discrepancy that can not be reproduced in a shop environment.
• There are faults that occur in one but not in another aircraft, possibly due to the coherence with other systems.

It is also possible that a Line Replaceable Unit has been removed from the aircraft unjustified. There is no way the test software can be able to determine unjustified removals and this will irrevocably cause a clean run on the test station, or result in reported errors that were not the cause of the original failure.

Problems like this require good communications between the technicians at Organizational and Intermediate level, i.e. the shop vs. the flight line. The decision by the Air Force to move the Intermediate level of maintenance away from the Organizational level, and thus hinder communications between the shop environment and the squadron, is therefore a questionable one.