First-Hand:Black Magic


Black Magic

Submitted by Anthony J. Iacono

While assigned to the A6E Program, the Engineering Manager, once asked me to investigate a problem with the aircraft's Fuel Quantity System (FQS). The FQS was a capacitance measuring device that pilots had complained of FQS indicator movements and inaccurate readings during flight operations. The crews reported resorting to estimating the quantity of fuel on board.

A production aircraft, ready for sell off, was provided for me to investigate the problem at Calverton, Long Island, New York. I was given no restrictions as to what I could do with the aircraft and maintenance personnel and shop personnel were on hand to provide assistance. My first task was to apply external power to the aircraft, and validate the reported problem. I climbed into the cockpit and began activating systems while monitoring the FQS indicator. I noted some systems causing minor indicator fluctuations.

After reviewing the aircraft's wire installation drawings for the FQS, I had maintenance remove the wiring trough that extended the envelope of the port fuselage. Once the trough was removed, all system cables on the port side were exposed, including those of the FQS. To inspect the installation and the wiring, the maintenance crew was requested to remove cable clamps and cut wire ties to allow access to the various individual system wires. I shuttered when I saw the rat's nest of wires hanging out of the side and wondered if I could solve the problem and have the aircraft put back to getter for sell off.

As cables and wires were moved in the fuselage, the fuel quantity gauge indicator in the cockpit moved quite dramatically. I initially thought I had finally found the proverbial black magic. Moving any wiring in the proximity of the FQS cables caused fuel indicator changes. A review of the FQS wiring installation drawings showed that triaxial cables from the fuel tanks to the cockpit were specified. I found that the correct cables were installed on the aircraft. Next, the drawings for the FQS connectors specified the use of coaxial connectors and the installation was again verified as correct.

Suddenly I wondered how the outer shield of the triaxial cable was terminated if only a coax connector was used. Eureka! The outer shields of the triax were stripped many inches away from the back of the connector, leaving the inner shields exposed to EM and capacitive coupling. Redressing and extending the outer shields of the FQS triax cable, resolved the issue of the FQS Indicator movements.

At the time, a redesign and reinstallation of the aircraft's FQS, with triaxial connectors, would have been cost prohibitive, so the field repair detail was passed on to management. Thus the solution to the problem was specified to management. Somehow, by magic, my test aircraft was later reassembled and sold to the Navy.