Aircraft Fiber Optic Maintenance

KITCO Fiber Optics

This is probably one of the most common fiber optic myths that still circulate today. Fiber optic systems and fiber optic cables have evolved as very durable and reliable systems. Even today I still catch people whispering around fiber optic cables, as if the sound waves emitting from their vocal cords are going to cause the optical fiber to shatter.

When optical fibers are made the tension strength of the optical glass is tested to 100,000 lbs/sq in. This test is performed on the bare fiber, i.e.,optical fibers with no buffer materials, Kevlar or outer jackets. If this same amount of tension were to be applied to a copper cable, the cable would stretch and then separate. As a comparison, the tensile strength of titanium is also rated at 100,000 lbs/sq in.

With fiber being held to such a magnitude of tension strength, I would hope we can put to rest the fragility issue, and continue by examining the benefits of a fiber optic system as compared to traditional copper systems. The following are some of those benefits:

  • No Electromagnetic Interference (EMI) issues
  • No Radio Frequency Interference (RFI) issues
  • Lightweight cables
  • Smaller diameter cables
  • Greater bandwidth
  • No grounding or shorting concerns
  • Upgradeable without ripping out & replacing cable harnesses

Sure those sound like great benefits, but how do you repair a damaged fiber optic cable? How do you repair a fiber optic connector? These are two very good questions.
Let’s start with the first question:

1. How do you repair a damaged fiber optic cable?
First you have to locate the damaged area of the cable. This is accomplished
either using a Visual Fault Locator (VFL) or an Optical Time Domain Reflectometer (OTDR). Based on the location of the damage and what type of repair (line level repair or depot maintenance level) either a splice, a pigtail assembly, (mechanical splices are under review for approval) a redundant spare fiber, or cable replacement can be used.

As for the second question:

2. How do you repair a fiber optic connector?
First you will need to perform a visual inspection of the connector endface
using either a handheld optical microscope or a fiber optic video probe. This procedure will determine the type of action needed to repair the connector. If the connector only has minor damage, (i.e., scratches, small chips, or small pits), then re-polishing the connector will usually fix the problem. If the visual inspection determines that the connector is shattered or there is no continuity, then either a pigtail assembly will have to be spliced onto the cable, or the cable will need to be replaced utilizing a redundant spare fiber for line level repair. A new fiber optic connector must be installed during depot maintenance level repair.

Another major maintenance concern is the time it takes to complete the repair of the fiber optic cable and or the fiber optic connector. This issue is especially critical when the airplane is on the line. To repair a damaged optical cable a mechanical splice can be installed in approximately fifteen minutes. The quickest way to repair a fiber optic connector is to replace it utilizing a redundant spare fiber. This can be accomplished in about five minutes (per termination end) assuming unimpeded access to the fiber optic connector. Of course you know there will always be some uncontrollable variable that could come into play, but typically line level repairs can be performed on both fiber optic cables and connectors.

I recently had the opportunity to perform some fiber optic terminations onboard a Boeing 767. I terminated fiber optic termini for a Max-Viz enhanced vision system. I terminated one cable harness at the forward pressure bulkhead back to the avionics bay, a cable assembly from the Max-Viz processor to an ARINC 404 connector, and a cable assembly from the forward pressure bulkhead to an infrared sensor in the radome. I was performing depot level maintenance procedures exactly as a fiber optic technician would do for replacing damaged fiber optic termini.

I first performed a continuity check on the fiber optic cable to ensure that the cable had not been damaged during installation. I used a VFL to accomplish the continuity check. I then prepped the cable harness for termination and placed the termini into the curing oven. After the curing process I polished the termini, inserted the termini into the connectors and then tested each cable harness for link loss. After all of the individual cable harnesses were tested I then connected all of the harness together and performed concatenated testing.

The termination process, per fiber optic cable harness, took approximately 1 ½ hours for each end of the cable harness. Keep in mind that one hour of this process was the epoxy curing time of the oven. Both ends of the shorter fiber optic cable harness can be cured at the same time since both ends can be placed together into the curing oven.

In conclusion fiber optic cables are designed to withstand the sometimes hostile environment encountered on board the aircraft. Fiber optic systems are currently installed on board various military aircraft platforms, including the F/A-18 Hornet, F-22 Raptor, F-16 Falcon, F-35 Joint Strike Fighter (JSF) and the EP-3E Aries. The Boeing 777 has fiber installed on board for the Electronic Flight Bag (EFB) system. Boeing performed fiber optic testing on the 757 platform and mounted the fiber on the axle of the landing gear and on the fire wall of the engine.

The fiber was intended to stay on board for 1 year, but due to availability of the aircraft, the fiber ended up staying on the plane for 4 years without a single fiber optic failure. Fiber optic technology has been thoroughly tested and will ultimately end up replacing many kilograms of copper cables. As new systems are designed for aviation, it is a good bet that fiber optics will part of that design.

page up