An airplane manufacturer continues to hear from airlines that the landing gear indicator keeps flashing off and on, confusing the flight crew. Which step of the design process should the designers of the plane revisit? Explain your reasoning.

Answer:

  the applicable design step will depend on the findings as to the cause(s) of the problem

Explanation:

First of all, the affected airplane(s) should be examined to find the proximate cause(s) of the indicator flashing. Once that is known, corrective action can be investigated.

If we assume the airplane has been manufactured and maintained in accordance with all released and approved procedures (a big assumption), then the next step might be to revisit the analysis that sets rigging position and limits--both for manufacturing and for maintenance. Attention should be given to all allowable tolerances anywhere in the linkage related to the gear position sensor, and to the sensor behavior in relation to gear position.

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As an engineer responsible for landing gear indication, I have had to deal personally with this issue. A number of factors are involved, including landing gear linkage and its tolerances; sensor rigging and its tolerances; sensor target size, position, and the geometry of its travel in relation to the sensor. Temperature can also be a factor, affecting both the mechanical linkage and the sensor behavior.

Up/down and locked/unlocked sensors for each gear can contribute to the problem. Each has its own geometry, which is not always easy to discern from the multitude of design drawings and different engineering groups involved. Sensor vendors like to work with a specific target geometry and motion that may not be duplicated on the airplane, so sensor behavior is not always well-specified. The specification to the vendor may need to be revisited.

When sensors are located on moving parts, wire routing and protection come into play. In some cases, wiring may be "in the wind" when landing gear is extended, so must be protected against a variety of assaults. When things rub on wires, damage always occurs eventually. The nature and extent of the protection provided can also be something to assess in the investigation.

Maintenance and repair procedures can also be scrutinized. We have seen issues related to the way splices and crimps to wiring are done, and where those are allowed to be located. Deicing fluid is corrosive to wiring, and travels up a wire as though it were a straw. So, careful protection is needed for wire ends exposed to areas where deicing fluid may be found.

Sensors that rely on magnetic properties of materials can be affected by residual magnetism. Manufacturing and maintenance procedures that detect and/or eliminate those effects may need to be reassessed.

The airplanes I worked on did not have a computer between the position sensor and the indicator light. I have worked with indicators that were computer driven, and that did flash. In one case, the intermittent flashing problem took years to solve, and was eventually traced to a poor design choice in the way internal wiring was routed in the computer chassis. So, the design of both computer software and computer hardware may also need to be revisited.

In short, every part of the design process may need to be revisited.