How To Detect Damaged Jack Screws in Aircraft?

On January 31, 2000, just off the coast of California, flight 261 lost pitch control because of damaged jack screws. One may assume that the malfunctioning of these tiny machine elements would not have much effect on a large aircraft, but this incident resulted in all 88 passengers on board dying upon impact. This tragedy left aircraft engineers, pilots, and people across the country stunned.

What followed: aircraft manufacturers and maintenance crews began devising ways in which jack screw failure could be prevented. Moreover, special focus was placed on finding a means of providing an advanced warning of potential failure to pilots. As such, failure detection systems for jack screws were developed as their failure can cause a complete loss of control during flight.

Like other aircraft parts, preventing failure begins with the physical examination of all areas. For jack screw threads, thorough inspection can only be carried out after removing large sections of the access panels during ground operations. Currently, there is no other feasible method to complete this check, either on the ground or in flight, without making a visual inspection first.

Once the access panels have been removed while on the ground, ground crews can look for damaged threads that are blanketed in grease. Furthermore, they must also feel the threads with their fingertips in order to assess for cracks, abrasions, or dents, but this is usually a hit-or-miss procedure. This method does not always work because there are too many threads on a screw, making it very easy to miss a damaged thread. 

Presently, monitoring the state of a screw during flight is not possible; thus, all jack screw threads must be manually examined during preflight and post flight procedures. That being said, there must be a way to detect and monitor damage to screw threads on aircraft without having to visually inspect the threads. Until such a method is developed, the current method must suffice.

One system that has worked for quite a while consists of wiring connected between a drive motor driving a screw on the aircraft and an ammeter equipped with an electronic display. The ammeter is configured in such a way that the current level associated with driving the drive motor of the screw is displayed. For example, high current levels indicate a drop in resistance which triggers an alarm circuit coupled to an ammeter to warn of possible screw damage.

Generally, jack screws are connected to a drive motor, meaning that it is partially responsible for controlling the movement of certain aircraft parts. They may also be affixed to the horizontal stabilizer, which is an aerodynamic surface that provides longitudinal and/or directional stability and control. Therefore, if the jack screw in this section is damaged, longitudinal stability is lost and the aircraft will likely crash.

With all that in mind, the previous detection system, wherein there is wiring connected to the drive motor, transmits an electric signal to an electronic display, warning pilots of potential damage if a change of resistance has occurred. If there is no damage, a baseline current level will be displayed. Furthermore, if more than one thread is damaged, a series of current spikes will be observed.

Another embodiment of this system consists of a subcircuit that detects current spikes by producing an audible signal like a horn or a video signal that flashes a text warning. During flight, these warnings provide in-time flight conditions which is valuable data to have. This allows pilots to prepare for and conduct an emergency landing prior to complete loss of aircraft control, and such information can be sent to the flight recorder for further analysis.

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