Sometimes a major incident is necessary before mankind’s awareness is pushed to the forefront. How many of us remember that fateful day back on April 28th 1988 when Aloha Airlines Flight 243 broke apart? In short, approximately 23 minutes after takeoff, a small section on the left side of the roof ruptured. The resulting explosive decompression tore off a large section of the roof, consisting of the entire top half of the aircraft skin extending from just behind the cockpit to the fore-wing area. The electrical wiring from the nose gear to the indicator light on the cockpit instrument panel was also severed. As a result, the light did not illuminate when the nose gear was lowered, so the pilots had no way of knowing if it had fully extended. Fortunately, the crew was able to perform an emergency landing whereupon they deployed the aircraft's evacuation slides and evacuated passengers from the aircraft quickly. In all, 65 people were reported injured, eight seriously.
A miraculous ending for this set of passengers for sure, but an investigation by the United States National Transportation Safety Board (NTSB) concluded that the accident was caused by metal fatigue exacerbated by crevice corrosion (the plane operated in a coastal environment, with exposure to salt and humidity). The root cause of the problem was failure of an epoxy adhesive used to bond the aluminum sheets of the fuselage together when the Boeing 737 was manufactured. Thus, water was able to enter the gap where the epoxy failed to bond the two surfaces together properly and started the corrosion process. The final conclusion was that the age of the aircraft was the key mechanism in the accident, and that in order to prevent the likelihood of future occurrences, all aircraft should receive regular fuselage maintenance checks going forward.
Aircraft health monitoring
There can be no doubt that the structural fatigue of today’s large fleet of aircraft is a serious issue and needs to be addressed. Fortunately, it is. This is being accomplished through more inspections, through improved structural analysis and tracking methods and by incorporating new and innovative ideas for assessing structural integrity. This is sometimes referred to as “health monitoring of aircraft.” This process incorporates sensors, artificial intelligence and advanced analytical techniques to produce real time and continual health assessment.
Acoustic emission detection is a well-established method of locating and monitoring crack development in metal structures. It can be readily applied for the diagnosis of damage in composite aircraft structures. A clear requirement is a level form of ‘go,’ ‘no go’ indications of structural integrity or immediate maintenance actions. The technology comprises low profile detection sensors using piezoelectric wafers encapsulated in polymer film and optical sensors. Sensors are bonded to the structure’s surface and enable acoustic events from the loaded structure to be located by triangulation. Instrumentation is then used to capture and parameterize the sensor data in a form suitable for low-bandwidth storage and transmission.
Thus, although wireless sensor modules are often embedded in various airplane sections for structural analysis, wings or fuselage for example, powering them can be cumbersome. Therefore, these sensor modules are more convenient and efficient when powered wirelessly, or even self powered. In an aircraft environment there are a number of “free” energy sources available to power such sensors. Two obvious methods are thermal energy harvesting and piezoelectric energy harvesting. Each has pros and cons and will be discussed in more detail.