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.
Another useful article to read, related to the topic and discussion:
Thermoelectric Energy Harvesting
With a 15degK temperature differential, it is possible to generate 1mW of power.
Subsequent to my comment above, I ran into this article at IDTechEx's Energy Harvesting Journal:
Energy harvesting sensors for aircraft
One of the approaches discussed in the Energy Harvesting Journal is to tap into the temperature differentials between the outside (coooold!) and inside of the aircraft and use thermoelectric materials that can be used as heaters, coolers and generators.
This article also leaves out details on how exactly the cracks are detected. Now a days, in addition to Aluminum / Titanium alloys, Carbon composites are used also in aircraft structures. The fracture mechanisms and the propagation of cracks are different in these materials and techniques suitable for one may not work for the others.
Needless to say, there is a lot to be done here before 'systems' evolve where energy harvested (or powered) sensor nodes are used to provide real time data on the integrity of the aircraft while in operation or at rest. In short, an area of opportunity!
Dr. MP Divakar
This is article starts on a good premise but makes the case for acoustic emission techniques in fatigue crack propagation rather poorly. Also, there are many inaccurate statements.
There are two major types of repetitive stressors in an aircraft's life: the high cycle fatigue (flutter in wings, aelerons and control structures are good examples) and low cycle fatigue (examples include de/pressurization of the cabin which expands & contracts the fuselage, landing gears...). The fatigue crack behavior is different for HCF & LCF, described in simple terms by the Paris equation.
The optimal monitoring methodologies and tecniques for LCF & HCF effects assessment can be different and the article doesn't distinguish that at all. Secondly, the behavior of defects 'detected' by such sensor nets have to be strongly correlated between multiple states of the aircraft -while in flight, while on the ground, etc. The behavior of defects will be signicantly different in these states.
The existence of defects in any assembly process is a given and the industry has found ways to keep their damaging effects to a minimum.
@mysterylectricity: I see your point! Safety critical systems should never ever be trusted to a system without redundancy! I am for energy harvesting as a backup for structural integrity monitoring thru sensor nets, not as the primary source. Better yet, use energy harvesting for powering things like cabin lights, fans, even laptop chargers...
Dr. MP Divakar
Very educational article, but seriously: is aircraft the proper testbed for this nascent technology? Whereas such self-powered sensors might seem reasonable during flight, can these systems be fully checked for operation before takeoff? Not without a blowtorch or some highly trained elephants in the hangar. Whereas loss of signal indicates a hard fault in most systems, such a fault in an energy-harvesting sensor leaves considerable ambiguity: is it the sensor proper, or is it simply a lack of harvestable power under the circumstances? I for one would not want to hear, "The pilot is waiting for the Sun to heat up the wings to the point where the sensors become active" while waiting for takeoff. C'mon. This must be a joke.
Don't forget turbulence!
Has this actually been tested on aircraft or is it just theoretical?
It seems almost pointless to do thermal harvesting around the engine since this area already has wiring. I can see some sense in doing it lesewhere though.
It also seems challenging stepping up from low-bandwidth measurement of slow-changing signals like building stress measurement and tyre pressure measurement to monitoring an airframe. I would have expected that airframe measurements would require high frequency measurements to be analysed via FFT etc.