Augmented Reality, a useful application for aircraft maintenance?

With the Microsoft HoloLens and the success of Pokémon Go, Augmented Reality (AR) is being developed more and more, the use of augmented reality for industrial applications is coming closer. Aviation is an industry with long certification processes for new technologies like this, but AR applications are being developed as we speak. The use of a Heads Up Display (HUD) is already widely applied to military aircraft and even is used in the newest civil aircraft. But what are the applications of AR in the area of aircraft Maintenance, Repair and Overhaul (MRO)?

(Image source: Netherlands Aerospace Centre)

Safety and cost savings are the number one and two drivers in Aviation (up to you to decide which comes first). Among others, because of extensive (and mostly expensive) maintenance activities, the level of safety remains sky high and the number of casualties as low as possible. However, due to human factors, enough incidents occur that could have expensive effects. Proper training and experience minimizes the risk of these incidents to occur, and AR could help minimize these risks even further. I found some interesting applications of AR that have been developed already in MRO and will discuss them in the next section.

Firstly, Microsoft has demonstrated the application of getting familiar with for instance a turbofan engine (link). From my own experience I could add to this that seeing something in real does add up to the theory you learn in books. Adding AR to this equation could combine these two and even go further. Secondly, Operose has developed an application for the HoloLens, in cooperation with Magnetic MRO, which displays liveries on aircraft (link). It is not an application that could be useful on a daily basis for every MRO, but it demonstrates the way AR could be used to see the application on 1:1 scale. And lastly, the Netherlands Aerospace Centre (NLR), in collaboration with KLM Royal Dutch Airlines, developed a demonstrator for using AR in maintenance training (link).

The use of AR could be applied even further, to the maintenance activities itself. Kingsee demonstrates in this video (link) the application of AR during maintenance tasks. The way certain things are displayed could be improved, but it shows the basic application of AR within the maintenance activities. I expect that steps will be taken to reach a way of working like this, however it has its pros and cons.

Starting with the pros, the major improvement that comes with using AR is a more efficient maintenance and more fault free system. The tonnes of paperwork that came with the old aircraft will become obsolete and all documentation could be done digital which saves a lot of money. With the development of more and more ‘Ditigal Twins’ and 'Big Data', these buzzwords could even be combined. Imagine a line maintenance manager doing a workaround during a turnaround with all the information he or she should have displayed on the focus spots itself? Countless applications could be imagined.

Digital Twins and AR, a golden combination?

Back to using AR in the maintenance process, it will also come with a lot of cons: when looking at the dirty dozen in human factors (link), some came up by imagining the application of AR. Development of a Lack of Assertiveness is one of the first Dirty Dozen that crossed my mind: When personnel gets familiar with using the HoloLens and, by a technical error, information is displayed wrongly, personnel could mindless follow the wrong instructions. Using physical manuals, this problem could be mitigated for the biggest part. Stress is a second Dirty Dozen that crossed my mind: due to being exposed to more inputs than in ‘normal reality’, the stress level could be enhanced that could lead to hazardous situations. When a mechanic has not slept well and a lot of noise is present in the hangar, the stress level could rise to an unwanted level. The last dirty dozen is not the most obvious, but personnel will experience fatigue problems faster when using an application like the HoloLens. The lens works with projecting the augmented reality around you by using a screen right in front of your eyes. Unconsciously, your eyes will switch from focussing to the screen to focussing on the surface where the object is projected on very fast which results in eye sore and higher fatigue for the personnel using the AR application. The research by NLR also mentioned this issue (link).

Due to all the cons I just mentioned, I cannot imagine a future of Aviation MRO without the application of AR. We, as Aviation professionals, should be aware of the Dirty Dozen and the new challenges in the Human Factor area that are linked with applying AR in MRO. In the meantime, developments like MRO AIR are a major stepping stone to the actual application of AR.

Why erosion is funest for helicopter engines

Recently, I have started my graduation internship at the Netherlands Aerospace Centre (NLR). During my internship I am trying to map the severity of erosion on a helicopter engine. In this article I will briefly explain why erosion is funest for gas turbine engines, especially for helicopters.

First let me start off by giving a definition on erosion from found literature: Finnie, Wolak and Kabil defined erosion as “The removal of material from a solid surface by the action of impinging solid or liquid particles” [Source 1]. Sundararajan and Roy defined erosion as “the removal of material from component surfaces due to impact of hard particles travelling at substantial velocities” [Source 2], which is a more applied definition to erosion.  Erosion is a broad term which also is used in the geology. The picture below is known as Árbol de Piedra, an eroded rock formation in Bolivia.

The rock is eroded due to grains of sand that have been carried away by the wind and impacted into this rock formation. One grain of sand does not make a difference, but if this happens often enough, one could see the effect: erosion.

This is also a problem for gas turbines, especially the ones operating in sandy environments. Sand that has been ingested in the engines will pass all the blades and vanes inside the gas turbine and will erode them on long term. Especially helicopter engines suffer even more from this erosive effect due to blown up sand due to the downwash of the rotors, also called a ‘brown-out’. The picture below shows a V-22 Osprey demonstrating a brown-out landing. 

Brown-out landings are mostly notorious for the hazard of the loss of situational awareness of the pilot. An example is the crash of a Sikorsky MH-60K Black Hawk of the US Army in Afghanistan, 2001 [Source 3]. The secondary effect of a brown out is excessive sand ingestion in the engine, with more erosion as a result.

What is the effect of erosion on gas turbine components? The late Professor Widen Tabakoff did extensive research into erosion and the effects on gas turbines. He described in his researches, along with other researchers that for the compressor, erosion resulted in (see also the illustration on the right):

• ·         Blunted leading edges (1);
• ·         Sharpened trailing edges (2);
• ·         Reduced blade chords (3);
• ·         Increased pressure surface roughness (4).

What is the effect of these four erosion results? Overall, the efficiency of the compressor will degrade. What is the effect of compressor efficiency degradation? Higher fuel flows. To explain this, let me start by explaining how a turboshaft engine works. The illustration below is a basic schematic of a turboshaft, a gas turbine engine type applied on helicopters. The green part is designed to deliver a high energetic gas to the purple part, the free power turbine (3). The Free Power Turbine extracts the energy from the gas and transfers it to a power shaft which is coupled by gearboxes to the rotors of the helicopter. The engine is designed in such a way that the Free Power Turbine always delivers the same amount of work on the Power Shaft.

If it happens, due to erosion, that the compressor efficiency drops, it means that the compressor (1) requires more energy which is ‘harvested’ from the compressor turbine (2). This requires more energy from the compressor turbine and, compared to a more efficient engine, would deliver less energetic gas to the Free Power Turbine. To keep delivering a constant amount of work to the Power Shaft, the Free Power Turbine requires more energetic gas and the engine will adjust this by injecting more fuel. More fuel usage means less range or bringing along more fuel which means more costs. Either way, no positive effect for the operator but no hazardous conditions.

A more hazardous effect is the potential of an engine surge, or compressor stall. A compressor is designed to do a very unnatural thing: moving air from low pressure to high pressure. To achieve this, the compressor is designed very precisely. When the compressor blades are off-design, for instance as result of erosion, disruption of air could occur within the engine and the high pressurized air will flow out of the front of the engine: an engine surge. 

This video [link] is an example of what happens during an engine surge. The GE90 engine was mounted on the first 747 during its test program. You could see (and hear) a violent bang with flames coming out of the front of the engine, which is the high energetic gas mixture from the combustion chamber. An engine surge damages the compressor section and will result in an engine shutdown. For a Boeing 747 with three other fine working engines this does not have to end catastrophically, but for a low-flying helicopter it certainly could.

To summarize: brown-outs cause excessive erosion to helicopter engines. Blunted leading edges, sharpened trailing edges, reduced blade chords and increased pressure surface roughness is the effect of erosion. This results in higher fuel usage due to compressor efficiency detoriation and could lead to the potential of engine surges which could end catastrophically to a low-flying helicopter.

Sources:

[1]       Finnie, I., Wolak, J., & Kabil, Y. (1967). Erosion of Materials by Solid Particles. Journal of Materials, Vol. 2, No. 3, 682-700.

[2]       Sundararajan, G., & Manish, R. (1997). Solid particle erosion behaviour of metallic materials at room and elevated temperatures. Tribology International Vol. 30, No. 5, 339-359.

[3]       https://aviation-safety.net/wikibase/wiki.php?id=77600

[4]       Hamed, A., & Tabakoff, W. (2006). Erosion and Deposition in Turbomachinery. Journal of Propulsion and Power, 350-360.

Older Article - Sky is the Limit?

Note: This is an older article, written by me, that was posted on Aviationtelegraph.com. Unfortunately, due to inactivity, the website was removed from the Internet.

Older Article - The detachable cabin, a good idea?

Note: This is an older article, written by me, that was posted on Aviationtelegraph.com. Unfortunately, due to inactivity, the website was removed from the Internet.