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21st July 2022 - Aircraft Safety ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Recently, I have been watching videos about aircraft accidents. This can be seen by people as strange but I got interested after listening to the yearly product safety training. There is a great person who puts out great videos which are clear, concise and conscientious. Their channel is called Green Dot Aviation [1]. Each video basically covers an accident and goes through the steps which lead to the accident. Most of the videos I had seen seemed to be due to pilot errors or human factor issues. However, one video concerning flight Cathay 780 [2] struck home. It concerned a Airbus A330 of which the majority of engines are Rolls-Royce Trent 700s. One of my first jobs as a graduate engineer was investigating pressure transducers on Trent 700 EECs (Electrical Engine Controllers). The EEC for an engine is essentially a computer which controls a tap for fuel. It essentially reads the throttle demand from the pilots and then determines how much fuel can be pumped to the engine without causing the engine to stall or flame out. Now, I got intimate with this EEC having worked on some legacy component issues as well as fault finding units which failed production test and then working on an attempted upgrade. Flight Cathay 780 was from Indonesia to Hong Kong and issues started with messages from the EEC about a fluctuating EPR (Engine Pressure Ratio). On the early Trent engines such as the 500, 700 and 800, the thrust is calculated using the EPR value. This is calculated by dividing the air pressure at the inlet to the engine by the air pressure leaving the exhaust. More modern Trent engines such as the 1000 and XWB do not use this but calculate thrust using the N2 shaft speed. When I first heard the EPR value was fluctuating, I worried it was related to one of my first jobs which was looking into replacing vibrating pressure sensors with an alternative technology. One of the issues with measuring the exhaust pressures was that you end up with rather nasty chemicals inside the sensor. It was found that one of these was becoming sulphuric acid which corroded parts of the vibrating cylinder pressure sensor. This caused the sensor to be inaccurate and so the EPR would be incorrect. The video progressed and it turned out the fault was actually due to a fault in the fuel system at the airport. A fuel filter stage had disintegrated and got through into the aircraft fuel tanks. This filter stage was one which removed excess water from the fuel and seemed to essentially be silica balls. These balls got stuck in the fuel metering valve and lead to the EEC on both engines struggling to manage the fuel flow. One engine flamed out while the other was stuck on a high thrust rating as a result of stuck FMVs. Somehow, the pilots managed to land safely with just 300m of run way left before they would of landed in the sea. No one died although there were accidents from a rather hard landing. I am amazed at how well these pilots coped and their successful landing. Every year I have to do product safety training and yet that video had hit me harder than any of that training. Mostly as it is the same old gumph which is repeated each year. Partly as it often does not relate to things I have worked on. Instead they show a child at a departure gate looking out of a window at a shiny aircraft. Instead, give me Green Dot Aviation videos and I will ensure I do not get goose bumps again as something I have worked on is described prior to an accident!! Engineers are strange people and we tend to want the details before seeing the bigger picture. The other issue with the product safety training is that they try to scare us with legal issues resulting from our work and having to justify our decisions in a court case. I had a manager who used to regularly ask if I would stand up in a court of law and justify this work. I mean of course I would. Mostly as he was the one who would of had to do the justifying as the only signatory on the report. I think it would be much more scary and worrying to see the news that an accident happened due to my design. This is what keeps me questioning the work of my team and myself to ensure that what we have produced is safe and follows the ALARP (as low as reasonably possible) principles. I still fly on aircraft regardless of the engine manufacturer so that says something! [1] https://www.youtube.com/c/GreenDotAviation [2] https://www.youtube.com/watch?v=tCrfrQ9CnUg