Miles O'Brien, the Trekkie fellow from DS9?
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He must get some stick with that name
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It's strange how it has 'never' been seen before. Okay, the exhaust temperature is over 1000 Degs Celsius, and I think that the air would 'ignite' if there was a 'fuel' and oxygen present (most likely that they are).
However, to have at least a dozen night landing missions where it has never been seen and it's only just starting to happen, well, if I was a manager or a engineer associated to the APUs, I would be asking a lot of questions.
The only thing I can think of has the APU been modified to run a different mix of fuel recently?