Comment by smcleod
1 year ago
I was speaking with a 787 pilot last Sunday, I told him that the week before when I was at an airport there were two pilots sitting next to me talking about how "This is the third bloody 787 rescue we've had this month... I can't believe we had full engine and <I think he said auxiliary?> failure at the same time" - I asked him if this is common and he said "I hear of it, but I haven't had that many major failures, but lots of little things - last time I flew in from <city> a few moments after we touched down we lost auxiliary power from the rear engine, all the cabin lighting went black along with a number of other things, thankfully we'd already significantly reduced speed and were straight and already lost most of the speed we were carrying, so we were fine and taxied to the disembark location, they had it up and flying again within the day - but it certainly was disconcerting to say the least".
I will be slightly paraphrasing from memory there, but certainly was quite surprised how calm he was about the whole thing, there's no way I'd board one of those things.
Modern two-engine planes like the 787 have an auxiliary power unit (APU) in the tail. This is a small turbine that runs a generator and a pump for the hydraulics. It’s typically only turned on when the plane is on the ground, or if there’s an emergency in mid-air. It is also needed to start the main engines so if the APU is faulty the plane will probably be stuck where it is. In theory a 787 can take off with just one engine but this is not very safe and wouldn’t be done in all but the most exceptional circumstances.
There are variations on this depending on the plane model, of course. Some older planes can use an external starter for their engines, but I think that’s very rare now.
Aircraft with INOP APUs can generally be "air started" with a ground-based high-pressure air system. It's relatively common and I've been on a plane that had to do the procedure. It was entirely undramatic other than engines being started before the pushback, but I doubt most passengers even noticed.
Now, interestingly, the 787 is a "bleedless" aircraft, so it doesn't use high-pressure air from the APU to spool up the engines. I believe it can use its hefty bank of lithium-ion batteries to start its engines if the APU (and associated electrical generator) is INOP.
Not a pilot/engineer - just an enthusiast. Someone more au fait with the 787 might be able to correct me on the above.
My understanding is that there was a push to modify the U shaped tow trucks they use to position planes to have a battery powered system to start the engines.
The idea being that the APU isn't particularly clean burning, not compared to power plant emissions. It's been a long while since I've heard anything about that plan, for or against.
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Yeah the 787 can be started electrically but it takes a ton of juice.
https://www.youtube.com/watch?v=1W_RtawHVvw
>Modern two-engine planes like the 787 have an auxiliary power unit (APU)
Where "modern" here includes jet airliners made in the 70s yes.
>It is also needed to start the main engines
The engines need an air source, and the APU can be an air source, but at one point at least, big airlines preferred using ground hookup provided air sources for starting, in order to save gas. Next time you fly, look at the jetway. There will be a large yellow duct system underneath it that can be hooked up to the plane to provide pneumatic pressure and air conditioning air without starting the APU. There are similar hookups for electrical power so that a plane won't drain its battery during routine turnover operations.
The bottom price flights I've taken recently don't seem to hook either up though, preferring instead to start the APU during taxi to the gate while shutting down one engine, shut down the other engine once they are at the gate, and reverse the process to taxi back out to the runway. The turnaround time is so short, and the required work to clean and restock the cabin so little, I bet they just don't pay for ground hookups.
APU failure maybe? That would be troublesome indeed; with no engines and no APU you'd lose most instrumentation and a lot of the hydraulics.
There is also a RAT at the back that can be deployed to generate some power(~5-10 minutes max) in case of severe emergency in Air. It is what you hear sometimes, when the aircraft is making a very shrill noise flying over your head.
However, if it is not a test flight, a RAT deployment should make you very uncomfortable and worried…
> RAT … It is what you hear sometimes, when the aircraft is making a very shrill noise flying over your head.
I’ve been around a lot of airplanes and I can’t say I’ve seen or heard a ram air turbine deployed in flight. There was a recent incident involving a Frontier Airlines flight in which the RAT was deployed when the aircraft was put in emergency electrical configuration. The deployment of the RAT would be quite rare.
https://en.wikipedia.org/wiki/Ram_air_turbine
I find it hard to believe that anyone reading this was within earshot of a plane in a severe emergency and heard this particular sound and since turbine engines are already quite shrill I am basically just sorta confused who your audience is for this suggestion.
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The chances of you being on multiple commercial flights where the Ram Air Turbine are deployed is infinitesimal, no?
Also, RAT can power limited systems indefinitely on most models, not all or limited systems for a limited amount of time.
Why would the Ran Air Turbine be time restricted? As long as the plane is moving there’s power.
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I thought the guy I was speaking mentioned something about instrumentation but I wasn't 100% sure and that sounded more serious so didn't mention it - but if the aux engine failing would do that - I guess that lines up!