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BreninTW
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RTO question

Wed Jun 15, 2016 6:17 am

Hi everyone,

I stumbled across this video of an A380 RTO at Hamburg, and one of the things that I noticed was that the spoilers came up while the engines still sounded like they were spooled up, and it was a few seconds before I heard the engines spool down.

Is it normal for the engines to remain spooled up after the spoilers are activated in an RTO, or was this just a sound/picture mismatch?
 
Tristarsteve
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Re: RTO question

Wed Jun 15, 2016 6:54 am

The engines are spooling up in reverse. In a RTO the first action is to select reverse on the engines, and this action deploys the ground spoilers, as the reverser levers operate the AGSB system.
 
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BreninTW
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Re: RTO question

Wed Jun 15, 2016 8:05 am

Tristarsteve wrote:
The engines are spooling up in reverse. In a RTO the first action is to select reverse on the engines, and this action deploys the ground spoilers, as the reverser levers operate the AGSB system.


Thanks Steve,

In the video though they don't appear to have used the thrust reversers ... and there was no instance where the engines spooled down. I'm assuming that the engines have to spool down before the thrust reversers can be activated.
 
Leej
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Re: RTO question

Wed Jun 15, 2016 8:58 am

Pretty sure it is just a mismatch - in as much as the sound takes that much longer to get to you from the viewing point. If you look at when she starts rolling, it is a good 5 or 6 seconds till you hear the engines spooling up. So the same effect when she stops.
I think RTO tests are performed without reverse?
 
boeingfixer
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Re: RTO question

Wed Jun 15, 2016 3:44 pm

Sound travels at 343.2 meters per second (1,126 ft/s; 1,236 km/h; 768 mph; 667 kn). Therefore if there is any distance involved the sound will have a delay from what is seen. A good example is lightning and thunder. What you see and hear in the video is just that.

Cheers,

John
Cheers, John YYC
 
boeingfixer
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Re: RTO question

Wed Jun 15, 2016 3:54 pm

Leej wrote:
I think RTO tests are performed without reverse?


Most RTO's below 100 knots won't need reverse thrust. RTO's are pretty aggressive as max braking is applied automatically in the majority of cases along with full ground spoilers. By the time the pilot is able to select reverse thrust the aircraft has usually slowed below RT stowing speed.

As part of our post C-Check flight test, we conduct a full RTO test at 100 knots on our 757's. Comes to a stop pretty quick with brakes alone and slams you into the seat harness. There's not enough time to even deploy the reverers and get them spooled up before the aircraft is stopped.

Cheers,

John
Cheers, John YYC
 
mmo
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Re: RTO question

Wed Jun 15, 2016 4:56 pm

The other thing to consider is the engines will not spool down to idle right away. The inertia in the fans will keep the engines spooled up while the thrust levers are at the idle stops. It will appear as if the engines are still putting out thrust but the thrust levers are commanding idle thrust.
If we weren't all crazy we'd all go insane!
 
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Florianopolis
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Re: RTO question

Wed Jun 15, 2016 6:52 pm

What's the reverser logic like? Can the reversers deploy with the engines going at take-off thrust, or would that break them? The engine has to spool down to idle, deploy the reversers, and spool back up? I'm guessing the levers can move faster than the engine, so you could be commanding reverse and waiting for the engine to slow down and deploy them?
 
pygmalion
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Re: RTO question

Wed Jun 15, 2016 8:30 pm

Two things... for certification and calculation of runway takeoff lengths etc... RTO as a test is done with no reverse and brakes worn to minimum servicable limits and at Max takeoff weights. This results in a maximum brake energy stop. Fire and tire fuse failure is allowed and expected really but the fire cant impinge on the passenger area for 5 minutes and airplane must be able to exit runway to pass.

Reversers work by changing the air from exiting at the rear of the engine out the nozzle to exiting out the sides of the engine and slightly forward. Its not like your car where you have to stop to not break something by going into reverse. The engine has to spool down enough so that the pressures from the bypass flow dont break the reverser cascades as they deploy. Idle RPM is not zero... its more like 40% of max. I dont know how low it has to be to deploy but is not idle.
 
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Starlionblue
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Re: RTO question

Wed Jun 15, 2016 11:12 pm

Florianopolis wrote:
What's the reverser logic like? Can the reversers deploy with the engines going at take-off thrust, or would that break them? The engine has to spool down to idle, deploy the reversers, and spool back up? I'm guessing the levers can move faster than the engine, so you could be commanding reverse and waiting for the engine to slow down and deploy them?


Correct.

On the thrust lever end, the levers have to be at idle for the mechanical lock to release the reverse levers. But as you say the levers move faster than the engines.

The engines themselves are limited to idle until reverser doors have opened past a certain degree. Once the reverser doors are open (or mostly open but this is type dependent) the thrust is increased by FADEc in response to reverse lever position.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
JFK31R
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Re: RTO question

Thu Jun 16, 2016 4:40 am

I've only experienced one RTO, it was on an A-320 and was pretty much exactly the same as the video.

I remember I noticed a good way in the takeoff roll the spoilers suddenly popping up about 5 seconds before hearing the engines spool down, or seeing the cascades open. I feel silly saying it, but those 5 seconds were pretty scary.
 
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7BOEING7
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Re: RTO question

Fri Jun 17, 2016 8:08 pm

During production testing (Boeing) the RTO is accomplished at about 90kts groundspeed (to ensure the system is armed), followed by idle reverse to check speed brake deployment and manual braking to confirm disconnect forces are higher than that required to disconnect the autobrakes on a normal landing. Because the airplane is relatively light with a production fuel load it comes to a stop rather abruptly and if higher than idle reverse is used there is a chance of damaging the engines due to FOD. I'm guessing what AB does is similar except they do the RTO test a day before the first flight and BA does it at the completion of the first flight.

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