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speedbird707
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RAT Question

Wed Jul 08, 2015 7:06 pm


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In this picture the AC 787-9 is seen landing with what I think is the RAT deployed. I am assuming that this was a test flight of some variety. My question is simply, I am guessing that the thing is deployed as part of testing, but can it not be stowed automatically?
 
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tb727
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RE: RAT Question

Wed Jul 08, 2015 7:38 pm

Quoting speedbird707 (Thread starter):
but can it not be stowed automatically?

Nope. If that thing comes out, it's out. If it comes out, you are in a situation that you probably need it until you land anyways.
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wingscrubber
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RE: RAT Question

Wed Jul 08, 2015 9:27 pm

Video here of another occurrence;
https://www.youtube.com/watch?v=Oo_ujmwe9iM

I'm not sure that this is a component that is routinely flight tested, RATs would be acceptance tested at the manufacturer, and then deployment tested on the ground at the airframer, if it's deployed in flight then something has triggered it.

I'll defer to someone with Boeing inside knowledge if these get purposefully tested? The Boeing 787 RATs are a hybrid electric/hydraulic unit, so they can provide power to both hydraulics and the electrical system, so a fault one with of those systems would normally need to occur to automatically deploy it.

On older RATs, like the VC-10 electrical unit, if left deployed in flight at speed, the unit would overspeed and then overheat because they cannot be stowed until the aircraft lands again, the new gen RATs might be designed to run for longer but a deliberate flight test deployment on a delivery flight/green flight seems awry to me.
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77west
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RE: RAT Question

Wed Jul 08, 2015 11:30 pm

Quoting wingscrubber (Reply 2):
I'm not sure that this is a component that is routinely flight tested, RATs would be acceptance tested at the manufacturer, and then deployment tested on the ground at the airframer, if it's deployed in flight then something has triggered it.

It is always tested in the air prior to customer delivery. It can be manually triggered. Can only be stowed on the ground.
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7BOEING7
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RE: RAT Question

Thu Jul 09, 2015 3:58 am

Quoting 77west (Reply 3):
It is always tested in the air prior to customer delivery. It can be manually triggered. Can only be stowed on the ground.

  

On all Boeing airplanes that have a RAT it is tested inflight. On the 777/787 it is dropped on the ground during the B-1 preflight checks by manually pushing the Ram Air Turbine switch -- in flight the generator control switches are turned off until the RAT drops and the APU autostarts confirming correct operation of the system.
 
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747classic
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RE: RAT Question

Thu Jul 09, 2015 6:41 am

All 747 series have no RAT installed, only the latest 747-8 series are equipped with a hydraulic RAT.

The 747-8 RAT supplies hydraulic pressure for syst #3 in case of an all engine failure situation, because the latest generation high bypass engines are not capable to deliver sufficient hydraulic pressure for the flight controls for pitch, roll and yaw control, when windmilling at approach speed.

In case 3 or more hydraulic system low pressures are sensed, the RAT auto deploys and (reduced) control in all 3 axis is still possible.
A manual deploy switch is present on the overhead panel and will be tested on the ground, before the first flight.
During the B-1 flight the auto deploy system is tested by depressurizing of 3 hydraulic systems.
Stowing only possible on the ground.
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OldAeroGuy
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RE: RAT Question

Thu Jul 09, 2015 1:26 pm

Quoting wingscrubber (Reply 2):
On older RATs, like the VC-10 electrical unit, if left deployed in flight at speed, the unit would overspeed and then overheat because they cannot be stowed until the aircraft lands again

RAT's on newer airplanes (A300/310, 757, 767) and on have featured variable pitch blades, allowing constant speed operation. This avoids the over-heating problem.
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BoeingGuy
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RE: RAT Question

Thu Jul 09, 2015 3:13 pm

Quoting 7BOEING7 (Reply 4):
On all Boeing airplanes that have a RAT it is tested inflight. On the 777/787 it is dropped on the ground during the B-1 preflight checks by manually pushing the Ram Air Turbine switch -- in flight the generator control switches are turned off until the RAT drops and the APU autostarts confirming correct operation of the system.

On the 777, the RAT only drops automatically if both engines go sub-idle. On the 787 the RAT will drop automatically for both engines going sub-idle, and also for some combinations of generator failures. Remember that the 787 is highly dependent on electrical power for everything.

For the 757 and 767 the RAT will also drop automatically if both engines go sub-idle.

As 7BOEING7 indicates, there is also a switch on the overhead panel to manually drop the RAT. In fact, the Dual Engine Failure checklists have a step to push the RAT Deploy switch as a backup to the automation.

The 787 RAT supplies electric power only. The 777 RAT supplies both electric and hydraulic pressure. The 757 and 767 RAT supplies hydraulic pressure only.

On the 777 and 787 the APU will automatically start if both engines go sub-idle. I wasn't aware that it will automatically start on the 787 for generator failures also, as implied above. Again, for the Dual Engine Failure checklist, there is a step to manually start the APU to backup the automation.

The APU will not automatically start on the 757, 767 when both engines go sub-idle. As you might imagine, there is a step in Dual Engine Failure checklist to manually start it if both engines fail.

[Edited 2015-07-09 08:14:36]
 
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7BOEING7
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RE: RAT Question

Thu Jul 09, 2015 3:50 pm

Quoting BoeingGuy (Reply 7):

On the 777, the RAT only drops automatically if both engines go sub-idle. On the 787 the RAT will drop automatically for both engines going sub-idle, and also for some combinations of generator failures. Remember that the 787 is highly dependent on electrical power for everything. Ets, etc etc .....

A lot of misinformation here or I'm reading it wrong.

There are 3 failures that will automatically drop the RAT on the 777 and 5 on the 787. The RAT on both the 777 and 787 provides both hydraulic and electric power throughout the flight envelope.

777:

Both engines have failed and center pressure low, or
Loss of both AC transfer busses, or
All three hydraulic system pressures are low

787

Both engines have failed, or
All three hydraulic system pressures are low, or
Loss of electrical power to the Captain's and First Officer's flight instruments, or
Loss of all four EMP's and faults in flight control system on approach, or
Loss of all four EMP's and engine failure on takeoff or landing
 
boeingfixer
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RE: RAT Question

Thu Jul 09, 2015 4:14 pm

Quoting BoeingGuy (Reply 7):
For the 757 and 767 the RAT will also drop automatically if both engines go sub-idle.

I don't work on the 767 but on the 757 the RAT will only deploy automatically if these 3 conditions are met.

1. aircraft not on ground

2. airspeed greater than 80 knots

3. both engine N2 speeds drop below 51%

Take any 1 of those out of the equation and the RAT will not auto deploy.

Cheers,

John
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BoeingGuy
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RE: RAT Question

Thu Jul 09, 2015 4:36 pm

Quoting boeingfixer (Reply 9):
Quoting BoeingGuy (Reply 7):
For the 757 and 767 the RAT will also drop automatically if both engines go sub-idle.

I don't work on the 767 but on the 757 the RAT will only deploy automatically if these 3 conditions are met.

1. aircraft not on ground

2. airspeed greater than 80 knots

3. both engine N2 speeds drop below 51%

Take any 1 of those out of the equation and the RAT will not auto deploy.

Cheers,

John

That's pretty much what I said. Both engines go sub-idle (N2 80.
 
BoeingGuy
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RE: RAT Question

Thu Jul 09, 2015 4:39 pm

Quoting 7BOEING7 (Reply 8):
A lot of misinformation here or I'm reading it wrong.

There are 3 failures that will automatically drop the RAT on the 777 and 5 on the 787. The RAT on both the 777 and 787 provides both hydraulic and electric power throughout the flight envelope.

777:

Both engines have failed and center pressure low, or
Loss of both AC transfer busses, or
All three hydraulic system pressures are low

787

Both engines have failed, or
All three hydraulic system pressures are low, or
Loss of electrical power to the Captain's and First Officer's flight instruments, or
Loss of all four EMP's and faults in flight control system on approach, or
Loss of all four EMP's and engine failure on takeoff or landing

You just cut and pasted this out of the FCOM. I was being more brief than pasting every last detail. My understanding was the 787 RAT only supplies electric power though and then the electrical driven pumps supply hydraulics.
 
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7BOEING7
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RE: RAT Question

Thu Jul 09, 2015 5:45 pm

Quoting BoeingGuy (Reply 11):
You just cut and pasted this out of the FCOM. I was being more brief than pasting every last detail.

Yes I did, but your being "brief" by saying "only" was an incorrect statement and, the 787 directly provides hydraulic power to center system flight controls, the electric power mainly going to the instruments, communication, navigation and fire detection.
 
boeingfixer
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RE: RAT Question

Thu Jul 09, 2015 11:33 pm

Quoting BoeingGuy (Reply 10):
Quoting boeingfixer (Reply 9):
Quoting BoeingGuy (Reply 7):
For the 757 and 767 the RAT will also drop automatically if both engines go sub-idle.

I don't work on the 767 but on the 757 the RAT will only deploy automatically if these 3 conditions are met.

1. aircraft not on ground

2. airspeed greater than 80 knots

3. both engine N2 speeds drop below 51%

Take any 1 of those out of the equation and the RAT will not auto deploy.

Cheers,

John
Quoting BoeingGuy (Reply 10):
That's pretty much what I said. Both engines go sub-idle (N2 80.

No! Actually that's the only thing you said. I was just expanding on it for those who might find the extra information interesting.

Cheers,

John
Cheers, John YYC
 
Max Q
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RE: RAT Question

Fri Jul 10, 2015 7:45 am

Quoting BoeingGuy (Reply 7):
The APU will not automatically start on the 757, 767 when both engines go sub-idle. As you might imagine, there is a step in Dual Engine Failure checklist to manually start it if both engines fail.

Also, the 75/67 Rat does not supply any electrical power, it's only purpose is to provide hydraulic power to the flight controls
in the event of a dual engine flameout.
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HAWK21M
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RE: RAT Question

Fri Jul 10, 2015 12:43 pm

Most RATs need to be stored on the Ground with RAT unpowered.

Quoting Max Q (Reply 14):
Also, the 75/67 Rat does not supply any electrical power, it's only purpose is to provide hydraulic power to the flight controls
in the event of a dual engine flameout.

True....The HMG [Hydraulic Mechanical generator] provides the Electrical output vide Hydraulic input
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BoeingGuy
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RE: RAT Question

Fri Jul 10, 2015 4:41 pm

Quoting Max Q (Reply 14):
Quoting BoeingGuy (Reply 7):
The APU will not automatically start on the 757, 767 when both engines go sub-idle. As you might imagine, there is a step in Dual Engine Failure checklist to manually start it if both engines fail.

Also, the 75/67 Rat does not supply any electrical power, it's only purpose is to provide hydraulic power to the flight controls
in the event of a dual engine flameout.

I didn't state differently. I stated above that the 757/767 RAT only supplies Hydraulic Power.

Quoting HAWK21M (Reply 15):
True....The HMG [Hydraulic Mechanical generator] provides the Electrical output vide Hydraulic input

Yes, but the HMG was only installed on ETOPS and ER 757s and 767s. It was not installed on all 757/767s, especially some of the earlier domestic only ones.

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