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tybin
06-14-2018, 03:59 AM
[DISCLAIMER]I'm not a pilot, I'm an enthusiast with a question that seems to not have an answer elsewhere, that's why I'm here

Hello,
my name's Jake, that's my first post here, thrilled to join the community!

Lately I've watched some paradocumentary about loss of power and it's consequences, and at some point the narrator started to explain that after full power loss (all engines out, only wind turbine/battery power) the airplane is much harder to control because of loss of the hydraulic system (exact words: hydraulics aren't working anymore). I thought about it for a minute and came to conclusion that regardless of having power or not, pilot flies using hydraulics, just that instead of pumps or servos pressurizing the whole system it's his muscles that introduces the nessesary pressure to whole system. Am I right, or in case of hydraulic loss (not the fluids, just the power) he operates by strings, directly affecting control surfaces like in older planes? Plane that's in question is 737-800, but I figured any plane with hydraulic system used to control the airplane would have a similar solution implemented.

PS. If the thread doesn't match the standards or the question should be disregarded as obvoius to people with experience, please give it a chance and move it to offtopic instead of trash. Thanks!


Packrat
06-14-2018, 05:22 AM
In a lot of systems there is a cable back up to the hydraulic system. In fly-by-wire systems there can be 3 or more independent hydraulic systems that use ram air turbines (RATs) to provide electrical power for the hydraulic pumps in case of electrical failure.

rickair7777
06-14-2018, 06:38 AM
Depends on the design and size of the plane.

Small planes have no hydraulics, only mechanical links (cables, pushrods typically).

As planes get larger, hydraulics become helpful and then necessary to operate the control surfaces because the aerodynamic forces increase with larger surfaces and faster speeds.

In some cases there exists a mechanical linkage which is boosted by hydraulics, this sort of design would allow some control without hydraulics.

New planes typically have no mechanical backup on some or all controls, so hydraulics would be necessary. In those planes there are typically three hydraulic systems, 5-6 pumps, and a wind-driven backup turbine. Also some pumps are electric, but some are driven directly by the engine. In case electrical power is lost, the engine-driven pumps will still work (the fuel pumps and engines can run without any external electrical systems).

About the only thing that get through all that redundancy is actual physical damage from objects penetrating the aircraft. This is what happened with UAL flight 232, but there were some re-designs to prevent a failed engine from damaging all three hydraulic system. Today about the only that should be able to do that would be a bad collision or enemy action.

Even without hydraulics, some control might be available with differential thrust (roll/yaw), flaps/power setting (pitch), or mechanical trim tabs (if installed). That's how flight 232 arrived at the runway.


Twin Wasp
06-14-2018, 06:46 AM
If the engines are still turning (windmilling) they will still provide some hydraulic pressure. But the system may not be at the pressure and flow it would be with the engines running. So the flight controls would not react the same which could make the aircraft harder to control. It really depends on the airplane. Older Boeings had "manual reversion" on some of the flight controls. It was a cable from the yoke to the flight control surface. The 744 doesn't have manual reversion but each of the 4 hydraulic systems has a backup pump to pressurize the system if the engine driven pump is inop. But on some 744s some of the backup pumps are driven by engine bleed air so if the engines fail there would be no hydraulic pressure in that system. You would lose the outer ailerons and elevators but the inner ones would still be powered so the airplane would respond slower to flight control input.

Flightcap
06-14-2018, 05:05 PM
Many modern airliners (Embraer 170/190 and Airbus series in particular, probably more) use a wind-powered turbine as a last resort to pressurize hydraulic systems in the event of total loss of engine power. It automatically deploys when the airplane detects a loss of engine power so the system sees very little if any loss of pressure. Many auxiliary hydraulic devices will be de-powered but key flight controls will continue to operate. Air Transat flight 236's glide to the Azores is a good example of this system in action.

UAL T38 Phlyer
06-15-2018, 05:39 PM
757 and 767 also have RATs.

tybin
06-16-2018, 04:51 AM
To summarize the thread:
1) Critical hydraulic systems are pressurized by RAT(s) after loosing all engine power
2) Unless specified, there usually isn't any auxillary mechanical system that could help in case of loosing hydraulic pressure and/or engine power at the same time
3) Unless the engine was badly damaged and stopped turning completely, it provides some power from windmilling

Thanks!

sailingfun
06-22-2018, 04:11 AM
To summarize the thread:
1) Critical hydraulic systems are pressurized by RAT(s) after loosing all engine power
2) Unless specified, there usually isn't any auxillary mechanical system that could help in case of loosing hydraulic pressure and/or engine power at the same time
3) Unless the engine was badly damaged and stopped turning completely, it provides some power from windmilling

Thanks!

Your summary is not really correct. Each different type of aircraft may have different backup systems. You have to be aircraft specific. Many aircraft have a mechanical backup like the 727. The MD-80 series are mechanical all the time.

TJBrass
06-24-2018, 03:04 AM
The RAT does not pressurize the hydraulic system, it is a wind driven generator providing electrical power that, in turn, provides power to a electrical driven hydraulic pump. Most modern engines have some type of flow control valve that disconnects or reduces the hydraulics on a windmilling engine. The idea it that it is easier to windmill start without the extra strain of powering a hydraulic system

sailingfun
06-26-2018, 06:03 AM
The RAT does not pressurize the hydraulic system, it is a wind driven generator providing electrical power that, in turn, provides power to a electrical driven hydraulic pump. Most modern engines have some type of flow control valve that disconnects or reduces the hydraulics on a windmilling engine. The idea it that it is easier to windmill start without the extra strain of powering a hydraulic system

Again this depends on the aircraft type. There are hydraulic only rats on aircraft. The 757/767 is a example.

joepilot
06-29-2018, 09:55 AM
I will chime in on the "everything is aircraft specific" bandwagon.

Some quite large aircraft (such as the DC-8) were designed without hydraulic power to some or all of the flight controls. This can work because control tabs are used to aerodynamically reduce the force required. The DC-8 elevator was strictly muscle powered via a very long and thick cable. The majority of pilots used two hands to flare. The horizontal stabilizer (trim) did use hydraulics, with electric backup trim.

Joe



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