38

Something should be pointed out here. Fume events are not related to Phosphate Ester hydraulic fluid and there is nearly zero risk of Skydrol or Hyjet getting into an air conditioning system (if there are airliners out there that use air conditioning source bleed, to pressurize accumulators or reservoirs, with an open return path back through the bleed ...


25

The linked article directly answers the question "Above 3000 psi, using aluminum as a pressure vessel becomes risky," says Galloway. "5000 psi systems almost exclusively rely on steel or titanium. To minimize stresses, the sizes of passages can be reduced. Computational fluid dynamics is also used to minimize pressure drop in critical areas." 3000 PSI ...


23

Skydrol was developed to provide a hydraulic fluid that does not catch fire when a fine mist of particles under pressure is released, upon a leak in the hydraulic system. Of the two evils, poisoning or burning, a case can be made to avoid the latter when considering aeroplanes. For ground based systems like full flight simulators, hydraulic systems are on ...


20

I'm sure it's possible to use servo motors to get precise control of the ailerons, elevators or the rudder. No, it isn't. Primary flight control actuators require both rather high forces and quick response time. Electric motors can provide either, but both at the same time is a problem. Hydraulics is still better for that combination. Note that other ...


18

The primary reason is availability -- the high pressure fuel pump is running whenever the engine is, while the airframe hydraulic systems may not be available in case of an accessory gearbox failure, hydraulic leak, or maintenance shutoff. Also, this keeps hydraulic lines from needing to be plumbed deep within the bowels of the engine -- this has benefits ...


17

According to the Boeing 737 Flight Crew Operation Manual, the "Standby Hydraulic System" only powers the following: thrust reversers rudder leading edge flaps and slats (extend only) standby yaw damper It does not power, for example, brakes, ailerons, spoilers and the landing gear. I speculate that the word is chosen to remind pilots that the system ...


16

Is it possible? Sure. Theoretically. Once. I can't speak to your Aeroflot scenario without more details, but broadly speaking any liquid will work as "hydraulic fluid" within certain limits: At the most basic level all that is required is that it be incompressible and flow at an acceptable rate (i.e. It is a liquid with a viscosity that falls within a ...


15

Screws can be either self-locking or overhauling. Jackscrews used for stabilizer control are designed to be self-locking, since their purpose is to make the stabilizer adjustable, but prevent it from moving by itself. A jackscrew (or screw jack) is pretty much by definition a self-locking screw. It should be noted that vibrations can induce travel in ...


15

Acme screw type screw jacks with the square threads, as used in stab trim systems (as opposed to a recirculating ball screw), are usually inherently irreversible because of the higher friction of the direct sliding contact of the square sided threads vs a rolling ball interface (as a sliding interface, it's totally dependent on the grease to keep friction in ...


13

Standby Rudder is a separate indicator light that was installed as part of the Rudder Enhancement Program, which was put in place after findings of the NTSB regarding the cause of accidents that occurred in the 90s. Standby Rudder pressurises the Standby hydraulic system which will power the separate Standby rudder PCU. Above and below pictures are from ...


12

I'm not sure of the full story here, but if you pull the controls of a 737 while the autopilot is engaged the autopilot will trip and the pilots can fly the plane manually. This requires about 25 lbs of force. There is also a button on the yoke where you can disengage the AP. So if the AP was controlling the plane and going into a violent descent the pilots ...


11

You can't. Most of these airplanes are flown with hydraulically powered control surfaces with no mechanical input possible from the cockpit flight controls. The flight controls just operate servo control valves in the hydraulic actuators, like the bucket on a front end loader but a little fancier. If it's FBW, the FBW system does the same thing at the ...


10

(A320 PTU (Vickers), source) The Power Transfer Unit is well known on Airbus due to its barking-like sound (refresher: Video of a PTU in action). The sound comes from a pump which creates a 3,000 psi pressure being quickly switched ON and OFF. (Source: Youtube) The PTU is a safety system to ensure hydraulic pressure is available, specially at takeoff, ...


8

Yes there have been, and some of them are still flying. This article is about the fully pneumatic actuation systems on board the F27s built by Fokker and by Fairchild. If there was a leak, it was leaking clean air. System pressure was 3,300 PSI = 228 bar. Some of the advantages cited: Many factors favored the use of pneumatic systems on the F-27, most ...


8

This depends on how the systems are plumbed, and while this is likely an exception, for some airplanes like the EMB-145, the answer is yes. The EMB-145 has 2 hydraulic systems. System #1 serviced the outboard brakes and system #2 serviced the inboard brakes and the parking brake. The parking brake actuated all of the brakes and this let you transfer ...


8

There is only one rudder on the aircraft, so speaking of "the standby rudder" would be incorrect -- to describe it that way suggests the presence of a "main" rudder and a second "standby" rudder, and that's not what's on the 737. What you do have is the standby hydraulic system, which is used to provide backup hydraulic power to a few components (listed in @...


7

I suspect @RonBeyer's comment is right and the fluids cannot mix. According to the diagram in this answer (explaining the 3 hydraulic systems in an A320) the rudder (for ex.) happens to be fed by the Green, Blue, and Yellow system. I found this description of the hydraulic system and control of a B737's rudder. They say The rudder PCU [Power Control ...


7

As mentioned already, hydraulic systems are fast and powerful but you need to also consider how extremely efficient and simple they are. Reducing complexity reduces maintenance costs which is a large factor. Flaps, trim devices, and landing gear doors are the only good places to use motors. I believe the specific example you're looking for was the Sioux ...


7

Most transport category aircraft, including B737, have the elevator, the main surface controlling pitch, attached to a horizontal stabilizer that is also movable. The elevator is controlled using the control column (yoke), while the stabilizer is moved using the trim wheels on the central console. This arrangement means the command from the elevator is ...


6

About 25 years ago, there was a major effort to revise MIL standards and specifications. The biggest change in MIL specs was to move away from a design or manufacture spec to a performance spec. The downside of the old specs were that they didn't allow for an improved product without getting the spec revised. By switching to performance spec (what it must ...


6

In short, yes! The thing is, a fluid is fluid. Any fluid that is similar to water or oil can be used. But, you would need to be on the ground and have enough of that fluid. Also, using such a fluid will introduce potential damage and corrosion to the system, so a flush should be done as soon as possible. In addition, there might be hydraulic systems that ...


6

I was working for Airbus before, never seen it on A300, A320, A330, A340, A380, A350 or A400M. Only Gear Up or Gear Down position. For confirmation on A320, here the FCOM section: NORMAL OPERATION The flight crew normally operates the landing gear by means of the lever on the center instrument panel. The LGCIUs control the sequencing of gear and doors ...


6

With typical hydraulic controls, the mechanical control cable circuit just operates control valves on the hydraulic actuators. The control valves normally only have fairly light centering springs that take only a few pounds of force to overcome. With no other devices in the control circuit, you could move the control column with your pinky finger. Control ...


6

Out of trim is no way to fly The issue with the B737MAX as detailed in the Emergency Airworthiness Directive from Nov. 7, 2018 is not that the plane is applying a nose-down elevator input through the autopilot servos — this would be trivial for the pilots to overcome, even if the servos failed to declutch when the pilots pulled the yoke. The problem ...


6

Only small planes can fly without power actuation. The largest size airliner that can be flown manually would be about a B737, which actually has manual back-up for the elevator & ailerons as in this answer. The B737 uses a balance tab to assist in lowering the hinge moments during manual actuation. In a much larger aeroplane like the DC-10, there is no ...


5

Airbus A320 (also A319) has three hydraulic systems — green, yellow and blue. Green is driven by engine 1, while the yellow is driven by engine 2 (there are electric pumps for yellow and blue, but that's beside the point). The PTU is a bidirectional unit which enables the yellow system to pressurize the green and vice versa. The PTU runs automatically when ...


5

As wikipedia mentions, "Electro-Hydrostatic actuators" (EHA) are self contained, the only input they need is electrical power and commands. The electricity powers up the hydraulic pump that is used to move the actuator piston, while the electrical commands are used to select what the actuator does through the use of a valve. What does electrically ...


5

Assuming it's zero fuel dual engine failure with no chance of powering up the APU, the plane is left with the Ram Air Turbine (RAT). 1. The emergency electrical configuration only powers ELAC 1, one of the two units that control the ailerons, and ELAC 2 is lost (see below). (A320 FCOM) 2. The RAT powers only the Blue hydraulic system. Add those two ...


5

Why is it designed like that? This design combines two advantages: The system works reliably with one filter instead of two, saving a bit of cost, weight, and complexity In case of a blocked filter, the gears can still be lowered (thanks @GgD for the hint!). How it works Gear down: When the flight deck gear selection handle is put in the gear-down ...


4

The direction of travel is controlled by a directional control valve for a double acting cylinder. The red line is high pressure from hydraulic pump. when the high pressure fluid is entering the right chamber and push the actuator piston to left. The low pressure fluid in left chamber (blue line) will leave the actuator and return to reservoir. If we ...


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