Questions tagged [hydraulic-system]

Hydraulic systems on aircraft use pressurized liquid to move components such as control surfaces and landing gear.

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How do the hydraulics in an airplane have constant pressures? What is creating resistance to the pump flow to create constant pressure?

my question comes from the A320 specifically, and I also suppose Boeing jets. Whenever you check the hydraulic system pressures on an A320 it says 3,000 psi, even when you move the flight controls. ...
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Why do the ailerons on an A320 droop when the engines are shut off and the 787 ailerons dont droop when the engines are shut off?

I have been wondering why the ailerons on the A320 (and other Airbus aircraft) droop when there is no power from the hydraulic pump. From my understanding of hydraulic actuators, the position where it ...
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Why would the 737 autopilot A disengage without engine-driven hydraulic pump during heavy demand?

Note: Loss of the system A engine-driven hydraulic pump, and a heavy demand on system A, may cause A/P A to disengage. Just finding it difficult to understand why the A/P would disengage in this case....
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Why does the hydraulic pump of retractable-gear GA aircraft run continuously throughout the gear cycle?

The POH of a Cessna T210N (which is a retractable-gear aircraft) states that the hydraulic pump will run 1 - 2 times per hour to restore system pressure bleed down, and whenever the gear up / down ...
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What is the hydraulic fluid capacity of the Cessna 210?

What is the hydraulic fluid capacity of the hydraulic fluid reservoir and the complete hydraulic system of a Cessna 210 ? (the N model, if it makes a difference) I've looked in the POH, in the service ...
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How to troubleshoot a noisy aircraft engine driven hydraulic pump? [closed]

I understand that engine driven hydraulic pump noise is caused by several reasons such as cavitation,aeration or mechanical defects but how to troubleshoot this problem using a flowchart from any ...
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9 votes
3 answers
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How does the landing gear downlock mechanism work?

How does the landing gear downlock mechanism work? Especially when it's operated without hydraulic pressure or other energy sources. How can it keep the landing gear down and locked, during a gravity ...
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9 votes
2 answers
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How common were (are?) hydraulically driven fuel pumps?

The XB-70 had a large number of fuel transfer pumps (something like 25) to transfer fuel from the 10 other tanks into the sump tank, and then three fuel "boost" pumps to pump it from that to ...
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How to determine the RPM of the rotating shaft of an A320 PTU?

The A320 PTU has an in-line variable displacement pump on one side and a bent-axis fixed displacement pump on the other connected directly by a shaft. what would be needed to know how fast the shaft ...
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Why was NWS shifted from green hydraulic to yellow hydraulic system in the A320?

In older Airbus A320s, the NWS (nose wheel steering) was on the green hydraulic system and now it's on yellow. What's the reason for the change?
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Why was blue hydraulic system chosen for RAT?

Q1. Why is blue hydraulic system chosen for RAT out of the three. Q2. If in case of hydraulic emmergency if I push RAT switch on Emer Elec pannel what will happen
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Does transport-type certification account for engine separation and its possible effect on hydraulic lines?

This question is inspired by the United Airlines Flight 328 where the No. 2 engine is damaged. Some multi-engine aircraft are design to withstand the detachment of an engine while some are not. Boeing ...
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Why does the 737 Original have an explicit hydraulic-quantity indicator for only the A hydraulic system?

The 737 (Original, Classic, NG, and MAX) has two primary hydraulic systems (A and B),1 which provide power to the aircraft’s flight controls and other aircraft components (like the wheelbrakes, ...
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How many flight control shutoff valves are there in Boeing 737?

I came to know from a source that there are total 6 flight control SOV 3 of which associated with each of system A and B hydraulic system .but in 737 FCOM under stand by hydraulic section in schematic ...
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Writing Research: What stick sensation would you expect in an Su-15 with a total hydraulic flight system failure?

The title pretty much says it all. I have a very specific research question in relation to a writing project I'm working on. The main character is in an Su-15 performing low flying military training, ...
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Why is the landing gear lever not set to OFF on the ground?

Several older airliners (e.g. Airbus A300, Boeing 737 until the NG series, Boeing 747 until the -400 series) have landing gear levers with three positions: UP, OFF and DOWN. OFF (or NEUTRAL for Airbus)...
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How does an accessory drive hydraulic system power a crop duster pump?

Aerial agricultural spraying uses pumps, which are usually powered electrically from the starter/generator. Now I have read, that these pumps can also be powered by an accessory drive hydraulic system....
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Why does Thrust Reverser need to be operative during the cargo door operation?

It is stated that when the Electric Pump push button is depressed, the electric pump is automatically running as long as the cargo door selector valve is set to either OPEN or CLOSE position. During ...
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5 votes
1 answer
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Where is the control loop of the actuator closed?

When electro-hydraulic servo-actuators are used for the actuation of control surfaces in aircraft, linear variable differential transformers (LVDTs) are used for position or velocity feedback. There ...
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What does the isolation valve do in the landing gear extension/retraction system?

The isolation valve is an integral part of the landing gear extension/retraction systems on most larger aircraft. What is its funtion?
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Why is the most common hydraulic fluid so poisonous?

The usual hydraulic fluid, Skydrol, is poisonous. If it makes it to the air conditioning system, it causes fume events that may have adverse health effects up to and including at least one fatality. ...
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What factors should be considered to select jet-pipe or flapper-nozzle technology for a primary control surface?

How is the servovalve mechanism chosen between these two mechanisms? what are their advantages and drawbacks?
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How is the bandwidth of an actuator selected for a primary flight control surface?

How the bandwidth of an actuator is selected for a primary flight control surface? Is there any standard? Based on what factors this decision is made?
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Do jackscrews suffer from blowdown?

With hydraulic control surfaces, at high airspeeds, there is a point where aerodynamic loads exceed the capability of the actuators. This limits control authority and can result in the control surface ...
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6 votes
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Why does the B737NG need electrical power for aircraft towing?

I have read the AMM for towing and one of the mentioned steps is to supply electrical power. Is it because you need power to turn on the hydraulics in order to pressurize your brakes? For B737NG
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7 votes
3 answers
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How strong someone should be in order to fly without "power steering"?

Planes these days use fly by wire systems while older ones had a mix of hydraulics with electronic assistance like the MD-11 (And maybe 737-800?). But what would happen if you lost the electronic ...
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4 votes
2 answers
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Why does only one hydraulic intake line from the reservoir have a filter on the Arrow II?

This is a portion of an Arrow II landing gear hydraulic system diagram. I noticed that there are two intake check valves, one located on each side of the pump, that would allow flow from the ...
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2 votes
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What is the difference between a pressure switch, high-pressure control, and low-pressure control in a landing gear hydraulic system?

This is an Arrow II landing gear diagram. In the diagram, the high and low pressure controls look like check valves. The pressure switch is unclear. What are these three items, and what purpose ...
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2 answers
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What is the purpose of the shuttle valve in a Piper Arrow's landing gear system?

This is the PA-28R-201 landing gear hydraulic system diagram. I am wondering how this shuttle valve functions. Research I have done indicates that a shuttle valve blocks one attachment to the valve ...
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2 votes
2 answers
139 views

Surface deflection if loss of hydraulics on combat jet

In a typical combat jet from the 1970ties, at loss of hydraulic pressure, how does the deflection surfaces behave? If it happens on the runway, will the weight of the surfaces make them deflect ...
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6 votes
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How can the F-16 remain stable when there is a loss of hydraulic power?

Edit for justification: This question is different from If the EPU (Emergency Power Unit) on a modern fighter jet fails, will the aircraft drop out of the sky? for the following reasons: The ...
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-4 votes
1 answer
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Are hydraulic retractible landing gear possible

One of the impediments and drawbacks to extending landing gear in light of building larger engines are the larger space required for the taller gear. Would hydraulic retractable gear that works as a ...
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14 votes
4 answers
9k views

Can the autopilot (or stall avoidance system?) of the B737 MAX 8 be overridden by sheer force?

A New York Times article from today about the Lion Air Flight 610 crash claims that in order to control the airplane the pilot could have braced his feet on the dashboard and yanked the yoke, or ...
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1 vote
1 answer
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Fly-by-wire vs hydraulic systems [closed]

So, the great old argument, flyer-by-wire systems vs conventional hydraulics. I was wondering which one was better and why. Which one is safer? Pros and cons in terms of safety? Basically, I just ...
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Is the 737NG hydraulically controlled or electronically controlled?

I know that the 737NG is not fly-by-wire, but are the control surfaces controlled with hydraulic systems that transfer the input from the pilot to the control surfaces via fluid (hydraulics), or is ...
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1 vote
2 answers
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What is the average aerodynamic load on a control surface of a commuter-sized airplane?

I'm doing a study about aircraft hydraulic pump sizing. In order to do that, I need to know the size of a flight control actuator, then determine the maximum flow and pressure that the actuator needs. ...
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6 votes
1 answer
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Why does the A320 right aileron float on dual engine failure?

On this question for the A320 after dual engine failure it says: When the hydraulic power is lost, the right aileron is lost, and is in the upfloat position. Why would this affect only the right ...
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Is it possible to 'retrofit' electric flight control actuators to replace existing hydraulic actuators on aircraft?

Read a bit about the potential cost savings of replacing existing hydraulic aileron actuators on large commercial aircraft such as the A350/380 with electrohydraulic/electromechanical actuators. ...
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1 answer
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How can I reduce the responsivness of my homebrew brake system?

I have a hydraulically actuated aircraft brake system. Each main landing gear are equipped with brake disk hydraulically operated. The actuation is realized independently for each wheels by means of ...
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5 votes
3 answers
985 views

Do Airbus aircraft have an "off" position for their landing gear?

I know some Boeing aircraft feature an up, down, and "off" position for their landing gear lever. I know that some Boeing wide-body jets do not have a 3rd lever position, but still have an "automatic" ...
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3 votes
1 answer
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What happens if an aircraft's hydraulic system starts leaking during a flight?

What will happen if hydraulic fluid starts leaking during a flight? What will happen to the overall performance of the flight, and how will the pilot compensate for the loss of the system?
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Why is it called "STBY RUD" on a Boeing 737?

(Source: b737.org.uk) Anyone knows why "STBY RUD" is named "STBY RUD" when the selection of the switch position activates the standby hydraulic system as well as the standby rudder system (by opening ...
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3 votes
2 answers
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How does hydraulic actuator servo valve feedback work?

The book Aircraft Systems uses the following diagram for a mechanically controlled hydraulic linear actuator (for moving a control surface e.g. an aileron): The following explanation is given about ...
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5 votes
2 answers
3k views

How many hydraulic systems does the Boeing 757 series have?

How many hydraulic systems does the B757 series have, does it have a Power Transfer Unit (PTU)? What is the operating pressure of the hydraulics in the Boeing 757? How big is the hydraulic reservoir?...
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2 votes
4 answers
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What is the difference between MIL-H-5606 and MIL-PRF-5606 hydraulic fluid?

what is the meaning of PRF here and can you differentiate them visually ?
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Have there been fully pneumatic aircraft?

Have there been any passenger aircraft that used pneumatics for all of their on board systems that required force actuation, such as: landing gear; wheel brakes; nose wheel steering; propeller ...
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Why is 3000 psi the most common hydraulic pressure used in commercial aircraft?

As per the article on this website (from 2002), most of the aircraft use 3000 psi as hydraulic pressure: Since the 1970s, military aircraft have been using 5000 psi hydraulic systems, which use ...
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What are the reasons for using hydraulic power on major aircraft systems? [duplicate]

What are the actual benefits or advantages of using Hydraulic energy in aircraft systems, compared to pneumatic and electrical systems.
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1 vote
1 answer
664 views

How is the direction of the hydraulic fluid changed so that the cylinders and pistons can move backwards and forwards?

How is the direction of the hydraulic fluid changed, so that components and actuators (cylinders and pistons) can move backwards and forwards?
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2 votes
1 answer
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What would be the potential effects of not having an accumulator in the hydraulic system?

In an aircraft, what would be the potential effect of not having an accumulator in the hydraulic system?
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