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40 votes

Why is the most common hydraulic fluid so poisonous?

Fume events are from two sources in the vast majority of cases: Deicing fluid, and engine oil. The Aviation Herald event linked was poisoning from (really severe) engine oil contamination (mainly the ...
John K's user avatar
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28 votes
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Why is the most common hydraulic fluid so poisonous?

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 ...
Koyovis's user avatar
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26 votes

Why is 3000 psi the most common hydraulic pressure used in commercial aircraft?

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. ...
user71659's user avatar
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17 votes
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Why is it called "STBY RUD" on a Boeing 737?

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 ...
kevin's user avatar
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17 votes
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Why are the speed brakes on the F-86 deployed when parked?

The F-86 Sabre Pilot's Flight Operating Manual is availlable on Google books site. It describes the speed brake system as being operated by the utility hydraulic system. Both extension and retraction ...
Jpe61's user avatar
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16 votes
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Do jackscrews suffer from blowdown?

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 ...
John K's user avatar
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15 votes

Do jackscrews suffer from blowdown?

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 ...
Jpe61's user avatar
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15 votes
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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” This ...
Mike Sowsun's user avatar
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15 votes
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How does an aileron actuator work considering the following?

You will see two methods of providing articulation for the hydraulic cylinder to accommodate the vertical shift of the attachment at the aileron as the aileron moves. In one method, one end of the ...
John K's user avatar
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13 votes

Why is it called "STBY RUD" on a Boeing 737?

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 ...
Koyovis's user avatar
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12 votes

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

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 ...
Douglas Pettersson's user avatar
12 votes

How strong someone should be in order to fly without "power steering"?

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 ...
John K's user avatar
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10 votes

How does the landing gear downlock mechanism work?

Short answer just in case details in the other answers leave anybody scratching their head trying to relate: Think of a folding table leg with its 45 degree angled brace, and the little sliding metal ...
Michael Hall's user avatar
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8 votes

Why is it called "STBY RUD" on a Boeing 737?

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 ...
Ralph J's user avatar
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8 votes

Have there been fully pneumatic aircraft?

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 ...
Koyovis's user avatar
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8 votes
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What is the difference between MIL-H-5606 and MIL-PRF-5606 hydraulic fluid?

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 ...
Gerry's user avatar
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Can the fluid from one hydraulic circuit go to another one (voluntarily or not) in flight?

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 ...
casey's user avatar
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8 votes
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Why do some aircraft's flight control surfaces not droop when hydraulic power is taken away?

There are two kinds of Power Control Units used on flight controls. The PCU is the hydraulic cylinder and input control valves and linkage that drives the surface, normally two or three per surface. ...
John K's user avatar
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7 votes

Can the fluid from one hydraulic circuit go to another one (voluntarily or not) in flight?

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 ...
PerlDuck's user avatar
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7 votes
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Can the autopilot (or stall avoidance system?) of the B737 MAX 8 be overridden by sheer force?

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 ...
Jan Hudec's user avatar
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7 votes
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How strong someone should be in order to fly without "power steering"?

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 ...
Koyovis's user avatar
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7 votes

How does the landing gear downlock mechanism work?

The main side brace of the gear leg is itself braced to hold it straight by the smaller brace in the middle that runs back to the main gear leg. The smaller lateral brace has an overcenter position ...
John K's user avatar
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7 votes
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Why does the hydraulic pump of retractable-gear GA aircraft run continuously throughout the gear cycle?

Since there should be no pressure loss. There is no fluid loss, but there most definitely is pressure loss. Pressure is energy. More precisely, energy is pressure times volume. A hydraulic actuator ...
Jan Hudec's user avatar
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6 votes
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Do Airbus aircraft have an "off" position for their landing gear?

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 ...
Chris Lau's user avatar
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6 votes

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

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 ...
John K's user avatar
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6 votes

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

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 ...
UnrecognizedFallingObject's user avatar

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