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Old 24th Aug 2004, 04:23   #1 (permalink)
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Hydraulic Fuses in Aircraft

Hear tell that some/many aircraft are now fitted with Hydraulic Fuses to isolate parts of the system and preserve partial operation.

Question arises as result of another thread which mentions water fuses as available for isolating washing machines and dish washers should a feeder hose burst. Saves a flooded house.

Could some knowledgeable engineer tell us something about hydraulic fuses?
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Old 24th Aug 2004, 08:31   #2 (permalink)

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A simple mechanical device that is fitted primarily wherever there is a risk of a ruptured hydraulic line causing total loss of fluid so they are typically found in the brake lines and u/c tilt actuator lines (if fitted!).
If there is a sudden high flow of fluid a spring loaded shuttle moves across to shut off the flow (sorry can't think of a better description at the mo')
The 747 also had them fitted in the hydraulic lines to the empenage after the failure of the rear pressure bulkhead and fin on the JAL a/c that was lost.
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Old 24th Aug 2004, 15:49   #3 (permalink)
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The description above is pretty spot-on. Any sudden increase in flow as a result of a burst/failed line downstream will cause the fuse to close thus preserving some/most of the system fluid.

The 737-3/4/500 has fuse's in the brake system, L/E flap/slat extend/retract lines, nose gear extend/retract lines and also in the thrust reverser pressure and return lines.

It is possible to inadvertently trip the thrust reverser fuses by cycling the reversers too quickly i.e repeatedly deploying and re-stowing as sometimes happens during system function checks in maintenance. The MM recommends a minimum of 10 sec's between cycles.

The 737 fuses are generally about the size of 2 "D" cell batteries and sometimes have a check valve as part of the union on one end.

Rgds, Eng
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Old 25th Aug 2004, 07:41   #4 (permalink)
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What happens on the return/suction side of a system if a pressure line fails causing the fuse to shut off?

Will not the system be then sucking air.

Perhaps the fuse has both pressure line and return line coincident and both shut off together.

Any known cases where an aircraft and occupants owe their longevity to a fuse?
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Old 25th Aug 2004, 10:29   #5 (permalink)
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Longevity link


Page 7 shows shows the parking brake schematic and You will see the fuse for the rtn line,great question though.
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Old 25th Aug 2004, 14:46   #6 (permalink)
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I can offhand think of one particular type that has not actually crashed thanks to hydraulic fusing...L1011/SV/DOH, early eighties.
In this particular incident, a tyre/wheel burst in the well at FL290 ruptured all four hydraulic systems, and the aircraft was successfully landed at DOH by the First Officer (mandatory, because only his column functioned) with less than one half of system D remaining.
All others were depleated.

Then again, can think of one particular type that has crashed, because it did not have hydraulic fusing fitted, as standard equipment...DC10/UAL/Souix City.

JAL has already been mentioned.

Lockheed got it right, during original design.
Some of couse might argue otherwise, but usually find that these folks have not flown the TriStar....a delightful aircraft, especially the -200's/-500's.
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Old 25th Aug 2004, 16:50   #7 (permalink)
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In the race to produce a working design 35 years ago, Lockheed lost out to MD, with the DC10. What with Lockheed's fusing (which MD considered, but discarded), the poorly designed cargo door (which blew out a number of times, collapsing the floor onto the flight controls, resulting in bent hulls & bodies) of the DC10 (Lockheed had an inward opening door, which was inherently safer), and many other items, Lockheed was a much superior design, but a financial disaster. Public gets what it pays for!
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Old 26th Aug 2004, 00:32   #8 (permalink)
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Right you are,411A,

The C-5 also has hydraulic fuses throughout the systems and have saved loads of problems. Many installed after the Saigon crash attributed to loss of flight controls after the rear pressure door failed. Regrettably, the ramps and doors must be pressurized by design and size. Loads of fuses and pins to ensure that won't happen again.

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Old 26th Aug 2004, 01:27   #9 (permalink)
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Back to the point of the thread.
Are 'hydraulic' fuses actually available for domestic water supplies?

If not, I claim the marketing rights

Last edited by andyb79; 26th Aug 2004 at 14:11.
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Old 26th Aug 2004, 01:42   #10 (permalink)
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Domestic water supply fuses.

Some around but inexplicably rare.
Mine have saved a house flooding which is a terrible event if house is unoccupied at the time.

Very simple in design. A half ball is held off a seat by a spring. Excessive flow drags half ball down on to seat for a complete cut off. Turn off tap upstream and fuse resets.

I would like to be the Aussie distributor.

Great to hear that hydraulic fuses are becoming the norm in aircraft systems.
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Old 26th Aug 2004, 11:31   #11 (permalink)
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I found a nice Picture of a Hydraulik Fuse called Safety Valve.


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Old 26th Aug 2004, 14:07   #12 (permalink)

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Thumbs up Costs too much and/or it weighs too much.

You would be surprised how many aircraft manufacturers and equipment manufacturers refused to consider the input of reliability engineering relative to the incorporation of hydraulic fuses. The two main reasons for not using them are it adds to systems cost and/or it adds to systems weight.

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Old 26th Aug 2004, 15:24   #13 (permalink)
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Except Lockheed, of course, with the TriStar.

Only those lucky enough to have flown the old girl could possibly understand.
For others, it was definitely second fiddle.
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Old 27th Aug 2004, 11:23   #14 (permalink)
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So, Lockheed put the fuses in because it was a good idea, Boeing put them in the 747 because of JAL123 (12 Aug 1985), and nobody thought this was a good idea for all a/c that would be uncontrollable without hydraulics.
Question: Did MDD/Boeing install fuses on the DC10/MD11 after Sioux City?

Has every type got to crash owing to hydraulic failure before that type gets hydraulic fuses?
This is surely generic to every wide-body that would crash without hydraulics, so why is the fuse safeguard not applied to all of them?
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Old 27th Aug 2004, 15:17   #15 (permalink)

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Thumbs up

The main reason for not installing fuses in the hydraulics systems of some aircraft is the reliance on the safety hazards analysis. This analysis considers the loss of a single system and depends on system design and redundancy to keep the other systems from going tits-up. This analysis does not consider an external force causing the loss of all systems at the same time.

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Old 28th Aug 2004, 14:30   #16 (permalink)
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For those wishing to see what they look like, these are on the aft wall the wheel-well of the 737:

S & L
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Old 29th Aug 2004, 05:19   #17 (permalink)
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Good one CaptainSandL

Nice pics. What sub sytems do they isolate?

What are the little white balls? Resets?

And what about isolation of the return lines at the same time, else the system is going to be sucking air if a line is ruptured.
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Old 29th Aug 2004, 10:31   #18 (permalink)
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Hi Milt,

They are both for the left normal brake system, one for the inboard wheel and one for the outboard. There are another pair on the other side of the aft wall for the right normal brakes and also one each for the left & right alternate brakes.

Yes the white balls are reset levers (see below).

There are no return lines. The fluid goes to & from the brakes along the same line. The fuses are the last things in the whole system before the brakes, (except for the shuttle valves to use the alternate braking system) so even if a wheel falls off, the fuses will close, no further hydraulic fluid will be lost and no air will be sucked back into the system. Brilliant.

Below is an extract from the 737NG AMM

S & L

“Brake hydraulic fuses prevent hydraulic fluid loss if there is an external leak downstream of the fuses.

Functional Description
During normal operation, the piston and spring are at the relaxed position. This lets fluid pressure go through the metering slot and over the bypass valve normally. When the pressure differential across the fuse starts to decrease below normal, the piston starts to compress the spring.

If 60 to 95 cubic inches (0.9-1.5 litres) of fluid goes through the hydraulic fuse metering slot and bypass valve, the piston compresses the spring until the fuse
is closed. When the pressure differential across the hydraulic fuse decreases between 0 and 30 psi, the fuse resets. This lets the spring push the piston to the normal position.

A reset lever permits manual reset of the hydraulic fuse. To reset the hydraulic fuse, move the reset lever in the direction shown on the placard near the fuse. This operates a bypass valve inside the hydraulic fuse that makes the pressure on each side of the fuse equal.

There are no visual indications of a closed fuse.”
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