UNREALISTIC HYDRAULIC SYSTEM FAILURE!

[Guest MB327]

Thanks for posting that link. I have seen a few documentary films about that flight but this was a very interesting story and it cleared up a few misconceptions about that flight as well. I found myself reading that long after bedtime last night.Thanks again,

[Guest]

>ALL big airplanes still have at least one area where all 3 hydraulic systems are close together I may be wrong (wouldn't be the first time!) but I'm sure I read that Boeing made a modification to the Hydraulics of the 744's as a result of the JAL123* incident to prevent the hydraulic systems losing pressure in the event of a similar incident. Unfortunately I left my PS1 docos at home so can't get at the extra hydraulics docos today, but I'm sure the -400's now have a kind of fuse to maintain (some) pressure... Ian? ;)(edit - not all hydraulics systems were lost, but "Because the rudder is powered by two hydraulic systems, the oil in the other system also escaped, basically leaving the aircraft with a lot of control surfaces inoperative")John*failure of rear pressure bulkhead due to maintenance error caused explosive decompression that blew parts of the tail off, and in the process ruptured the 2 hydraulic systems feeding the rudder.

[Guest]

Having said that, I found an unzipped version (I do do work at work, really I do!)Apparently the "fuse" detects flow rather than pressure...Regards;John

[Guest]

Which prompts a question - was such a device retrofitted to other Boeing models as well (specifically 767)... and if not, what might have been the reasons... ? Cost?Cheers;John

[Guest Iz]

Hi Ian,Well I don't know about the 744. I know that it has many of the same system design principles as the 757/767/777 series (though completely different from any 737 or 747 classic).Normal trim, that you find in any aircraft from a Cessna 152 is factually not trimming for a pitch attitude but for a speed. Add power, the nose will go up to achieve the same speed, decrease power, nose will drop. So the only time you really need to change your trim, is when you change your airspeed. Not when climbing, not when descending, not when leveling off, only when airspeed changes.Because in a jet you have such a large airspeed regime, they incorporated this autotrim system that helps you as you fly manually.Again, it does nothing when your airspeed is constant (even in climb, descent, turns or when leveling off or even when changing flaps/gear/speedbrakes while keeping a constant airspeed).The only time it is activated is when you accelerate or decelerate. It doesn't eliminate all need for extra trimming but when I accelerate manually from flaps up speed of 210 knots to 315 knots cruise speed in one go, I just need a few taps of the trim switches nose down.Boeing airplanes still behave like normal airplanes. If you're in level flight, manually and the airplane is trimmed, reducing thrust will cause the nose to drop in order for the plane to maintain the trimmed AIRSPEED. It may go over it for a bit but after a bit of oscillating, it stabilizes at the same airspeed.This is different from Airbus, which does trim pitch attitude, since it's all computer aided anyway. This does, however, give the pilot a completely new principle of flying, something you'd have to get used to (and hope that you will not forget the REAL way of flying when you rent that Cessna with your family). :)Iz

[Guest]

As far as I know (and I fly them) the 757/767 have nothing to prevent a massive fluid loss. (Don't take that as the last word though, they teach us surprisingly little about the nuts and bolts systems that we cannot deal with from the cockpit! Some of you folks apparently have maintenance manuals and probably know more about it than I do! Thank goodness we don't have to know all that stuff anymore. Just flying them is enough!) Neither did the A300, although there is some kind of a fuse somewhere in that aircraft, which apparently does not have anything to do with the kind of failure that 587 sustained. We haven't had 74's for years , so I don't know about that fleet. Again, the 777 is new and it might have whatever mod the previous poster said Boeing came up with for the 74. Ditto the newer 767-400 and 757-300.What is needed, of course, is a system of fast acting, crew reversible shutoffs, or fuses, that can isolate any and all systems at any major point in case of fluid loss. But the odds are so long against this sort of thing happening that there is apparently no certification requirement to do this. Bottom line is the same for all hydraulic airplanes without manual reversion - no hydraulics, no real control.

[Guest]

According to "Air Disaster vol 2" by Macarthur Job, all 4 hydraulic systems were lost, for whatever reason.

[Guest Iz]

You're right, it is really unlikely.Should there be anything in the emergency procedures what to do if a wing falls off? The vertical stabilizer being a wing or control surface as well."Chapter Unanunciated Non-Normals---------------------------------------LOSS OF CONTROL SURFACECondition: Loss of either main wing, horizontal or vertical stabilizer.MENTAL CONDITION - Be afraid, be very afraid"Even though the Sioux City DC-10 had damage to the vertical stabilizer, the thing was still there. If you think that the AA Airbus would've flown fine without the tail, I wonder why. Remember a JAL 747 that lost its tail when the rear pressure bulkhead exploded due to incorrect maintenance (riveting)? Virtually uncontrollable, it collided with a mountain.

[Guest]

I was of course mistaken; a quick glance at the diagram in the doco was looking at before does indeed show all 4 hydraulic systems link into the rudder actuator; in fact just turning to the Hydraulics section of the PS1 manual shows that too... :)I stand suitably corrected...Thanks for the comments on current configuration of other airliners... interesting to know... maybe something we will see in the next gen of aircraft...Regards;John

[Guest Ian_Riddell]

"As far as I know (and I fly them) the 757/767 have nothing to prevent a massive fluid loss. (Don't take that as the last word though, they teach us surprisingly little about the nuts and bolts systems that we cannot deal with from the cockpit! Some of you folks apparently havemaintenance manuals and probably know more about it than I do!"After a quick browse through the 767 Boeing Maintenance Manual.... :-)Yes, they actually do have hydraulic fuses. I was thinking that there would be one per hydraulic system, but there are actually several... located at strategic points around the aircraft (tail, gear, etc). I couldn't find them in the chapter on Hydraulics... I had to search through other chapters for systems which use hydraulics, such as "Flight Controls" and "Wheels & Brakes".Hope this gives you a little more faith in the aircraft you fly :-)Cheers.Ian.

[Guest Ian_Riddell]

"From my 757 AOM: Automatic Trim. The stabilizer is controlled automatically by the autopilot or by a Mach speed trim system when the autopilots are not engaged."Thanks, Iz.I discovered, last night, in the 767 Maintenance Manual, that the 767 also has a similar setup, with the Mach Trim operating when the autopilot is not in use. Actually, there was surprisingly little on this subject (no explanation of what it was there for, or any tables of trimming values for various speeds). It was also sometimes referred to as "speed/mach" trim.I've only ever heard the 747-400 (non-Autopilot) trim referred to as "speed trim". Mach trim is not mentioned.Cheers.Ian.