Douglas DC-9
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Heart Of The DC-10...

Wed Mar 19, 2003 1:56 pm

Is it true that the "heart" of the DC-10 is the tail engine? I hear without the tail engine working that hydraulics of the aircraft are lost, such as: landing gear...ect. Anything that is hydraulically driven?

Let me know what you know of,
Douglas DC-9
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RE: Heart Of The DC-10...

Wed Mar 19, 2003 2:00 pm

You've been reading too much about UA232.... in that case, the uncontained failure of the tail engine severed a hydraulic line, which paralyzed the rest of the aircraft's system.
Faire du ciel le plus bel endroit de la terre c'est impossible sans Concorde!
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RE: Heart Of The DC-10...

Wed Mar 19, 2003 3:38 pm

Actually ConcordeBoy (and Douglas),
UA232's uncontained #2 engine failure severed *all* hydraulic lines in the tail, not just one. All means... primary and backups. It was an unanticipated design flaw that put all of the hydraulic system lines in the same place at the tail.

These lines were either moved or reinforced in some way after UAL232 so that uncontained #2 engine failure wouldn't cause the same thing again.

So the answer is yes and no. If #2 fails, the failure itself will not cause loss of hydraulics. If #2 has uncontained failure in a old (unaltered) DC-10, there might be loss of hydraulics if the lines are severed, however all three system lines would have to be severed to lose all hydraulics.

However, on DC-10s that were retrofitted, it's no longer possible. All DC-10s in the United States were retrofitted by FAA mandate.

Up, up and away!
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RE: Heart Of The DC-10...

Wed Mar 19, 2003 6:53 pm

Since I was a flight engineer, and later F/O on the DC-10, let me help;

To start - the three hydraulic systems are completely separate from each other. As said before, the reason the United DC-10 lost it all was because the exploding engine took out all three lines in the tail. And after that accident, a retrofit put valves in the #3 system forward of the tail engine to prevent that sort of thing from happening again.

The #2 hydraulic system on the DC-10 is powered from the tail engine, and is actually the least busy of the three. All control surfaces have redundant power, and the only thing (for the most part) the #2 system drives are parts of the rudder, elevator, ailerons, flaps, and spoilers. The #1 and #3 systems also drive the landing gear, steering, braking, slats, some flaps, and the stab trim, in addition to the control surfaces.

The #2 system does power a backup non-reversible motor-pump, that provides pressure (but not fluid) to parts of the #1 system in case the #1 engine pumps give out (stab trim & rudder).

So in short - no. The loss of the tail engine does not appreciable affect the hydraulics on the DC-10. In fact, the loss of one system is treated as an abnormal procedure. Only if you lose two systems does it become an emergency. And finally, the hydraulic pumps are driven off the accessory drive from the N2 section of the engine. If the engine quits because of a fuel problem or something else that allows the engine to windmill, you'll still have hydraulic pressure in that system as the rotation of the engine keeps the pumps working.

One smooth landing is skill. Two in a row is luck. Three in a row and someone is lying.
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RE: Heart Of The DC-10...

Wed Mar 19, 2003 9:46 pm

Ten years before United 232 , there was the American Airlines disaster in Chicago. It also had to do with hydraulic lines. Shortly after take off, the aircraft lost it's right or left engine (I don't remember which) but not the tail engine. That was obviously not the only cause of the crash because if it was, the crew would have been able to return safely to O'Hare with two engines out of three. What really happened is that the engine failure took all the hydraulic lines on the wing, according to what HAL explains about hydraulic systems on the DC-10, I figure that the n#1 and/or #3 systems were damaged. The aircraft lost control because of the severely damaged hydraulic lines, and plunged into the ground. It was bound for Los Angeles.

Ben Soriano
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Ben Soriano
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RE: Heart Of The DC-10...

Wed Mar 19, 2003 10:53 pm

Okay, I'm going to start a new thread on 232.
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RE: Heart Of The DC-10...

Wed Mar 19, 2003 11:55 pm

As far as I remember, the AAL DC10 at OHare was the engine ripping off upwards pivoting on the front pylon lug... and it ripped the leading edge slats... given the slow speed and the resulted asymetric thrust, the left wing lost lift, hence the plane rolled to the left, and into the ground.

The loss of the left engine resulted in loss of all hydraulics connected/associated with that engine.

HAL, pls correct me if I'm wrong on this one...

When losing situational awareness, pray Cumulus Granitus isn't nearby !
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RE: Heart Of The DC-10...

Thu Mar 20, 2003 3:09 am

Just to expand slightly on HAL's informative post.

The protective hydraulic shutoff valve isolates the #3 hydrualic system from the tail should #3 hydraulic quantity decrease below a certain threshold in its reservoir (located in the right wheel well).

The AA ORD crash, the left wing outboard slats retracted as the hydraulic lines were ruptured.
Balance relief valves were installed with the wing slat drive units to prevent the slats from again retracting if pressure is lost. The difference between a balance relief valve and a hydraulic check valve is this.
A check valve allows fluid flow in only one direction, not usefull for this application. The balanced relief valve allows flow both ways, but closes if a pressure loss is sensed.
You're only as good as your last departure.
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RE: Heart Of The DC-10...

Thu Mar 20, 2003 3:12 am

You're right. Poor maintenance caused the AA DC-10 in Chicago to break one of the pins holding the engine pylon onto the left wing. The engine rotated up and over the wing, tearing out the hydraulic lines on that side, causing the slats (which are held out by hydraulic pressure) to retract. The pilots just thought they'd had an engine failure, and maintained the slower engine-out climb speed. At that slow speed with one side of the slats retracted, the left wing stalled, and they spun in. Simulations later showed that if they'd kept even 10 more knots, they could have made it.

One smooth landing is skill. Two in a row is luck. Three in a row and someone is lying.

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