Topic Author
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Weird Dc-10 Gear Design Question.

Mon Jan 14, 2002 11:37 am

Ok here is a question i cannot get answered right by people.

Reference for my question, is on page 74 in Terry Waddington's Mcdonnell Douglas Dc-10 book.

Question is: Aircraft 9Q-CLI of Air Zaire found in the picture at the top of the page of 74 has it's gear box doors open, but the weird thing is the main outer gear is hanging like a 767 gear. Why is that? In person, I have never seen such a thing on Dc-10's when the gear doors open. Also the way the gear box is design i know for a fact with the gear hanging as such, it would not even fit in the gear box. I have pictures in my personal collection, of gear doors opening or shutting with the gear extending or retracting, and no sign of the gear making a movement like this is visible.

Can someone clarify something here, or did i just miss something on the gear design? As you notice in this pic and on all other Dc-10s the gear is flat, but go to Terry Waddingtons book and you will see that 9Q-CLI has forward tilting bogies.

Click for large version
Click here for full size photo!

Photo © Brian Stevenson - Orlando/Tampa Aviation Photography

So any of you former or current Dc-10 Mechs, please help. Also if you do not know or you are just going to speculate please do not bother to respond to this post. I would like a valid answer and maybe a link to reasons for this with diagrams of some sort... Thanks

Aaron Mandolesi
Denver Spotters Hub-Founder


RE: Weird Dc-10 Gear Design Question.

Mon Jan 14, 2002 1:14 pm

If you look at the video of United flight 232 (a DC10-10, N1819U), making its descent into Sioux City, Iowa, you will notice the gears are hanging forwards as well. I just thought it was because that aircraft had a complete total hydraulic failure.
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RE: Weird Dc-10 Gear Design Question.

Mon Jan 14, 2002 2:06 pm

Is that video avilable for download online? RY
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Gear Dilema

Mon Jan 14, 2002 7:49 pm

Ok Ill try to answer this question but I have not seen this DC-10 book your speaking of. Your question was kinda unclear about the doors. The DC-10 has 2 sets of doors, flying doors which attach to the gear and make the gear more aerodynamic when retracted. The second set of doors can be opened and closed on the ground these are the Landing Gear Bay doors. Second question, The DC-10 has what is called a Bogie Trim Cylinder, located on the front of the bogie assy. The trim cylinder positions the truck beam during extension and retraction, it also decreases truck beam movement while on the ground when brakes are applied or when you move the aircraft. When the aircraft is in a landing configuration the gear bogie is designed to cock back to allow back wheels to contact the runway first. If the bogie trim cylinder becomes defective it has an internal lock that will not allow the gear to retract into the well. BTW this aircraft has no center gear, i.e. there is no gear between the left/right main landing gear, this aircraft is a DC10-10 or 15 series. I hope that was helpful.

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RE: Gear Dilema

Tue Jan 15, 2002 3:20 am

I'll just add a little to what TechRep said.

I've seen the photo (I have the book), and the picture was taken during the gear extension cycle. The gear doors you see are the main doors under the fuselage, and normally cover the wheels when the gear is retracted. They open to let the gear out, then close again after the gear is extended. The position of the gear bogies in the picture is how the gear is stowed in the fuselage, and is held there by hydraulic pressure in the bogie trim system. Once the gear is out, one of the last steps in the extension process is the bogie trim system (hydraulic) moves the bogies back to the normal position seen in most of the photographs here. The gear is not held in an extreme rear-wheel-down position (such as the 747 mains) because of the relatively nose-up landing attitude, which places the gear naturally in the rear-wheel-down position.

One other note - the picture in the book is that of a DC-10-30, and you can see the center gear already extended. Because of the size of the cylinders (and the mass to be moved) the nose gear and center gear get down and locked before the mains.
One smooth landing is skill. Two in a row is luck. Three in a row and someone is lying.
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RE: Weird Dc-10 Gear Design Question.

Tue Jan 15, 2002 4:07 am


Your assumptions are correct. The DC10 main gear bogeys should always be level when all the wheels are off the ground, being positioned this way by the bogey trim cylinder.

This is, again as you said, to allow the gear to fit into the wheelwell.

I would say the reason the gear is hanging as you describe, especially the fact it was both left and right bogeys, would be due to an emergency "alternate extension"

The gear doors remaining open would signify the gear being lowered in ALTERNATE, this done by raising the "alternate gear extend handle".

When the handle is pulled, #3 hydraulic pressure is isolated from the landing gear extend/retract mechanism.

As this happens, the main gear droops and rests on the gear door. As the handle is fully raised, the main gear doors mechanically unlock and drop open.

When the gear doors open, the bogeys resting on them freefall to down and locked.

The gear doors will remain open.

I don't think dropping the gear in "alternate" removes pressure from the bogey trim cylinder so their might have been a loss of #3 hydraulic pressure.

The bogie remains level right up to the point the aft wheels contact the runway, at which time the weight of the aircraft forces the front wheels down.

Their is a hydraulic pressure relief valve on the underside of each bogie which vents a little (a pssst)hydraulic fluid during touchdown. This because as the bogie is forced out of its normal level attitude, the trapped fluid must be able to vent as pressure builds to a certain threshold.
You're only as good as your last departure.
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RE: Weird Dc-10 Gear Design Question.

Tue Jan 15, 2002 6:16 am

I reckon you've got that sussed. Same as I thought - alternate extension due loss of hyd 3.
One point - the main gears are not locked up on the 10 so they always rest on the door (which *is* locked) during normal ops.
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RE: Weird Dc-10 Gear Design Question.

Tue Jan 15, 2002 11:54 am

I've got to respectfully disagree on that last point. Normally, the DC10 main gear is held in the up and retracted position with hydraulic pressure.

Should hydraulic pressure (#3 system) be lost, the gear will settle on the gear door which is locked.

You're only as good as your last departure.
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RE: Weird Dc-10 Gear Design Question.

Tue Jan 15, 2002 1:09 pm

FDXmech is right on with the #3 hyd pressure. In fact, there used to be a procedure, similar to some Boeings in which the gear handle is moved partially out of the "Up" position to check the door locks. As for the original question, I would have to agree that the trim system was inop for some reason.

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