B733 Hard Landing

Sun May 21, 2000 2:22 am

Is there any indication on the main landing gear that the airplane has experienced a hard landing?
(Of course you can ask the passengers !)
But seriously ..........
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Joined: Sat Feb 26, 2000 6:18 pm

RE: B733 Hard Landing

Sun May 21, 2000 2:31 am

Well, inside the strut there is a part which can be pushed together to absorb a too hard impact. This is very helpfull, because this way mainenance only needs to replace (a part of) the strut, i.s.o. the complete landing gear and the beam that supports it. You could compare it to a kind of a very strong spring, but with one big difference: it doesn't unfold anymore, so it needs to be replaced.
(BTW: A normal landing is absorbed by oil only, there are no springs used like on a car)

RE: Sabenapilot

Sun May 21, 2000 6:41 am

The landing gear strut is filled with nitrogen gas under pressure to provide the springing action, the gas compresses, the oil provides damping. I'm not familiar with the hard landing indicator you mentioned, where is it located? I have the 737-300 IPC available. Hard/heavy landing inspections are described earlier in this forum use the search function under "heavy landing checks".
Posts: 2451
Joined: Sat Feb 26, 2000 6:18 pm

RE: Sabenapilot

Sun May 21, 2000 7:40 am

My answer was not specifically related to the B737, but rather to a general idea about how a strut works.
I'm not familiar with all the details of B737 systems since I don't have a typerating for that type.
(I fly AVRO RJ85 and RJ100).
I have however a DC-10 manual here at hand which I used to describe how a hard landing is dealed with.
I've looked it up once more and the part I've described is called a 'crush tube'. You can find it at the top of the nitrogen chamber.

Just to make clear to others how the system works:
The stut (= leg of the landing gear) is made out of 2 concentric cylinders. The upper cylinder is attached to the fuselage, the lower cylinder to the gear.
When the landing gear makes ground contact, the lower cylinder is pushed into the upper cylinder which is somewhat wider. Both are filled with oil and the upper cylinder also contains some nitrogen gas. Because of the reduction in volume inside the lower cylinder, part of the oil from the lower cylinder must go through one or more small holes to reach the upper cylinder, thus loosing energy. Also, the way through is progessively blocked by a cone-shaped metering pin attached to the lower cylinder and moving upwards as well. This makes it more and more difficult to compress the strut even further. The nitrogen gas in the upper cylinder acts as a spring which is compressed and will slowly expand again to its normal volume, pushing the oil back into its chambers in the lower strut. However this is done much more slowly, so only about 10% of the impact energy is released again. The rest is transformed into other forms of energy like heath.
If the nitrogen gas is compressed to its mimimum volume and this still isn't enough to absorb all impact energy, then a diaphragm on top of the nitrogen gas chamber folds over and a crush tube is used to absorb the rest of the energy and to act as a rebound control.
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Joined: Sun Mar 19, 2000 9:48 pm

RE: B733 Hard Landing

Sun May 21, 2000 8:35 am

The crush tube is only on the center gear on the
DC10-30 and MD11. The main and nose gears will bottom out without any telltale sign (unless visually damaged). If the crush tube is crushed, there is a rubber disc on the upper rear part of the strut that will rupture indicating the centerline gear has bottomed out.
You're only as good as your last departure.

RE: B733 Hard Landing

Mon May 22, 2000 1:49 am

Thanks guys !
How hard (in g's) does a landing have to be in order to make visible effects?

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