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Topic: How Is This Possible?
Username: 777DadandJr
Posted 2007-07-02 03:27:16 and read 7948 times.

Saw this pic on the top photos:


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Question?
How can you retract the landing gear when the a/c is on the ground?
I didn't think that was possible.

Russ

Topic: RE: How Is This Possible?
Username: Lowrider
Posted 2007-07-02 03:30:21 and read 7948 times.

There are safety interlocks to prevent this sort of thing. They can fail or be bypassed, however. The nose gear could have collapsed for other reasons, however. Perhaps someone familiar with the 767 could shed some light on the finer points of the nose gear.

Topic: RE: How Is This Possible?
Username: JetMech
Posted 2007-07-02 05:50:34 and read 7881 times.

Quoting 777DadandJr (Thread starter):

Under normal circumstances you shouldn't be able to, due to the interlock device that Lowrider mentioned. This interlock device blocks the retract lever from moving to the retract position on the ground. Perhaps it was a maintenance snafu? The interlock can be overridden manually, which was perhaps what occurred. Maintenance crews may have been applying hydraulic pressure to the landing gear to check for leaks. Perhaps they neglected to fit the nose gear retraction lock-out pin?

Perhaps the flight crew accidental moved the landing gear lever to the retract position (which would have required manual override of the lock-out) with hydraulic pressure on? In this case, it is likely that no landing gear retraction lock-out pins were fitted, which caused the nose gear to collapse. The main gears on a 767 retract sideways, and even with hydraulic pressure to the retract cylinders, there is no way they would be powerful enough to drag the main gear sideways against the weight of the aircraft, which is why they did not collapse.


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In the above photos, you can see the flags attached to the main gear retraction lock-out pins. The main gear leg has two large braces, the fore-aft brace and the side brace. Each of these braces has smaller braces called jury struts. The jury strut holds the side and fore-aft braces in alignment when the gear is extended. The jury struts are folded by the two hydraulic cylinders that can be seen in the photos.

You can also see that the jury struts have large springs attached. These springs are arranged such that they are always trying to straighten out the jury struts, which tends to straighten the fore-aft and side braces which in turn tends to extend the gear. In the even that hydraulic pressure is lost, these springs help the gear to gravity drop, and hold the gear in the extended position when there is no hydraulic pressure.

The gear retraction lock out pins fit between the two parts of the jury struts, and due to the arrangement of the strut and braces; physically lock the main gear in the extended position. The nose gear has a similar arrangement on it's drag brace.


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In the above photos, you can see the gear retraction handle lock override button, and a 777 nose landing gear which is similar to the 767. It is hard to see from the photo, but the nose landing gear also has a jury strut which goes back from the apex point of the upper drag brace (white coloured triangle) and the lower drag brace (light brown coloured beam). This jury strut also has a spring and lockout pin arrangement similar to the main gear.

Regards, JetMech

Topic: RE: How Is This Possible?
Username: AirframeAS
Posted 2007-07-02 06:43:25 and read 7859 times.

The 737's have the same lock-out pin function as well.

Topic: RE: How Is This Possible?
Username: Ceph
Posted 2007-07-02 09:58:39 and read 7796 times.

This is not the first time something like this happened in China if I did not remember wrongly

Topic: RE: How Is This Possible?
Username: N231YE
Posted 2007-07-02 12:31:42 and read 7733 times.

It has happened before. I remember an incident at CLE about 2 years ago, when an XJ ARJ85's nose gear collapsed at the gate. I believe it was caused by faulty pins or was improperly (hydraulically) locked.

Topic: RE: How Is This Possible?
Username: Scooter01
Posted 2007-07-02 12:43:14 and read 7724 times.

...and this is what happens when the main-gear legs retract at night when nobody notices:

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Photo © Magne Roalkvam



Scooter

Topic: RE: How Is This Possible?
Username: Starlionblue
Posted 2007-07-02 13:21:18 and read 7700 times.

Quoting Scooter01 (Reply 6):
...and this is what happens when the main-gear legs retract at night when nobody notices:

If a plane falls on the tarmac and no one is there, does it make any sound?  Wink

Topic: RE: How Is This Possible?
Username: TristarSteve
Posted 2007-07-02 13:44:38 and read 7690 times.

The most probable cause is that someone moved the Landing Gear selector lever on purpose, not realising that the hyd system was pressurised. I believe that on a B767 you have to move the LG selector to up to remove the right hand MCDU. Of course you should fit the nose pin first, but things go wrong.
BA have had two incidents in recent years on B747 where the nose leg has retracted. In both cases the mechanic moved the selector to up, knowing that he had installed the safety pins, but they were not there and the NLG leg retracted.
One incident happened in CWL, ex maint. The pins were fitted, and the aircraft was outside nearly ready to position back to LHR. One mech was running the hyds and moving the LG selector to check the MLG door operation. At the same time another mech took out the pins ready for departure.
The other incident was at LHR on a Korean? B747. Some work had been carried out on the landing gear, and the pins were fitted so that the LG selector could be moved to operate the MLG doors. As the NLG doors opened and closed, the door caught the red flag from the NLG pin, and pulled the pin out of its hole. The NLG retracted. Any of you who work B747 know that it is very difficult to fit the NLG pin, as it is a long way up and access is limited with the doors closed.
When I worked in a hangar, I always positioned the Main Nose Jack in position, before I operated the LG selector. Now I work on the ramp, and have no Main Jacks, I always tie the NLG pin in place, and get a push back tractor parked under the nose.

Topic: RE: How Is This Possible?
Username: 777DadandJr
Posted 2007-07-02 15:07:37 and read 7605 times.

Wow! The things you learn on here!

When the nose gear retreacts, does it just lower the a/c to the ground softly, or does the a/c come down with a bang?

Is this incident a cause of major damage?

I remember back in the mid 80s, at BWI, a tug pulled the nose gear off a BA L1011 and the a/c came crashing down on the tarmac.
Word has it that the incident caused about $1 million in damage to the a/c.

Russ

Topic: RE: How Is This Possible?
Username: HAWK21M
Posted 2007-07-02 15:56:54 and read 7531 times.

Quoting 777DadandJr (Reply 9):
When the nose gear retreacts, does it just lower the a/c to the ground softly, or does the a/c come down with a bang

3000psi moves an Actuator Fast.

Its Important not to use the Overide on the L/G lever without the L/G pins installed.

regds
MEL

Topic: RE: How Is This Possible?
Username: Starlionblue
Posted 2007-07-02 19:21:04 and read 7212 times.

Quoting TristarSteve (Reply 8):
One incident happened in CWL, ex maint. The pins were fitted, and the aircraft was outside nearly ready to position back to LHR. One mech was running the hyds and moving the LG selector to check the MLG door operation. At the same time another mech took out the pins ready for departure.

What happened to the mech? Was he under the aircraft at the time?

Quoting TristarSteve (Reply 8):
As the NLG doors opened and closed, the door caught the red flag from the NLG pin, and pulled the pin out of its hole. The NLG retracted.

Talk about Murphy's Law...

Quoting TristarSteve (Reply 8):
When I worked in a hangar, I always positioned the Main Nose Jack in position, before I operated the LG selector. Now I work on the ramp, and have no Main Jacks, I always tie the NLG pin in place, and get a push back tractor parked under the nose.

Hängslen och livrem. A good philosophy.

Quoting 777DadandJr (Reply 9):
When the nose gear retreacts, does it just lower the a/c to the ground softly, or does the a/c come down with a bang?

As MEL points out, it's pretty fast. As I understand it (and I may be wrong), as soon as the leg starts folding there is really no force keeping it upright anymore. That is, gravity will "help" the retraction mechanism and the nose will essentially free fall to the ground.

Topic: RE: How Is This Possible?
Username: JetMech
Posted 2007-07-03 04:22:34 and read 7040 times.

Quoting TristarSteve (Reply 8):
Any of you who work B747 know that it is very difficult to fit the NLG pin, as it is a long way up and access is limited with the doors closed.

Tell me about it! You can just see where this pin fits (there is actually provision for two pins) in the photo below.


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Photo © Jeroen Stroes



If you follow the red flag upwards, it turns into a thin cable. This cable leads to the handle of the pip pin which you can just make out. A Steve said, with the gear doors closed, there is no way you can physically fit through the remaining gaps to fit the pin. It is actually highly dangerous to enter the nose wheel bay of a 747 with the doors open, as they are powered by a separate actuator.

This door actuator was released by the striped pole "T" handle you can see poking out of the fuselage to the right of the nose gear. If #1 hydraulic system was pressurised, and the gear handle was in the down position, the only thing preventing the nose gear doors from slamming shut is the proper manipulation of the "T" handle! The proper position of the "T" handle was actually quite subtle, which is why it was dangerous to be in the path of the doors unless you had a stand physically blocking the movement of the doors if they did try to shut.

We used long poles about 5 feet in length with a special trigger mechanism to hold the pin and insert it from the ground. This required you to lie back against the nose wheel with your arm at full extension holding the pole! A somewhat acrobatic manoeuvre yet surprisingly easy with practice.

Regards, JetMech

Topic: RE: How Is This Possible?
Username: Viscount724
Posted 2007-07-03 05:28:21 and read 7015 times.

It's certainly not the first time:


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Photo © Frank skinner

Topic: RE: How Is This Possible?
Username: Starlionblue
Posted 2007-07-03 13:58:42 and read 6904 times.

BTW what happens to the main landing gear suspension when the plane is resting on the forward fuse. Can it bend that far or will it get damaged?

Topic: RE: How Is This Possible?
Username: AJ
Posted 2007-07-03 15:09:37 and read 6872 times.

Ansett did exactly the same thing with a 767 at the gate in Sydney.

Topic: RE: How Is This Possible?
Username: IFIXCF6
Posted 2007-07-04 03:17:42 and read 6744 times.

I'm sure some of you have seen the "other" 747 incidents. The ones where the L/G handle is lifted without main gear pins installed, the wing gear cannot retract inboard but the body gear (aft) without the brakes being set, will happily retract forward dropping the 747 on her rear end. I've seen the results, not the action, twice.

Mike

Topic: RE: How Is This Possible?
Username: AirframeAS
Posted 2007-07-04 04:13:44 and read 6720 times.

The same thing happened to a HP 752 in PHX...the Arizona Cardinals aircraft. Somebody didn't follow procedures.....

Topic: RE: How Is This Possible?
Username: RoseFlyer
Posted 2007-07-04 07:45:29 and read 6663 times.

Quoting AirframeAS (Reply 3):
The 737's have the same lock-out pin function as well.

The 737 also has a four bar linkage system on all the landing gear. The system makes it so that supporting bars will essentailly over rotate so that the gear cannot retract unless a force is applied. So even if the hydraulic system has no pressure, and the pin is removed, the gear will not retract. It will stay in place. This is true for the main landing gear and the nose wheel to, but the main gear won't rotate since the tires would have to slide.

But if the hydraulic system is deployed, then its hard to say. But this feature doesn't work if the gear is manually deployed. The gear may not have enough force to essentially lock itself in place.

Topic: RE: How Is This Possible?
Username: JetMech
Posted 2007-07-04 08:42:48 and read 6652 times.

Quoting RoseFlyer (Reply 18):
But this feature doesn't work if the gear is manually deployed. The gear may not have enough force to essentially lock itself in place.

Every landing gear I have ever seen on a commercial airliner, both A and B has some sort of spring or bungee mechanism to ensure the gear locks down fully in the event of a manual deployment (as well as provision for ground lock pins). The activation of the manual deployment in almost all cases releases the landing gear up-lock hooks (or drag brace mechanism on some nose gears) and the gear doors. Gravity causes the gear to drop. The springs and or bungees then act on the jury struts or drag brace to force the gear mechanism over centre and lock it in the extended position.

Regards, JetMech

Topic: RE: How Is This Possible?
Username: RoseFlyer
Posted 2007-07-04 08:48:12 and read 6649 times.

Quoting JetMech (Reply 19):
Every landing gear I have ever seen on a commercial airliner, both A and B has some sort of spring or bungee mechanism to ensure the gear locks down fully in the event of a manual deployment (as well as provision for ground lock pins).

The four bar system that I am talking about is in addition to the spring mechanism that holds the gear in place. When the hydraulic system moves the gear, then it will move past the neutral point on the system and add an extra layer of security. But that feature requires a force that is usually not generated if the gear is manually deployed. So there is a layer of safety that may not exist in that case that I am referring to. But I am not completely certain that it will never occur. It is just a way that the gear is inherently designed to not retract on its own.

Topic: RE: How Is This Possible?
Username: TristarSteve
Posted 2007-07-04 09:02:17 and read 6645 times.

Quoting JetMech (Reply 19):
The activation of the manual deployment in almost all cases releases the landing gear up-lock hooks (or drag brace mechanism on some nose gears) and the gear doors.

Of course the Lockheed Tristar is different. The MLG had no uplock, the uplock was on the doors, and with the gear depressurised in flight it rested on the doors.

Talking about the B737. I still think the Boeing NLG design is marvelous. The way it uses a single uplock/downlock actuator that locks the gear up, and down. You have to watch it in action to understand it.

Topic: RE: How Is This Possible?
Username: JetMech
Posted 2007-07-05 03:52:36 and read 6542 times.

Quoting RoseFlyer (Reply 20):
The four bar system that I am talking about is in addition to the spring mechanism that holds the gear in place. When the hydraulic system moves the gear, then it will move past the neutral point on the system and add an extra layer of security. But that feature requires a force that is usually not generated if the gear is manually deployed. So there is a layer of safety that may not exist in that case that I am referring to. But I am not completely certain that it will never occur. It is just a way that the gear is inherently designed to not retract on its own.

Yes, in normal operation, the jury struts are usually forced to the extended position by a hydraulic actuator. The springs and or bungees are so arranged that the actuator must work against them to retract the gear. Presuming that a manual gear extension is carried out due to loss of hydraulics, it is the springs and gravity that provide gear extension.

Gravity acting on the gear weight causes most of the extension. The last part of the extension to the locked down position is provided by the springs or bungees acting on the jury struts. In most cases I have seen, the springs and or bungees and jury strut hydraulic actuator all act on the jury strut. I suspect that the combined jury strut / side brace / drag brace mechanism is the same as the four bar system you are talking about.

Quoting TristarSteve (Reply 21):
Of course the Lockheed Tristar is different. The MLG had no uplock, the uplock was on the doors, and with the gear depressurised in flight it rested on the doors.

Talking about the B737. I still think the Boeing NLG design is marvelous. The way it uses a single uplock/downlock actuator that locks the gear up, and down. You have to watch it in action to understand it.

Yep, and the B767 also. The 767 MLG is also held in the up position by the gear door. It is the gear door that is equipped with an up-lock roller that is held by a hook. The 747 nose gear also has no up-lock hook, as it is the upper drag brace that locks the nose gear in the extended and retracted position. It looks as though the 737 is similar in concept but different in execution.

The 747 has the lower tripod brace and upper tripod brace as one piece items, with the drag strut being articulated. The 737 appears to have the upper (drag strut) and lower drag braces (lower tripod brace) as one piece items, with the triangulation piece (upper tripod brace) being articulated. This design allows the nose gear to be triangulated and hence lock in both the extended and retracted position by the same mechanism.

http://i94.photobucket.com/albums/l118/Jet-Mech/NLG1.jpg

http://i94.photobucket.com/albums/l118/Jet-Mech/NLG.jpg

B747 classic nose gear mechanism



http://www.tc.gc.ca/CivilAviation/ce...ontinuing/Alert/Images/2004-04.gif B737 nose gear mechanism.

Regards, JetMech


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