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dongkiyu

main landing gear should not tilt when using alternate gear extension

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when the center hydraulic system fails and alternate gear extensin is used to lower the gear, the main gear shouldn't be able to tilt. however, it tilts now. please fix this issue.

--Dongki Yu

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Hello!

 

First, please sign your posts with your full name here.

 

Second, I encourage you to send a support ticket to PMDG, including documentation substantiating your point, and they will be happy to fix any problem!

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http://www.flickr.com/photos/7702824@N03/3605420057/

 

Based on the picture above. I reckon it looks just like what it should be with a normal landing gear extension. I don't recall I have seen any video on youtube filming at Paine field showing a 777 with ALTN landing gear extension coming to land with now tilt .

 

Referring to my very limited technical knowledge, I thought the tilting of the landing gear is caused by the natural extension of the oleo strut which is independent from the HYD system of the airplane. Because otherwise it wouldn't make any sense, landing gear tilt is a way to improve shock absorbance on touch down, although some airplane ie 787 will have less tilt or no tilt in their design and some like the A330 will have more. because it is most likely constrained by the size of the landing gear bay .

 

Moreover, a complete loss of CTR HYD fluid, for complete lost I meant absolute zero in all the pipes and accumulators, theoretically according to FCOM , it's impossible to lower the landing gear even with the use of ALTN gear extension, unlike the airbus which unlocks the gear directly using an electric motor, the 777 uses an electric motor to drive a HYD actuator to unlock the gear as a mean of ALTN gear extension. There are some accumulators within the HYD pipes to conserve the fluid for this exact purpose, I don't know how exactly it works, I was explained by the engineer in ground that according to Boeing the chance of a complete total loss of CTR HYD fluid is extremely remote.

 

However if your statement is correct then that truly something to me.

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This is the first time I have ever heard this. I don't believe it is true. I have NEVER seen any 777 landing with no tilted bogies, alternate, normal, never... but happy to be proved wrong.

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Thank you all for the comments. I saw this statemen in a 777 Computer Based Training...


Thank you all for the comments. I saw this statement in a 777 Computer Based Training...

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Thank you all for the comments. I saw this statemen in a 777 Computer Based Training...

Thank you all for the comments. I saw this statement in a 777 Computer Based Training...

Did you fail the Center Hydraulics first?

 

Because the idea of using Alternate Gear extension is ofcourse the loss of Center Hydraulic pressure.

The Alternate gear extension system then only unlocks the uplocks and the gear free falls to down.

 

No hydraulic pressure is then available to tilt the gear for landing as far as I remember!

 

If you just dropped the gear with alternate gear extension but without failing the Center Hydraulic system......then I guess it WILL tils because pressure IS available

 

 

Ps I am talking 777-200 here.......I I am not sure if the 777-300 is different in respect to landing gear tilt.

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HI, 777simmer, yes I failed center hydraulics first. It's 777-200LR. But unfortunately the main landing gear truck tilts...

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HI, 777simmer, yes I failed center hydraulics first. It's 777-200LR. But unfortunately the main landing gear truck tilts...

Tilt requires hydraulic pressure, so without it there's nothing to positively drive the gear trucks to the flight position. However it's possible they might move that way under gravity, it depends where the balance point of the truck is.

 

The fact that the gear might not tilt after alternate extension is not an operational issue and in the PMDG sim only affects the external model. So in that sense it doesn't matter much. However PMDG went to a lot of trouble to model alternate gear extension, for example the gear doors stay open as they should, so I expect they would like to get the tilt right too.

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http://www.flickr.com/photos/7702824@N03/3605420057/

 

Based on the picture above. I reckon it looks just like what it should be with a normal landing gear extension. I don't recall I have seen any video on youtube filming at Paine field showing a 777 with ALTN landing gear extension coming to land with now tilt .

 

Referring to my very limited technical knowledge, I thought the tilting of the landing gear is caused by the natural extension of the oleo strut which is independent from the HYD system of the airplane. Because otherwise it wouldn't make any sense, landing gear tilt is a way to improve shock absorbance on touch down, although some airplane ie 787 will have less tilt or no tilt in their design and some like the A330 will have more. because it is most likely constrained by the size of the landing gear bay .

 

Moreover, a complete loss of CTR HYD fluid, for complete lost I meant absolute zero in all the pipes and accumulators, theoretically according to FCOM , it's impossible to lower the landing gear even with the use of ALTN gear extension, unlike the airbus which unlocks the gear directly using an electric motor, the 777 uses an electric motor to drive a HYD actuator to unlock the gear as a mean of ALTN gear extension. There are some accumulators within the HYD pipes to conserve the fluid for this exact purpose, I don't know how exactly it works, I was explained by the engineer in ground that according to Boeing the chance of a complete total loss of CTR HYD fluid is extremely remote.

 

However if your statement is correct then that truly something to me.

The picture certainly looks like it's conclusive. In a normal extension the gear doors would start to close before the gear was down locked and tilted. However take a look as this video of a 300ER test flight.

 

 

In the first approach the gear doors are open implying an alternate extension (the RAT is out too). The gear tilt is minimal. Hard to tell exactly, but certainly less than on the second approach where tilt is markedly apparent. I think the best you can say is that tilt angle is indeterminate if there's no hydraulic pressure.

 

Gear tilt is deliberately designed in and has its own hydraulic actuator (two for the -300). For Boeings at least tilt is mostly about fitting the gear into the wheel bay, not shock absorption though that is present too. That's the reason why the 767 gear tilts forwards in flight. In the 747 the wing gear tilts much more than the main gear because of how it must fit in the wing structure. The 777 main gear has to tilt forward for retraction as well, but when extended it tilts rearwards to improve touchdown dynamics. Touching down rear wheels first is more directionally stable.

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KevinH you have the point. I don't have certain answer on this one because not like the airbus Boeing had taken out most of the details description off the FCOM1 and and the crew famoliarization book, and I did my CBT ages ago. So I derive my assumption based on my old A330 knowledge knowingly that the Airbus had a special design to ensure the tilt is maintained with what they called "gravity " extansion which is the same as ALTN in Boeing terms. And also the assumption that there shouldn't be a huge different in terms of landin gear geometry between the normal and ALTN extension method.

 

Since the main gear maintained a 5 deg of forward tilt anyway when both stowed inflight and being retracting/extanting. I think I found this in the crew familiarization book or FCOM I don't remember. In theory there will always be at least some tilt.

 

I still remember that I had an assignment when I was in university which was about landing gear design, and the topic itself was already wide enough to have a text book written solely for the exercise.

 

Anyway thanks for bringing this up.

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