B777 landing gear

[cowpatz]

I did a reply to a post some time ago regarding the attributes of the 777 undercarriage. For those that are interested I took some photos of the actual gear to help explain some more of its unique features.



In this side view you can clearly see the tilt actuator at the front of the bogey assembly. This has 3 functions:

1. Tilt the gear for optimum position for touchdown
2. Tilt gear to correct position to enter the wheel well
3. Lock the gear into the level position that you see here. More on the reason for this later

This is the tilt actuator.



Only the wheels on the rear axle are steerable. They turn to face the opposite direction to the nose wheels. This reduces both the minimum turn radius and tyre scuffing. You can make out the steering acuator that runs fore and aft on the RH side. This connects to a bellcrank assembly that pivots about the far left of the bellcrank. In between those 2 connection points, is a steering pushrod that runs from the bellcrank to a point on the rear axle just inboard of the left hand wheel. The axle pivot point is just inboard of this. With a bit of imagination you can see how this works.
When lining up on the runway there needs to be a slight roll forward to get the gear back into alignment. If thrust is greater than 60% N1, with the gear not aligned, then there will be a TO config takeoff warning and the take off must be rejected. It is for this reason that our take off procedures require us to advance the thrust levers symmetrically to approx 50 to 55%, with a brief pause to allow the engines to stabilise (and not set off the TO config) before pressing the TOGA switch to engage the autothrottles and apply TO thrust. This procedure allows the aircraft to roll forward just enough to align the real wheels.

The 777-300 variants have an interesting feature. During the TO roll the tilt actuator locks into its current position seen here. The reason for this is to help increase tail clearance during rotation and reduce the chance of aft body contact with the runway. Without the tilt actuator locked in position the gear would remain flat with all 6 wheels per bogey having contact with the ground. As the nose rotated the gear strut would rotate around a pin in the bogey fore/aft axle support beam. With the tilt actuator locked the gear strut remains in the same position so as the nose rotates only the aft 2 wheels on the bogey have contact with the ground. The geometry of this raises the fuselage and provides a small increase in height of the tail from the ground. Rotation is around the aft wheels and not the centre point of the bogey. The 777-200 does not have this feature, or even the tail skid that some of the older 777-300s have. Later 777-300 models do not have this tail skid (the retracts and extends along with the undercarriage) but have a software system that assists instead. On our Air NZ 777s you may notice that OKR and OKS do not have this tail skid.

[Charl]

Superb post, and I'll come back here and look some more.
Having seen these pics, my esteem of the flightsim models has dropped somewhat.
They do not show the half of it - well maybe PMDG but that is above my pay grade.

Design playing small percentages: wonder who made the call that tandem wheel scuffing was OK but not 3 in a row?
And the guy that figured out that standing up on the back wheels could save a whole redesign of the landing gear... hope he got a small Christmas bonus.

Given a clean sheet of paper, I still wonder why they don't spin up those wheels with an electric motor.
Things have moved on since the designers decided against this, back in the 1940's.
It's a no-brainer, you save on tyre wear but also can have a lighter tyre carcass because you don't go 0 to 140kts in an eyeblink - which translates to more fuel or pax.
And you can drive yourself off the ramp which saves fuel (= weight) and no tow truck (= landing fee cost)
And you use regen to save the brake pads, and get free charging for your electric (see 787) airplane at the same time!

[emfrat]

Charl, the Vickers Vanguard was meant to have main u/c bogies which could be rotated (about 15 degrees if memory serves) to offset the typical crosswind at Edinburgh. It was soon dropped, as the weight penalty was about two less pax, and in any case, tyres were much cheaper. If all the wheels are pointed in the same direction, you would move sideways which is hardly the desired effect, rather than following a smooth curve.

[cowpatz]

The weight and complexity of 12 electric motors would simply outweigh any advantage saved in tyre wear. The brake units are housed within the wheel hubs so I’m not sure where the drive motors would go. Still top marks for thinking outside the square Charl.

Emfrat I believe the B52 has swivelling main gear for crosswind landings.

[Charl]

The weight and complexity of 12 electric motors would simply outweigh any advantage saved in tyre wear.

Yeah but it's more than just tyre wear, it's about reversing the weight spiral at design stage. Maybe a compound motor/brake rotor with the brake taking the first bit of high energy and then handing off to regen before it deconstructs...even the initial massive heat energy could be diverted and captured. The physics side is clear, the engineers simply haven't been motivated to see it through.
Back in the day when forklift truck lead acid batteries were the only way to power an EV, they too were seen as too complex and heavy! It takes a Musk (unfortunately) to change the paradigm.

And, final thought: as a passenger, no matter how greased the landing, I HATE that jolt when a stationary tyreset meets the tarmac!

Matter of observation, CP: in your first photo, it seems the brake dust accumulation is successively heavier, moving toward the front pair.
Any thoughts?

[deeknow]

Fascinating re the TO config sequence and tilt stuff, cheers for sharing CP

Oh, and can I say congrats to the cleaning crew, that has to be the cleanest under-carriage on the ramp that day

[cowpatz]

In a previous life, when I was an aircraft engineer working out on the international line at Auckland, I was tasked with changing a mainwheel on a Pan Am 747-100. The whole wheel and undercarriage leg was covered in thick brake dust. After I put the new shiny wheel on, and having some spare time, I decided to give the main leg a good clean so that it looked as clean as our aircraft were back then in the day.
A few days later I got summoned to go to the Chief Line Engineers office. When there he asked me if I had changed a mainwheel on the Pan Am aircraft. I immediately thought the worst. Did I not tighten the wheel nut, forget to lockwire it or under/over inflate the wheel etc. Did it cause damage or at worst an accident? It was then that he handed over a letter from the Pan Am station engineer in Honolulu (a very busy man I might add). In the letter he stated that I had interfered with Pan Am's anti corrosion measures and that I should be reprimanded.
"What are these anti corrosion measures". I asked.
"To leave it dirty, so consider yourself reprimanded. Now get out of my office" He replied but in a more colourful engineering slang.
Indeed, they never cleaned their aircraft. The underside of the fuselage, aft of the body gear, was covered in a thick, green, oily, slime. It almost needed antifouling. What with that and huge quantities of speed tape repairs, it was not a good look at all. No wonder the airline collapsed.

[Charl]

Hah great tale... 'course what you did was to make a whole lotta work for their maintenance guys.
11 dirty wheels and one shiny one plus leg? Sticks out like a sore thumb.

[deeknow]

that's hilarious CP .. as I was reading I was thinking you'd be scolded for wasting time, not being too clean

[Splitpin]

CP, all the above compliments. So much precise information in a relatively short piece, and like Charl, it will be a re-read.
You really do this stuff well....technical explanations and the anecdotes about your job past and present.
Thanks for taking the time....great post.