A340 and B747 Main Undercarriage

[Spektor]

A340 and B747 Specialists/Pilots,

Another one of these difficult Interview questions I would really appreciate your help with. Actually a couple of questions.

- How many boogies (I hope I spelled it right) are there on the A340/B747 respective Main Undercarriage assemblies?

- Also, my understanding is that it is possible on the A340 to leave some of them up for landing (?) Is it true? If so, which ones and what would be the reason for that?

Thank you so much in advance for the explanation!

Spektor.

[TheOddOne]

Spektor,

A 'boogie' is a dance you do to music!

A 'bogie' is one of many terms for a part of a vehicle with wheels arranged in a set. Railway carriages and large aircraft sometimes have 'bogies'. Some American manufacturers prefer the term 'truck'. More generically, any wheel assembly, or the whole system, on an aircraft is referred to in America as the 'gear' whereas in the United Kingdom and some other places it's called the 'undercarriage'.

Now, all models of B747 have 4 main gear legs, each terminating in a truck with 2 axles, with 2 wheels on each. The 2 outer legs are attached to the main spar on the inboard portion of the wing and the 2 inner legs are attached to the fuselage and are known as the 'body gear'. The body gear is aft of the wing gear. The body bogies or trucks have some limited steering capability. I've seen a 747 land with one body leg 'hung up' or not deployed - not a big drama - in that the aircraft was able to vacate the runway and taxi to stand. At a heavier weight, no doubt the additional loads placed on the wing gear by one body leg missing would put too much loading on the tyres on that side. I've also seen a 747 with the oleo seals on a body leg burst, so there was effectively no weight on those wheels - again no real drama for landing and vacating, except for all the tyres locking up every time the brakes were applied during taxying, huge clouds of smoke!

The A340 has 2 legs attached to the wings and one centreline body leg, like the DC10-30 and variants. Again, this is required to support departure masses but the 2 main legs can support landing stresses - I've witnessed that, too, no problems.

Having an aircraft arrive with any leg retracted that is normally used is regarded as an Emergency by us ground types and we react accordingly, despite being aware that the manufacturers have made allowances for this operation. We regard it as being similar to an overweight landing, in that any tyre problem is likely to have greater consequences than a tyre problem within Standard Operating Procedures.

Good luck

The Odd One

[404 Titan]

TheOddOne
The A340 has 2 legs attached to the wings and one centreline body leg, like the DC10-30 and variants. Again, this is required to support departure masses but the 2 main legs can support landing stresses - I've witnessed that, too, no problems.
Actually the A340 can operate with the centre bogie MEL’d with the appropriate weight penalties applied for both take-off and landing. Engineers just tuck it up into its well and deactivate it.

[TheOddOne]

404 Titan,

Okey doke, thanks for that. We don't gat many A340 operators at LGW, Emirates & Virgin mainly, most go to the 'other' London airport, the one out on the Great West Road...

The Odd One

[barit1]

The DC-10-30 & -40 center main has a separate selector switch and, like the A340, can operate at reduced GW in this condition.

JAL had a sub-fleet of DC-10-40's dedicated to shorthaul, and actually removed the center gear for weight & maint. cost saving.

[Spektor]

TheOddOne, 404 Titan, barit1,

Gentlemen, thank you for the detailed answer and interesting operational facts.

Lets see if I got it straight:

B747 has two main gear legs (now these are the terms I know!) attached to the fuselage for the total of four, A340 has one with the total of three.

A340 can be operatred normaly (takeoffs and landings) with the center bogie (I am learning!) MEL'd and deactivated in the retracted position, taking appropriate weight penalties - makes sense.

Can one or both fuselage main gear legs be MEL'd on the B747 as well? Or is it an ubnormal procedure every time that happens?

Thanks once again,
Spektor.

P.S. Yes, Sir, I can boogie... Nah, I am more of a Tango person. ;)

[18-Wheeler]

Can one or both fuselage main gear legs be MEL'd on the B747 as well? Or is it an ubnormal procedure every time that happens?

Nope, we need ALL gear at ALL times.
It's possible to land safely with any one unit in the up position but it's an emergency and we certainly can't depart like that.
With the two body gear retracted there's a chance that the aeroplane may sit on its tail after stopping so it's a good idea to get the CoG as far forwards as possible. (Transfer fuel, pax, etc)
With the two wing gear up, there's only a very narrow track width and so you just stop on the runway and do not try to turn the plane at all. Just park it and leave it there.

[Spektor]

18-Wheeler,

Thank you for clearing it up for me. Much appreciated!

Spektor.

[Dan Winterland]

As mentioned, the 747 has body gear steering. The body gear steers in the opposite direction to the nose gear at about one tenth of the deflection. The nose gear steers to a max of 70 degrees, the body gear 7. On the classic 747, the steering is switched manually by the crew and typically it has to be switched off take off and is switched back on below 30kts on the landing roll. The 747-400's steering is automatic. It disarms above 20kts and re arms again below 15.

This gives the 747 a tight turning circle. In one company I worked for which operated both 747s and 340s, it was the 747s which flew to the smaller Carribean airfields as they were more manouevreable on the ground.

The 777 also has rear wheel steering. Th bogies have three axles and the aft axle steers in the same manner as the 747's body gear steering.

[Spektor]

Dan,

You are the man!

Spektor.

[r304ndy]

The centre gear on the A340-200/300 stays retracted in the emergency freefall situation, whereas the four-wheel bogie centre gear arrangement on the A340-500/600 extends like the others.

[AirRabbit]

Ladies and Gentlemen

I was always of the impression that the reason Boeing decided on the configuration of the landing gear for the B747 was more than just structural support spread across the fuselage and wing spar – it was also for distribution of the weight on the ground across the larger “foot print” of 16 tires (vs. a smaller number – like 10 or 12 with a single body gear) and for the additional brake surface area for stopping purposes. Certainly, there are considerations for weight for both takeoff and landing but to my understanding, its not just structural – its structural, weight distribution on the surface, and braking - all together.