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The A380 has 20 main and 2 nose wheels. Apparently the nose wheels have no brakes (compare with Are there any aircraft with a nose wheel braking system?).

While researching for this question (How are brakes cooled on heavy aircraft?), I assumed that all of the 20 main wheels are braked. I looked at a few pictures of A380 landing gears. As far as I have seen, all main wheels are equipped with brake cooling fans (BCF), except the four rear wheels in the inner 6-wheeled undercarriages. (e.g. here).

This picture below clearly shows that there is no brake unit inside the rim:

enter image description here

There is another picture here whithout BCFs, but the same four wheels apparently have no brakes.

What is the reason for omitting the brakes at these four wheel positions?

Is this a common configuration, or quite unique to the A380?

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    $\begingroup$ I've got no facts for this, so a comment: Here you can read that carbon brakes have less wear when used to apply heavy braking forces. Perhaps some of the brakes are omitted, such that the others have to 'work harder' and thus, wear less? $\endgroup$
    – ROIMaison
    Commented Dec 7, 2016 at 13:52
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    $\begingroup$ Because it doesn't need them. Why fit what's not needed? $\endgroup$
    – Simon
    Commented Dec 7, 2016 at 13:57

2 Answers 2

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Because they are not needed (for the passenger variant). Addition of brakes increases the weight and complexity of the system, which is undesirable. Why add something that is not required?. If it is required, additional brakes can always be added, like in the (proposed) freighter version. According to flightglobal.com:

Sixteen of the A380-800's 20 main wheels are braked, with the two aft wheels on each body landing gear of the passenger version not having brakes (unlike the freighter variant, where all 20 main wheels are braked due to its increased weights).

Also, Aft Axle of BLG is steerable to ensure better maneuverability for small radius turn (Body Wheel Steering); braking a steerable axle would require special linkages, increasing weight and complexity. This is not insurmountable, however- Boeing 777's landing gear is similar, but has brakes on all its wheels.

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    $\begingroup$ Out of curiosity: would the 'needed' braking force account for action of thrust reversers? (I'd imagine so under normal operation, but that the braking system could do the job in the event of failure at the cost of higher wear?) $\endgroup$
    – msanford
    Commented Dec 7, 2016 at 14:44
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    $\begingroup$ @msanford The aircraft is required to be able to stop without the use of thrust reversers, if that is what you are asking. $\endgroup$
    – Ron Beyer
    Commented Dec 7, 2016 at 14:47
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    $\begingroup$ @bogl Your V1 speed will depend on the runway lenght. It is not a fixed value. So yes, any runway. $\endgroup$
    – 60levelchange
    Commented Dec 7, 2016 at 16:04
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    $\begingroup$ @bogl The actual RTO certification test isnt done based on runway length, its done based on the maximum kinetic energy dissipation of the braking system the aircraft manufacturer wants the aircraft certified to. Page 33 on faa.gov/documentLibrary/media/Advisory_Circular/25-7B.pdf $\endgroup$
    – Moo
    Commented Dec 7, 2016 at 18:50
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    $\begingroup$ @Moo Great! This is what I was looking for! $\endgroup$
    – bogl
    Commented Dec 8, 2016 at 8:48
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According to this presentation by Claude Lelaie (Head of A380 test), the brakes of the A380 were tested before certification with 89 MJ, and after certification with 120 MJ.

According to your calculation here the RTO of an A380 sinks 1.7 GJ to the brakes.

Therefore either configuration would be sufficient with respect to the RTO energy:

  • 20x 89 MJ = 1.78 GJ
  • 16x 120 MJ = 1.92 GJ

It is not clear if this setup was planned from the beginning, or maybe the brake design turned out to be better than expected.

During braking, the vector sum of brake and weight forces is pointing forward. This means increase of force on the front wheels, and less force on the rear wheels. This is probably the reason why the engineers have chosen to omit the brakes on the 4 rearmost wheels, and not a different location.

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