Hello friends,


my question is dead honest, but it was inspired by this picture:




I know that this mishap wasn't caused by braking (per the other thread, Last Year B777 Missed Landing (by Artc May 5 2012 in Civil Aviation)), but I humorously wondered if one could do a 360 on a (partially) iced runway, and below a certain speed, because of the vertical stabilizer authority.

Well, for that matter, what is the reason behind split braking systems, one for each side? There have been runway excursion because of failing braking systems, like the Air France 747 mishap some years ago...


Kind regards,


David

Quoting flyingturtle (Thread starter): Well, for that matter, what is the reason behind split braking systems, one for each side?

I don't quite understand the point of the question - differential braking exists because it improves directional control quite considerably, especially when maneuvering on the ground. It's better in pretty much every conceivable way to a non-split system, which is why even GA aircraft have it.

Differential braking does seem a little out of place on some of the bigger aircraft with nosewheel steering, steerable bogies etc, but it has always been a method of steering assistance even on some of the very earliest airplanes. On some smaller types, differential braking is your only means of ground manouvering at low speeds, particularly on tail-draggers or aircraft with castering nosewheels.

Further to my previous comment; addressing brake failures, generally this is addressed by failsafe design of the hydraulic system on a transport category airplane. You typically have at least two different hydraulic systems powering a given brake, so one system will be powering every other caliper piston, this is to prevent loss of the entire brake should you have a hydraulic failure in that system.
Differential brakes are also very nice to have when your nosewheel steering fails, and give you a slightly better chance of avoiding swerving off the runway and collapsing your landing gear when you hit the dirt.

On a Cessna 172 however, or Piper Cub, or Cirrus SR22, or you name it, this is not so - you might only have two brake caliper pistons, one for the left main wheel, one for the right, each connected to a separate master piston through its own dedicated line, unlike the brakes on a car, where braking force is distributed evenly. (Unless it has a clever ABS/traction control system, but that's off-topic)

What this means for a small plane though, is if you lose one brake system, you lose that wheel brake entirely, and thus may find your landing a bit more swervy than usual. The difference is, slowing down a small, light airplane from 60 knots is a slightly less critical situation than slowing down a multi-ton behemoth from 140 knots, so the same level of redundancy isn't designed into the systems to avoid undue weight penalty.

Quoting wingscrubber (Reply 3): You typically have at least two different hydraulic systems powering a given brake, so one system will be powering every other caliper piston, this is to prevent loss of the entire brake should you have a hydraulic failure in that system.

That's not the way it works on the aircraft I've worked on. What you are stating would reduce your braking effectiveness by 50% if you lost a single system.

This is mainly for Boeing products but I'm sure Airbus is relatively similar. Each brake is supplied with pressure from a single source at any one time and only has a single hose going to each brake assembly. It's the hydraulic components up stream from the brakes that determine the pressure source to the brakes. Modern Boeings have predominantly 4 sources providing pressure to the brakes... Normal, Alternate, Reserve and in the event those three fail there is the brake accumulator. Only one of these sources provides pressure to the brake system at any one time.

Cheers,

John

I didn't think differential braking was (still) a useful feature on airliners...


Thank you all for your replies! I've searched for that topic and haven't found anything...

Kind regards,

David

As a run/taxi qualified mech. on multiple different aircraft over the years, differential braking is still taught as an effective way to steer on slick/icy surfaces. It can be as or more effective then nosewheel steering on ice, I have had times even at very low speeds that when the tiller was turned the aircraft did not. The nose tires just slid, put a little left or right brake into it and it does quite well.

Quoting wingscrubber (Reply 3): You typically have at least two different hydraulic systems powering a given brake, so one system will be powering every other caliper piston, this is to prevent loss of the entire brake should you have a hydraulic failure in that system.

Quoting boeingfixer (Reply 4): That's not the way it works on the aircraft I've worked on. What you are stating would reduce your braking effectiveness by 50% if you lost a single system.

Quoting boeingfixer (Reply 4): Each brake is supplied with pressure from a single source at any one time and only has a single hose going to each brake assembly

I suspect you are both right, but for one of the major manufacturers each!

Airbus brake units have two hoses supplying pressure to alternate pistons as stated by wingscrubber, but IIRC, even if both systems can supply pressure, only one of them does at a time. The pistons connected to the other hydraulic source do not move. Hence, there is no reduction in clamping force if one of the hydraulic systems should fail. Apparently, the primary and alternate sources of brake pressure swap between the green and blue system each time the gear is extended.

Boeing brake units tend to have one supply hose as stated by boeingfixer. The 747 for instance has three sources of brake pressure #4 is primary with #1 and #2 as alternates in that order. IIRC, a set of shuttle valves switches between them if and when needed, with all the pistons powered by a single system at any one time.

http://www.smartcockpit.com/pdf/plane/airbus/A330/systems/0016/
http://www.smartcockpit.com/pdf/plane/boeing/B777/systems/0010/

Regards, JetMech

Quoting wingscrubber (Reply 2): Differential braking does seem a little out of place on some of the bigger aircraft with nosewheel steering, steerable bogies etc, but it has always been a method of steering assistance even on some of the very earliest airplanes

Even on the very largest aircraft, differential braking is necessary for the aircraft to achieve its minimum turn radius.

nobody even mentioned a crosswind landing.

Quoting MrChips (Reply 8): Even on the very largest aircraft, differential braking is necessary for the aircraft to achieve its minimum turn radius.

Looking at a/c turn diagrams, that makes complete sense. There's no way the steering tiller alone will achieve turns required.

Quoting FlyASAGuy2005 (Reply 10): Looking at a/c turn diagrams, that makes complete sense. There's no way the steering tiller alone will achieve turns required.

The turn diagram will match maximum nosewheel deflection so, in some sense, the tiller will get you all the way there. However, in practice, the nose gear is going to skid if you try that at anything more than tiny groundspeed. Differential braking helps a lot with getting the nosewheel around cleanly.

Tom.

Quoting 737tdi (Reply 6): I have had times even at very low speeds that when the tiller was turned the aircraft did not. The nose tires just slid, put a little left or right brake into it and it does quite well.

what types were you referring to in this example?.

Quoting flyingturtle (Thread starter): I humorously wondered if one could do a 360 on a (partially) iced runway, and below a certain speed, because of the vertical stabilizer authority.

Does this video answer your question?  
http://www.youtube.com/watch?v=KSWjjjHb89M

Quoting zkojq (Reply 13):

...weeeee... thanks! 

Quoting HAWK21M (Reply 12):

DC8 (61,62,63,71,72,73), 727 (100/QF, 200), 737 (all), and DC9/MD80 series. I only drove the 747 a couple of times during qualifications and decided I did not feel comfortable driving that bird. I was also certified on the DC10 but don't recall ever having to use diff. braking on it. I do recall on the DC10 that it was also uncomfortable to taxi, you are so far past the turn point where you are sitting. Hope I answered what you were asking.

Quoting 737tdi (Reply 15): I do recall on the DC10 that it was also uncomfortable to taxi, you are so far past the turn point where you are sitting.

The orientation of the NLG below the Flight deck can give a totally different view when taxying.

Quoting CosmicCruiser (Reply 9): nobody even mentioned a crosswind landing.

Why would anyone?

Quoting Fabo (Reply 17): Why would anyone?

Because it can be difficult to maintain directional control with strong crosswind, especially on a contaminated runway. This is due to weathercock-effect (aircraft turning nose into wind due to wind pressure on vertical stabilizer). If you have friction problems on the nose wheel, you can assist with differential braking.

Ah, makes sense. I have just never heard of using differential braking as a control method for crosswind landings... learn something new every day...

I was just referring to controlling a straight rollout as the speed gets slower. I don't specifically think about, just do what it takes. If a little downwind brake can help, use it. I was only speaking of ldg.

Anyone ever had a real-world encounter with untentional differential braking? I remember landing a Cessna 210 one day at CUU (we landed kind of hot because we had an MD-80 on approach behind us), and going to apply the brakes, to find out only one of them worked, in a blinkin' crosswind   We almost ended up in the sagebrush. Finding a mechanic in Mexico who was willing to work on a GA plane was quite the chore, and expensive too...