End of the runway

[Mosley]

If you get in a tight spot you can always turn off anti skid and stop a little shorter, crew chief might get pissed about the tires tho.

[Eddie]

Disabling anti-skid should not result in a reduced braking distance, it'll actually increase the distance if the pilot applies sufficient braking force to lock the wheels.

[Tailgate]

As I explained in the other thread regarding landings, you need to stop aerobraking; it doesn't work in the A-10. Get the nose on the ground and with full speedbrakes, the airplane will coast to a stop without wheel brakes on an 8,000 ft runway.

 

I guess people keep forgetting that the A-10 is not a F-16 and the fuselage wasn't designed for aerobraking. ;)

[Yskonyn]

If anything the anti-skid system is a measure to aid in safeguarding shorter landing distances, not increase them.

It's like an ABS system on a car. Preventing the wheels to lock up keeps tabs on friction and prevents sliding the wheels over the runway which would increase distance due to reduced friction.

 

Like Eddie said; if the pilot would apply max manual brake and lock the wheels. The distance would increase.

If applying proper braking conditions (wheels have good friction and no skidding is present), the anti-skid system does not affect landing distance one way or the other.

 

Many people think its solely a wet or contaminated runway conditions measure, but its not.

For example, on some aircraft the wheels are also monitored in relation to eachother. Preventing any wheel from skidding.

Dry or wet.

 

The low speeds of the Hog make trying to 'aerobrake' rather useless. Besides, it's not a delta wing aircraft.

Edited September 10, 2012 by Yskonyn

[peter]

Also you should be touching down no more than 1/3 of the way down the runway. You cant expect to stop on concrete when you try to milk a nice landing and end up putting rubber on the concrete with 2000ft left. Just put the thing down, you dont get paid extra for a greaser.

[Echo38]

Wait, how does anti-skid decrease stopping distance compared to skidding tires? Anti-skid/ABS makes it safer to stop, by preventing the aircraft/automobile from losing lateral control. But how would skidding tires reduce friction? I'd expect that to be max friction.

[159th_Viper]

Wait, how does anti-skid decrease stopping distance compared to skidding tires?

 

It does not.

 

Anti-Skid engaged doubles your In-SIM stopping distance. Conversely, switch anti-skid off and stop in half the distance:

 

Kutaisi test:

 

Anti-Skid On:

 

 

Anti-Skid Off:

 

 

Above pics illustrating a dead-stop.

Edited September 10, 2012 by 159th_Viper

[GGTharos]

That's actually a tough one to explain. A rolling tire dissipates energy faster because it constantly deforms as a new part of it makes contact with the surface. This means that at some point additional energy is applied to deform the tire where it begins to contact the pavement. A locked up tire doesn't have that 'problem', its deformation happens once and it stays that way - therefore that additional energy is not dissipated.

 

Wait, how does anti-skid decrease stopping distance compared to skidding tires? Anti-skid/ABS makes it safer to stop, by preventing the aircraft/automobile from losing lateral control. But how would skidding tires reduce friction? I'd expect that to be max friction.

[Yskonyn]

I stand (partially) corrected; ;)

 

I looked it up in our manuals (B737)

Anti-skid is mainly a measure to keep controllability of the aircraft. Using anti-skid on snow/ice covered runways actually increases landing distance compared to skidding, but controllability is much better with anti-skid.

 

The reason I thought having it disabled increased landing distance initially was the non-normal landing distance table with anti-skid inoperative showing far higher numbers.

But checking the checklist revealed the probable cause;

 

'Do not arm the speedbrakes for landing. Manually deploy the speedbrakes immediately upon landing'.

 

Combined with the unavailability of the autobrake system (and hence requiring input from the pilot with the well known 'second needed to react' we humans have) might explain the higher values.

 

Now the A-10C does not have an autobrake, nor automatic speedbrake deployment. So we are back at square one. :D

 

Still, the remark from GGTharos must be taken into account as well.

 

Secondly; it depends on the surface you are braking on. High grip surfaces (like most runways are) have shown favorable effect on stopping distance with an anti-skid system vs no anti-skid.

The reason why is complicated and lies in the realm GGTharos touched.

 

Regarding the friction; I meant to say grip. Once your tires start to skid they loose grip.

It seemed only logical to me to conclude distance must increase, but further looking up revealed that (on a car) ABS has negligible impact on brake distance on a dry surface.

 

So anti-skid is a controllability safety feature in aircraft as well as cars, more than anything.

 

Why there is such a big difference in landing distance in the sim then, I don't know. It seems rather off.

The tire model is probably off (heck, even racing sims have difficulty modelling good tyre physics).

Edited September 10, 2012 by Yskonyn

[marcos]

Flaps down, airbrake on, thrust to idle and wheel brake on. You should easily stop the A-10 in a fraction of the runway.

[GGTharos]

Combined with the unavailability of the autobrake system (and hence requiring input from the pilot with the well known 'second needed to react' we humans have) might explain the higher values.

 

You are right, there is a lot of human factor. Without ABS for example you have to use threshold braking, which means you have to spend time finding the threshold. This makes a significant difference for something as heavy as an aircraft or a truck.

 

It seemed only logical to me to conclude distance must increase, but further looking up revealed that (on a car) ABS has negligible impact on brake distance.

 

Actually in practice, it is said that a skilled, alert driver would have trouble matching or getting better distance than a driver stomping on the brake pedal with ABS available. There is also an implication that the locked-wheel skid will go on for a longer distance.

 

But as you said, this is very, very dependent on the types of surfaces that are coming into contact - ie. in our case rubber and pavement ... the state of the treads for example can change everything, as can the temperature of the road ... never mind rain/snow/ice.

 

The tire model is probably off (heck, even racing sims have difficulty modelling good tyre physics).

 

I imagine it's as easy to model to model as rotor/tail airflow interaction for a heli :P

[Yskonyn]

Threshold braking, is that proper english for finding the sweet spot of highest friction without going into a skid by locking the wheels?

I am not a native english speaker, so found it hard to explain it.

Learned something new here, thanks! :thumbup:

[GGTharos]

Yes, that is what threshold braking is. You're welcome :)

[BlueRidgeDx]

Anti-skid is, and has been porked in DCS for a long time. On a dry runway, braking distance is almost exactly double what the airplane is capable of.

 

In the real world, without anti-skid it is exceedingly difficult to max perform the airplane without locking the wheels due to a lack of tactile feedback. The problem with this, of course, is that a locked wheel has a friction coefficient much lower than one rolling but partially skidding.

 

When the anti-skid is doing its job, you're leaving skid marks on the runway because the wheels are neither locked, nor rotating as fast as the aircraft is traveling.

 

If the wheels do lock early in the landing roll or during a rejected takeoff, you have about 2-5 seconds - depending on groundspeed - before the locked wheel blows its tire. Without a tire, the brake is useless and your stopping distance increases exponentially.

 

In the A-10, the anti-skid system is not as sophisticated as newer systems. If either wheel approaches a locked condition, brake pressure is reduced to both brakes simultaneously and equally. Each wheel brake is not individually modulated.

[sobek]

The reason for a rolling wheel providing a shorter run is that static friction is higher than dynamic friction, that means, adhesion of the tire to the concrete is best when its surface is not in relative motion to the concrete, ergo when it is turning without skidding.

 

If you don't believe me, put a piece of paper on your desk, apply some pressure on it and try to move it around. You will find that it takes a lot more sideways force to get it moving than it does to keep it that way once static friction has been overcome. I don't remember right now what the physical cause is for this effect, but it is the reason that maximum braking performance can be achieved just short of locking the wheels up.

Edited September 10, 2012 by sobek

[FreeFall]

Disabling anti-skid should

 

.... show you that the runway is actually kinda slippery thing...

 

For fun, I've trained landing without anti-skid. Especially if you engage your wheel brakes very soon after the touchdown (with anti-skid disabled), you'll find out that phenomenon.

[ShuRugal]

But how would skidding tires reduce friction? I'd expect that to be max friction.

 

 

incorrect. explanation follows.

 

first, there are two types of friction: Static and kinetic. Static friction occurs when two objects are in contact but are not moving relative to one another. In this condition, the objects settle fully together and all the minute surface variations that cause friction mate together as fully as possible. Kinetic friction occurs when two surfaces are sliding past one another. under this condition, the minute variations do not get a chance to fully mesh, and so a full grip is not developed.

 

In short, two objects have a higher coefficient of friction under static conditions than under kinetic conditions.

 

When a tire is rolling, the contact surface of the tire is stationary against the ground. This means that while a tire is rolling, it experiences static friction on the ground. when the wheel has been locked up, the contact patch is now sliding along the ground, and kinetic friction applies. since kinetic friction has a lower CoeF, a skidding wheel applies less force to slowing the plane.

[Yskonyn]

.... show you that the runway is actually kinda slippery thing...

 

For fun, I've trained landing without anti-skid. Especially if you engage your wheel brakes very soon after the touchdown (with anti-skid disabled), you'll find out that phenomenon.

 

It seems to be in-sim, yes. Which is weird.

Although I suspect it has got more to do with the anti-skid / braking logic than the actual surface being slippery. ;)

But yes, with anti-skid on, it does give you the impression you are landing on something which is kind of 'slippery'.

[Echo38]

As I explained in the other thread regarding landings, you need to stop aerobraking; it doesn't work in the A-10. Get the nose on the ground and with full speedbrakes, the airplane will coast to a stop without wheel brakes on an 8,000 ft runway.

 

Hey, I finally got a chance to try this out, and, sure'nough, works like a charm. Stopping in time without breaking the brakes is no longer a problem. Thanks awfully for the tip!

 

I suppose it must be because the A-10's mass gives it about a third more friction with the third wheel down, and the reduced AoA means less lift, so more pressure on each wheel. That sound right?

[Echo38]

Nah, it was my silly P-38-style aerodynamic braking that was giving me trouble, just like Blue said.

[Echo38]

Stopping in time without breaking the brakes is no longer a problem.

 

Err, um, well, it looks like I lied. Whenever I come down fast enough for a gentle landing (over the numbers at 130), I still have to apply a fair amount of wheelbrake near the end of an 8000-foot runway. I'm going about 60 knots when I start applying them, and I apply half brakes. This allows me to turn off at a relatively reasonable speed (about 20 knots, which is still faster than I'm comfortable with, but at least I haven't blown a tire).

 

I suppose I should make a video of one of my normal landings, so that everyone can tell me what I'm doing wrong. My landings which are slow enough that I can coast to a stop without wheelbrakes (over the numbers at 120) are too hard on the mains for my taste, as below 120 I come down very hard, like I'm landing on a carrier. I should mention that I'm practicing with nearly full internal tanks.

Edited September 13, 2012 by Echo38

[159th_Viper]

Err, um, well, it looks like I lied. Whenever I come down fast enough for a gentle landing (over the numbers at 130), I still have to apply a fair amount of wheelbrake near the end of an 8000-foot runway. I'm going about 60 knots when I start applying them, and I apply half brakes. This allows me to turn off at a relatively reasonable speed (about 20 knots, which is still faster than I'm comfortable with, but at least I haven't blown a tire).

 

I suppose I should make a video of one of my normal landings, so that everyone can tell me what I'm doing wrong. My landings which are slow enough that I can coast to a stop without wheelbrakes (over the numbers at 120) are too hard on the mains for my taste, as below 120 I come down very hard, like I'm landing on a carrier. I should mention that I'm practicing with nearly full internal tanks.

 

You are doing nothing wrong - that's just the way it is at present In-SIM: There is absolutely no ways you are going to coast to a stop with full internal tanks. Your speed at touchdown seems about right (could probably do it 5 knots slower with a clean airframe but that is neither here nor there).

 

I ran a quick test with airframe at 90% GW and touchdown at Batumi (8000 feet) is at 130 knots and there is, as said, no way you are coasting to a stop. With braking (no aero) you can/do stop within 3500 feet at 90% GW if you really have to, but that's not the question - fact is you are doing nothing wrong - it's a bug with the SIM.

[PeterP]

... I didn't read all the posts. So I don't know if this procedure "How to do it right" is already mentioned:

[Ironhand]

... I didn't read all the posts. So I don't know if this procedure "How to do it right" is already mentioned:

SWEET! :megalol:

 

 

Rich

[BlueRidgeDx]

If the Mission Editor is not lying, then the runway length at Kutaisi is 2500m, or roughly 7600ft.

 

I landed there with a left engine failure and forgot to pull the emergency brake handle, so I had no brakes. I had 5,500lb of fuel and a gross weight of 38,000lb.

 

I landed normally and coasted to a stop on the runway using only speedbrakes. Eddie was there, so he can confirm the story.

 

I think maybe peeps are afraid of using the brakes, or something? There's nothing wrong with using the brakes... It's not always necessary, beneficial, or desired to roll out to the end of the runway. There are plenty of situations that don't involve visiting the EOR pad before you park the jet.