Max Auto sets the highest autobrake deceleration rate. With full manual braking you are applying maximum brake pressure, within the limits of the anti-skid system, so you will stop quicker even without using reverse thrust. Using autobrake the deceleration rate is not affected by reverse thrust, the brake pressure will be modulated to achieve the commanded deceleration rate. If you use manual braking, reverse thrust does increase the deceleration rate of course.

 

According to an archived thread on pprune.org, autobrake deceleration rates are as follows

 

1. ........4 ft/sec²

2. ........5 ft/sec²

3 ........6 ft/sec²

4. ........7 ft/sec²

MAX ..11 ft/sec²

 

So MAX AUTO is significantly more powerful than the normal autobrake rates.

Boeing is not consistent between their different planes. For example the 737 has Level Change button where the 777 is Flight Level Change. The 737 has a Fasten Belts switch where the 777 is Seat Belts. That's just two things off the top of my head.

 

That is no difference :P

Because max auto is not the maximum that the aircraft potentially has.

 

Added to max reverse thrust [rarely used], one would get the minimum stopping distance

Using reverse thrust with MAX AUTO braking will not change the stopping distance unless the brakes can't achieve the selected deceleration rate.

Using reverse thrust with MAX AUTO braking will not change the stopping distance.

 

I didn't say it did, I said with manual braking.

I didn't say it did, I said with manual braking.

Your post that I quoted doesn't mention manual braking. That's why I felt it needed commenting on.

Because max auto is not the maximum that the aircraft potentially has.

 

On 737 you can rapidly switch autobrake selector from any landing position to RTO after touchdown and possibly get maximum autobrake power.

On 737 you can rapidly switch autobrake selector from any landing position to RTO after touchdown and possibly get maximum autobrake power.

 

lol

If the system provides maximum braking pressure then how can a pilot do any better? He can't exceed the maximum pressure that the hydraulic system can handle.

 

The switch is labelled "Max AUTO" for a reason. The pilot can do better by standing on the brakes. The only autobrake position that applies maximum braking is RTO.

 

Paul

Wow, thwere sure a lot of experts on this thread!

The reason for that Max Auto is less than Max Manual is to "Guarantee" the required deceleration rate. Max auto, may well apply full brake pressure, if the runway surface was slippy, or it may not, but if you were to base your landing calculations on maximum pressure as opposed to max auto, you don't know exactly if you'll get the full rate, or reduced rate depending on runway surface. It must be borne in mind that brake pressure and decel rates do not always match from one runway to another, ie you can't say, 50% brake pressure = 1m/s on every single runway, some may take 55% some 60% to achieve that rate of decel.

 

So basically, since max auto doesn't get the same rate as max manual on a dry runway, is basically just an in built margin to ensure that on contaminated runways there's still enough wiggle room to achieve the requested decel rate that was used for landing calculations.

 

Regards,

Ró.

The switch is labelled "Max AUTO" for a reason. The pilot can do better by standing on the brakes. The only autobrake position that applies maximum braking is RTO.

 

Paul

 

Next logical question then ... for short runways, can you select RTO as the autobrake setting for a landing ?

Next logical question then ... for short runways, can you select RTO as the autobrake setting for a landing ?

 

Doubtful. RTO is meant for rejected takeoffs. The fact that you can grab it after landing is more of a lack of idiot-proofing than it is because it's supposed to be a selectable option.

I probably mistook Max and RTO as largely the same thing. Thanks for clearing it up.

I probably mistook Max and RTO as largely the same thing. Thanks for clearing it up.

They are in the Airbus... :wink:

 

Regards,

Ró.

Next logical question then ... for short runways, can you select RTO as the autobrake setting for a landing ?

 

No you cannot select RTO in the air. The switch will just flip back to OFF. I'm not sure if you can select it during the landing roll, but I really don't see anything to prevent it. Of course, then you'll have to explain why you have 12 deflated tyres.

 

Paul