B737 Autobrake Calculations
 

Hi. It maybe asked before but made a research and couldn't find it.

On the QRH normal configuration landing distance tables there are calculations for every autobrake setting. But on the non-normal configuration tables Max Manual and Autobrake Max are calculated for every table, but for dry runway and good reported braking action only AB2 is calculated, and for medium and poor only AB3 is calculated.

What's the reason behind this?

It has to do with the braking distances and the runway length available at a normal airfield.

For example, with all flaps up landing and BA poor, the reference distance for AB 3 is almost 3500m. Do you really need the data for AB1, which would give you, let's say 5000m? How many such long runways are there?

If an auto brake setting is used, read it as a minimum auto brake level required for the subsequent stop to be made

Let's say there's a runway 2500 meters. On dry and good reported action the table only gives me AB2. Let's assume AB2 calculation gives us 2600 meters. I can guess that AB3 will work for me. But it's not in the table.

In such condition as I can't set my autobrake depending on assumptions I need to set my autobrake to max or I'll decide on manual braking. Doesn't make sense at all.

i would assume they reduce the autobrake setting on good surface conditions to reduce the likelihood of tires bursting.
when doing a non-normal landing the speed is usually higher than normal i assume?

so this could make sense to me but i guess there would also be a caution not to use higher autobrake setting if possible.

Well, I guess this is the best shot we have.

Thanks for the brain storming guys.

wiedehopf, unless stated as a limitation of the aircraft certification, you can discount the likelyhood of tyre bursting; e.g. no restriction on max manual brake.
Abnormal landings may have to account for max tyre speed and/or max brake energy, but again if limiting, should be stated in the documents.

rbnsncrs, you should be able to compare the untabulated data by working out the distances from the AFM charts. However, for abnormal conditions or system failure, any restrictions may be given in the AFM or FCOM procedures.
Check the abnormality against the availability of reverse, spoilers, etc, they all affect braking distance.

Safety rule of thumb; never take an assumption into the air. If in doubt check with the manufacturer.
Brake for safety, not for comfort; and not as a guess for system operation or the landing distance required / available.