B737-800 AUTOBRAKE Adjustments
 

Hi friends,
Just taking a look on my QRH PI section normal landing distances,
I just wonder why with Autobrake in 3 F30 dry runway there is NO correction if thrust rev is not used, and with Autobrake MAX, the adjustment is 5 meters is reverses are not used??
Anyone has a idea about why there is no effect using or not the reverses with autobrake 3??
Thanks

An autobrake selectiion will apply brake pressure to achieve a specific deceleration rate. If you add thrust reversers to the mix, the autobrakes will reduce brake pressure (since part of the deceleration is achieved by rev thrust). In theory, the used runway length will always be the same (on a dry runway)

Added to clarify - Autobrake MAX is not modulated so using reverse will increase available stopping effort.

MAX should be modulated, to 14 ft/sec^2 above 80 kts and 12 ft/sec^2 below. However, MAX modulates up to 3000 psi brake pressure, therefore there could be a small difference with reverser.

Thanks, Denti - didn't know that (obviously:)) Thus it cannot be 'Max' in reality? Where does RTO fit in?

This presentation may help understand the relationship between brakes, reverse and runway condition:-
http://www.scribd.com/doc/75487636/Slippery-Runways
Also see:-
http://www.scribd.com/doc/95073485/U...unway-Overruns

Thanks to all for your info, but I wonder why if with Ab3 (2000psi) and a deceleration 7.2 Ft/s2 NO adjustment is made if reverses are NOT used and with AbMAX (3000 psi) and deceleration rate 12ft/s2(gs<80kts) 14ft/s2(gs>80kts) an adjustment has to be consider if rev are NOT used?
Thanks

No idea now, having learnt that Max is modulated. Safetypee - those presentations have very pretty ppt slides but no content to answer the OP's question!

BOAC, perhaps dazzled by the graphics?
Ka8, #2 provides the short answer.
The ‘Slippery Runways’ file graphically illustrates the theory and practice for a range of runway conditions.
Page 25 shows that when AB is used, and the deceleration target is met, then reverse has no further (decelerating) effect because the AB ‘backs off’ the brake component to maintain the required deceleration. N.B. the diagrams only show deceleration, not distance.

The landing distance charts (-700 shown on page 12) consider the component of each decelerating device and the time taken to deploy them, thus pure AB might provide marginally better performance than with AB + Rev, and a lower AB setting might ramp up the deceleration slightly slower than AB MAX. The distances depend very much on the assumptions made in use; technical systems might be closer to the theoretical limits, whereas pilot selection (Rev) might assume greater time delays.

MAX Manual is slightly better than AB MAX (and even better with Rev) as there is no limiting deceleration target, but the ultimate deceleration available and thus distance will still be limited by a range of other brake / runway factors – page 19.
These aspects are summarised, in simple words, on page 31, but the performance stills depend on ‘reasonably attainable assumptions’ (page 17, in the ‘Overrun’ file).

@BOAC, RTO apparently is simply full brake pressure without any modulation. Quite honestly even MAX is pretty hard and will probably lead to some dented foreheads for the passengers in high density configurations. Used it only a few times, 3 with manual override is enough for pretty much every case on our route network.

This is indeed the reason why, as it takes (I recall ~.25 seconds) less to reach the predefined PSI before modulation takes place.

Safetypee - I think you and ka8 need to revisit the OP where he/she asks "and with Autobrake MAX, the adjustment is 5 meters is reverses are not used??". I have seen no explanation yet, although Denti suggests a possible answer. Ka merely explains how AB works in general. Do you know the answer?

Denti - yes - I have been sat in Row1-right on a positioning 737 where the operating Captain used 'MAX' and I finished up talking to the pax next to me with my head pressed against the bulkhead:). We stopped absolutely 'dead' in the middle of the LDA with the decel apparently so rapid that he was not able to over-ride the braking using foot pressure. Once the rear galley had been moved back from the front we taxied off...................

Safetypee - I think you and ka8 need to revisit the OP where he/she asks "and with Autobrake MAX, the adjustment is 5 meters is reverses are not used??". Thus " thus pure AB might provide marginally better performance than with AB + Rev," appears to be incorrect. I have seen no explanation yet, although Denti suggests a possible answer. Ka merely explains how AB works in general. Do you know the answer?

Denti - yes - I have been sat in Row1-right on a positioning 737 where the operating Captain used 'MAX' and I finished up talking to the pax next to me with my head pressed against the bulkhead:). We stopped absolutely 'dead' in the middle of the LDA with the decel apparently so rapid that he was not able to over-ride the braking using foot pressure. Once the rear galley had been moved back from the front we taxied off...................

Max manual might be better than AB max or RTO only if the hydraulic pressure available is greater than 3000psi, which it sometimes is.

Actually, max manual is always better than MAX as MAX modulates brake pressure and max manual doesn't. RTO on the other hand is max brake pressure anyway and the best course of action is to not interrupt auto brake function during an actual high speed RTO.

RTO is 3000psi unmodulated, if there is 3250psi available RTO isn't max pressure.

B737-800 AUTOBRAKE Adjustments
 

What's meant with "modulated"? Until the previous post I thought "regulated"....

JeroenC, many posts imply that modulated only refers to AB control - the regulation of brake pressure to achieve the selected deceleration. ‘Unmodulated’ braking in this sense is without AB limit or influence; however braking systems still have some form of modulation (controlled or regulated brake pressure) for the anti-skid function, which may be an integral part of the overall AB system.

LSM, 3250psi hydraulic pressure might be available to the braking system, but not necessarily the pressure used for RTO, thus 3000psi is max? Yet even this may be reduced due to anti-skid protection – which is a function of runway surface etc. Is this what you were implying?

BOAC, as I understand the question, the problem relates to the small differences in distance irrespective of the use of Rev, and in particular to no difference when using AB 3, for a particular aircraft model.
I do not have a precise answer; nor do I disagree with Denti #4.
Many views appear to be restricted to the pure technical (theoretical) aspects of AB / Rev, whereas the distances involve ‘time’ assumptions in selecting and operation of rev, - the time to achieve full reverse and for AB to stabilise the deceleration; I do not know what these assumptions are. In addition there are other assumptions in the distance calculations involving runway condition/brake system characteristics which limit the deceleration from the brakes, but do not apply to the reverser contribution to the overall deceleration.

Thus although page 25 shows that with/without rev the MAX AB decelerations are identical (theory), the advisory distance data (practical) can differ, and also that these distances / differences may vary with aircraft/engine model and the certification standard (FAA/EASA), but sometimes practice = theory = '0'.

@LSM: interesting, there seems to be a difference in our respective documentation then. In ours it says that RTO uses maximum available brake pressure whereas the landing settings are modulated up to a maxmum of 3000 PSI in case of MAX.

However i am the first to admit that our documentation is woefully bad and that could just be a transcription error and not the real technical explanation. Sadly we as pilots do not get access to the AMM.

Denti: is correct, as per AMM.