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Old 16th December 2008 | 18:56
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PEI_3721
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Joined: Mar 2006
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From: England
bobrun, take care that your economic view does not cloud the more important safety view of braking – and brake for safety not for comfort. Habits are difficult to change, and the conditions in which change is necessary are not easily identifiable.

Not all autobrake systems give a quicker brake application than manual braking. Most auto systems ramp up to full braking to ensure good wheel spin-up, enabling correct anti skid functioning.
Neither does autobrake provide better stopping distance than correctly applied manual braking. Most ‘Max’ autobrake settings result in a deceleration below that achieved by full manual braking; where fitted the ‘RTO’ setting might equal manual braking, both in speed of application and stopping distance, and often eliminating mistaken human actions.
However, as runway conditions deteriorate, the difference between manual and auto is not as large and in some cases there is none.
The two effects above are described and shown diagrammatically in the ALAR tool Kit Briefings 8.4 — Braking Devices , 8.5 — Wet or Contaminated Runways .

Although some Boeing manuals indicate that Max autobrake provides the shortest stopping distance, the document which I have suggests that this only applies to wet / slippery operations.
Other references, i.e. 737 QRH page 22.1 clearly shows the advantage of max manual braking; note the that the distances are unfactored.

A potential pitfall of autobrake is that when braking in very wet conditions with reverse thrust, you may not get any body/foot-force feedback of the brake’s effectiveness as the majority of the braking effort comes from reverse until lower speeds. Thus the crew might not gain early warning if the runway is more slippery than expected and a higher level of braking should have been selected. The effect is shown in fig 3 ALAR 8.4 Braking Devices.
Also, note that wet landing performance probably does not assume reverse use, whereas contaminated / slippery performance does (JAR-OPS 1).
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