Nathan,
This is the quote from the B747-400 Performance Limitations manual 'Background Information - Landing / Certification' section:
Quote:
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Reverse thrust is not employed to establish the Landing Field Length required. However, CASA require that a major retarding device be available in reserve; or failing that, the required field length be increased (by 15%). Reverse thrust provides the required margin.
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Our company adds an LDR margin of 200m if one reverser is inoperative, and requires a 300m LDR surplus in order for idle reverse to be used.
The Autobrake system provdes fixed rates of deceleration with or without reverse selected (it varies the brake pressure to achieve the target deceleration). However, if full manual braking was applied, the use of reversed thrust
in addition to the braking would result in a shorter stopping distance over the no or idle reverse case.
In any case, I digress!