Speed additives during an approach with autothrottle engaged depend on aircraft type / system performance; hence refer to the manufacturer’s advice.
The additives may only be required in particular conditions or at certain stages of the approach. The additives provide additional safety protection, but also require consideration of the landing distances required, which if excessive, present an additional and potentially greater hazard to safe operations.
Speed may be bleed off before the threshold enabling ‘standard’ performance to be used, but any speed change may destabilize the A/T system or trim condition – invalidating autoland performance, or result in higher crew workload, or violate stabilized approach criteria.
Boeing provides guidance for increased landing distances due to speed excess (also see AC 91-79), but these data assume manual thrust reduction. Some A/T’s have a variable retard initiation height or rates-of-closure resulting in different flare distances whilst attempting to give a consistent speed loss (normally 7kts). However, Boeing’s landing performance assumes a constant flare distance of 1000ft for manual landing (1200ft for big jets), which with A/T may not be achieved, or 1500-2500 ft for autoland/HUD which may be more realistic for manual landing with A/T engaged.
Thus, when using A/T during a manual landing consider carefully the implications on landing distance and the actual runway conditions, particularly where gusting / cross winds can also add to the actual stopping distance.
JT respectfully disagree with your para (e).
Whilst landing distance factors provide a safety margin for small variations in normal operations, they are not intended to accommodate ‘deliberate’ speed increments.
See
AIC 91_06. Note that the ‘15kt margin’ for higher speeds at the threshold refers to the ‘reference’ method of certification which is no longer used; and
AFAIK no modern commercial aircraft uses this datum for landing performance.
Also, it should be remembered that the actual safety margin on a wet or contaminated runway is not the same as that for a dry runway even though the certification distance factors are greater. Recent Canadian research suggests distance factors of 2.2 – 2.4 may be required in contaminated conditions to provide the equivalent level of safety as on a dry runway.
“The margin of error for the threshold crossing airspeed is +5/-0 knots. If the pilot has planned to carry additional airspeed beyond the threshold due to gusty surface wind conditions, or other factors, then the effect of this additional airspeed must be included in the actual landing distance”. (
AC 91-71 Runway Overrun Prevention.)