B737 manual throttle approach speeds and over-run risk management

[safetypee]

Re #7 “… then I doubt a few knots over the typical Vref additives would be that important …”

AC 91-71 “… an aircraft carries 10 knots of excess airspeed through the threshold window on a wet runway surface. The wet runway increase in landing distance would be at least 500 feet, and the extended flare would add another 2,500 feet to the landing distance for a cumulative addition of 3,000 feet.”

These sort of numbers become very important especially if the additives are already greater than 10 kts; but with the assumption “… so long as there's minimal float associated with the flare,” this involves higher workload and appropriate skill to achieve a manoeuvre which is probably not the norm.

For accuracy, the certification requirements for landing state: “The landings may not require exceptional piloting skill or alertness.” (CS 25.125) In addition there is a caveat defining the approach speed:“A stabilised approach, with a calibrated airspeed of not less than Vref, must be maintained down to the 15 m (50 ft) height.”

Any speed additive, altitude deviation, or non standard maneuver, will reduce the margin of safety provided in the landing distances. The margin of safety stems from the factored landing distance.
On a dry runway, the margin is based on measured distance, but on a wet runway, the margin may be significantly less even though the ‘factor’ (physical distance) is larger. In part, this is due to the majority of the distance data used in certification being based on calculations which use a nominal friction. Some manufactures may provide a range of data, i.e. wet or wet/slippery. However, to use this with any confidence crews have to know the runway friction, which is one of the great unknowns in aviation today.
There are also many other variables in the calculation which crews may not have knowledge of, such as runway surface texture and depth of water.
So on a limiting or near limiting wet runway “… a few kts …” might suddenly be very important, particularly if the safety margin is exceeded by the combination of unknowns and excess speed.