framer, et el “… to confirm that the 'Target Threshold Speed' is indeed Vref+5 on a nice day, and not Vref.” (#37)
In my experience (several operators and manufacturers), ‘Target Threshold Speed’ (TTS) can be used in different ways. Generally, TTS is Vref+5, but this could be a corruption / extension of the ‘calm conditions approach speed’, another ‘target’ speed. Note that the UK AIC (#34) avoids confusion (contention), by using 'basic reference approach speed' (Vref).
The safety view (certification - ignoring add-ons), requires the aircraft to cross the threshold at Vref.
Where manufacturers recommend an approach at Vref+5, then they may have assumed that either the 5kts is bleed off before the threshold, or lost during the landing flare, i.e. a particular technique, not applicable to all aircraft. It might be difficult for a manufacturer to argue otherwise if the aircraft is to retain its certificated landing distances.
If manufacturers do not make these assumptions or have some other justification, then more often than not, the crew’s responsibility for a safe landing is increased by having to consider their ‘calm air’ 5 kt addition in their landing assessment – user beware.
In practical terms for the majority of everyday operations this is not a problem (excess runway length), but occasionally in limiting conditions this small loss of ‘distance’ might be overlooked, which increases the risk of an overrun. (General assumption that +5kts is worth 250ft, but with an extended flare this could be 1250ft; ref AC91-71).
Some regulators/operators accept the status quo without further consideration; the extra 5kts is part of the ‘normal variability in operations’. However, operators should define the non-normal conditions by specifying an acceptable approach and touchdown (i.e. stabilised approach, touchdown position, use of brakes, etc), which IMHO also requires pilots to understand the factors effecting the risks as the conditions change.
BOAC, associating the ‘buffer’ (actually a safety factor in the certificated landing distance) with threshold speed can be misleading. The ‘buffer’ can be associated with any of the ‘normal’ variations in operation, but not all at the same time! A limiting landing cannot tolerate multiple errors, i.e. speed error, height error, touchdown position, spoiler/brake/reverse operation, and runway condition, the latter being one of the great unknowns in aviation.
BelArgUSA “Vref+5 seems to be the norm of the industry, as the target speed for landing. Plus winds additives, as described above, ....”
The UK AIC stresses that any wind addition is added to Vref, not the industry norm (Vref+5).
The short summary is we should follow the advice in the manuals, but use all available information in our landing considerations.