Some of the relevant FARs if it helps.
§ 25.125 Landing.
(a) The horizontal distance necessary to land and to come to a complete stop (or to a speed of approximately 3 knots for water landings) from a point 50 feet above the landing surface must be determined (for standard temperatures, at each weight, altitude, and wind within the operational limits established by the applicant for the airplane):
(1) In non-icing conditions; and
(2) In icing conditions with the landing ice accretion defined in appendix C if VREF for icing conditions exceeds VREF for non-icing conditions by more than 5 knots CAS at the maximum landing weight.
(b) In determining the distance in paragraph (a) of this section:
(1) The airplane must be in the landing configuration.
(2) A stabilized approach, with a calibrated airspeed of not less than VREF, must be maintained down to the 50-foot height.
(i) In non-icing conditions, VREF may not be less than:
(A) 1.23 VSR0;
(B) VMCL established under §25.149(f); and
(C) A speed that provides the maneuvering capability specified in §25.143(h).
(ii) In icing conditions, VREF may not be less than:
(A) The speed determined in paragraph (b)(2)(i) of this section;
(B) 1.23 VSR0with the landing ice accretion defined in appendix C if that speed exceeds VREF for non-icing conditions by more than 5 knots CAS; and
(C) A speed that provides the maneuvering capability specified in §25.143(h) with the landing ice accretion defined in appendix C.
(3) Changes in configuration, power or thrust, and speed, must be made in accordance with the established procedures for service operation.
(4) The landing must be made without excessive vertical acceleration, tendency to bounce, nose over, ground loop, porpoise, or water loop.
(5) The landings may not require exceptional piloting skill or alertness.
§ 25.473 Landing load conditions and assumptions.
(a) For the landing conditions specified in §25.479 to §25.485 the airplane is assumed to contact the ground—
(1) In the attitudes defined in §25.479 and §25.481;
(2) With a limit descent velocity of 10 fps at the design landing weight (the maximum weight for landing conditions at maximum descent velocity); and
(3) With a limit descent velocity of 6 fps at the design take-off weight (the maximum weight for landing conditions at a reduced descent velocity).
§ 25.479 Level landing conditions.
(a) In the level attitude, the airplane is assumed to contact the ground at forward velocity components, ranging from VL1to 1.25 VL2parallel to the ground under the conditions prescribed in §25.473 with—
(1) VL1equal to VS0(TAS) at the appropriate landing weight and in standard sea level conditions; and
(2) VL2equal to VS0(TAS) at the appropriate landing weight and altitudes in a hot day temperature of 41 degrees F. above standard.
(3) The effects of increased contact speed must be investigated if approval of downwind landings exceeding 10 knots is requested.
§ 25.481 Tail-down landing conditions.
(a) In the tail-down attitude, the airplane is assumed to contact the ground at forward velocity components, ranging from VL1to VL2parallel to the ground under the conditions prescribed in §25.473 with—
(1) V L1equal to V S0(TAS) at the appropriate landing weight and in standard sea level conditions; and
(2) V L2equal to V S0(TAS) at the appropriate landing weight and altitudes in a hot day temperature of 41 degrees F. above standard.
A reposting from
Vref=1.3Vs? [Archive] - PPRuNe Forums and is the answer given by a performance engineer.
Usually the 1.2Vs and 1.3Vso figures will be specified in the flight manual as it is important for the manufacturer to obtain best field length performance data to help sell the aircraft - the slower the aircraft the better for this consideration.
For some aircraft, depending on the certification basis, the stall speed may have been replaced by the minimum steady flight speed - which may be slightly different to the stall speed - no prizes for guessing in which direction. I believe, for example, that this was the case for the B727-200 and was cited by some as being part of the reason for the difficulty that some pilots (i.e. most of us) had in flaring that model to a nice landing unless a few knots extra were carried.
Also, there may be the odd aircraft where the AFM figures have an increased buffer for other reasons - one has to keep in mind that there are several certification requirements which have to be met simultaneously and it sometimes can lead to unhelpful confusion if one specific requirement is looked at in isolation. For example, the Vmca handling requirement on multis might well cause this to occur.
Another thing to keep in mind is that the manufacturers consider their data to be confidential. As a result, it is usually difficult to find out all (much ?) of the story relating to any SPECIFIC aircraft certification program. Occasionally, the manufacturer will sensibly engage in some horse-trading with the certification authority and this sometimes results in apparent anomalies. Also the use of the grandfather certification basis often leads to the situation where a pilot looks up the CURRENT design standard (e.g. FAR 23 at amendment so and so) - which has not a lot to do with the aircraft in question which was designed to an earlier standard - and then has a hard time rationalising apparent discrepancies.
It is usual to find that the manufacturer is quite considered in his development of AFM and other manuals - one always has a mind to the potential for litigation when including more data than the minimum required by the certification authorities. This then, quite naturally, encourages pilots to engage in speculation.
(b) One has to be careful that calculations to work out buffers over stall are done in CAS - strictly this should be EAS but the difference is not a concern for stall speeds. If done in IAS then the PEC (position error correction for static source errors) can make for strange results.
I think that we all have seen the small Cessnas deeply in the stall with the ASI indicating VERY low figures - which means only that the position errors have grown dramatically when the aircraft is well within the stall - and has little to do with the aircraft's actual speed.
One has to keep in mind at ALL times that the ASI is measuring pressures, NOT (and NEVER) speeds. The pressure reading's being scaled to speed is based on quite limited and limiting assumptions - if these assumptions don't apply for some reason, then the ASI reading is total high quality garbage - how many jets have had static obstructions and then been flown into the stall while the pilots have erroneously believed the aircraft to be overspeeding ? The same logic applies to the need to calculate TAS - the basic ASI doesn't give TAS to the pilot. It is the old airmanship consideration - get the whole story (or as much of it as might be reasonably available) before over-reacting to something which looks or feels strange or rather out of the normal experience.
It is worth noting that the flight test determination of PEC near the stall takes a considerable proportion of the certification flight testing program.
For light aicraft.
§ 23.73 Reference landing approach speed.
(a) For normal, utility, and acrobatic category reciprocating engine-powered airplanes of 6,000 pounds or less maximum weight, the reference landing approach speed, VREF, must not be less than the greater of VMC, determined in §23.149(b) with the wing flaps in the most extended takeoff position, and 1.3 VSO.
(b) For normal, utility, and acrobatic category reciprocating engine-powered airplanes of more than 6,000 pounds maximum weight, and turbine engine-powered airplanes in the normal, utility, and acrobatic category, the reference landing approach speed, VREF, must not be less than the greater of VMC, determined in §23.149(c), and 1.3 VSO.
(c) For commuter category airplanes, the reference landing approach speed, VREF, must not be less than the greater of 1.05 VMC, determined in §23.149(c), and 1.3 VSO.