Vref=1.3Vs?
 

The "definition" of Vs is 1.3Vs, but in a light aeroplane (e.g PA28) with Vs (flap) at 44 kts, this only gives a buffer of 13 knots over the stall. It seems usual to use an "absolute" buffer, such as 20 knots over the stall... Comments??

I am not a performance engineer, so contacted one who is also a current pilot. His answer is attached:



(a) I presume that the correspondent, in this case, is talking about book final approach speeds.

Some Background -

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For some aircraft, depending on the certification basis, the stall speed may have been replaced by the minimum steady flight speed - which may be slightly different to the stall speed - no prizes for guessing in which direction.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.


(c) I am not aware of any requirement to have a fixed minimum buffer (eg 20 kts as suggested).

Perhaps the correspondent here is confusing the conventional jet operations 20 kt maximum speed additive (for wind and gusts) to Vref ?

Or, if he is a light aircraft pilot only, perhaps the question somehow arises from the often seen practice of making approaches in light aircraft at "standard" speeds considerably faster than book

The question was from a light aircraft point of view, thanks for the comprehensive answer.

The "20 knots" was an arbitrary figure for illustration of what I meant in this regard, not related to the buffer you mention. The question arises from what appears to be the intent of carrying the 1.3 factor, which is to say, give a margin over the stall (or other minimum speed as is applicable) during an approach.

Hudson's chum knows his business, but from the perspective of another performance Engineer.

It is not permitted in certifying an aircraft for Vref to be less than 1.3Vs, (again in CAS, not necessarily IAS) - but in light aircraft particularly it may be best practice to use something higher.

They never do in airliners because Vref determines landing speed determines runway length determines the number of airports it can use. So, airliner manufacturers do everything they can to bring Vs down and thus keep Vref as low as possible.

Light aircraft rarely have this problem so a good approvals team may well decide to go for a higher Vref. In microlights, where it's far easier to lose speed, Vref of about 1.7Vs is not unusual. The bottom line is that the aircraft must touch down just before the stall, not just after!

G