737-800 max Vref additives reduced to 15 knots?

[sheppey]

I understand that Boeing has recently amended the maximum additives that can be applied to VREF for landing from 20 knots down to 15 knots for the 737-800. What is the reason for this change and does it apply to all models of the 737 including the Classics?

[sleeve of wizard]

In some cases Vref +20 may have been close to the flap limit speed, after sim and aerodynamic testing it was found Vref + 15 gave almost identical performance but with an increased margin to the flap limit speed.

[RAT 5]

It was the case that Vref included an extra 5kts for tail strike protection in the flare. Thus +15kts is in fact +20 on the required Vref.

[ImbracableCrunk]

Originally Posted by RAT 5

It was the case that Vref included an extra 5kts for tail strike protection in the flare. Thus +15kts is in fact +20 on the required Vref.

The max additive has changed. How does your comment apply?

[tttoon]

It may be applicable to all Boeing types. The reduction is applicable to at least the 737, 767 and the 787 (if you would fly it with manual thrust). The reason I heard was that you have more than adequate manoeuvre margin with Vref +15, and that all those 5 kts above that are doing is getting you closer to the stall max flap speed and increasing the risk for long landings and the associated problems.

[NGsim]

5 knots faster is taking me closer to the stall?? Jeez I need to get back in the books.....

[safetypee]

Does this amendment apply to all Boeing aircraft?

I wonder if it Is associated with the recent introduction of Operational Landing Distances, re TALPA and the worldwide harmonised view of landing performance. As per #5, +15kts should be sufficient, if not then question the choice to make the approach.

An associated aspect is that the general definition of a stabilised approach is based on the ‘Boeing’ +20 kts, which with some interpretations implies allowed at the threshold. Other views prefer +15 limit at the threshold as this better represents an extreme limit of the safety margins in landing performance, but not always.

[DogSpew]

The Vref+20 has been around since the introduction of the B707 well over 50+ years ago. Back then engines had slow spool-up rates, flight deck instrumentation was 'steam driven' analogue dial gauges and the pitot/static system was rudimentary to say the least. Today we have engines that have good spool-up rates and loads of excess thrust, instrumentation is digital coupled with a much improved pitot/static system, means that there is no need to be carrying an extra 5 knots (Vref+15 is ample).

As tttoon says: why place your Vfly close to flap placard speeds and risk a flap load relief?

Moving ahead with the times!!

[Judd]

inexperienced pilots often make the additive, then also proceed to fly the approach well above the (already increased) bug speed;

Some operators mandate a call-out by the PM if the airspeed is more than ten or minus 5 knots from the selected approach speed. Thus, if wind additives are included to the VREF you can feasibly get the situation where the additives are already up to the maximum of 20 knots (Boeing).
If, for whatever reason, the airspeed increases (say) by another three knots, the PM (depending on the operators call policy) would call "Bug plus three; meaning the aircraft is now three knots above the selected approach speed (sometimes called Bug speed). Three knots doesn't sound much but it sure should raise alarm bells in a pilot's mind since the real speed is now VREF+23 knots. In all cases like this, I would have thought it would be more appropriate for PM calls to be reference VREF and not selected approach speeds. A call of VREF +23 becomes a bit scary whereas Bug+3 would hardly raise an eyebrow unless you are aware of its significance.