737-800 max Vref additives reduced to 15 knots?
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737-800 max Vref additives reduced to 15 knots?
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?
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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.
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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.
Last edited by tttoon; 11th May 2018 at 06:09. Reason: Thanks NGsim, slip of the keyboard
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.
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.
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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!!
As tttoon says: why place your Vfly close to flap placard speeds and risk a flap load relief?
Moving ahead with the times!!
inexperienced pilots often make the additive, then also proceed to fly the approach well above the (already increased) bug speed;
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.
