B737 Ops Airpseed Exceedance Support Calls on Final

[A37575]

Operators may specify their own exceedance calls with regard to planned airspeed. Examples vary from +10 knots to -5 knots from planned manoevre speed to +15 knots minus zero (ie no tolerance if below selected speed)

Boeing define a stabilised approach to include a maximum of +20 knots above Vref as an upper exceedance limit presumably before a call is necessary. This does seem rather liberal. Original Boeing philosophy dictated exceedance calls on final approach as based upon Vref. More common now, calls are made based on selected approach speed including wind and gust additives whre pertinent. For example, if Vref +15 was the selected approach speed, I wonder what exceedance over or under that bugged speed should generate a warning support call? Clearly in this case an exceedance of only six knots would be sufficient for a warning support call that Vref +20 had been exceeded. Would then that warning call by the PM be "Bug plus six?" After all Bug+6 sounds pretty reasonable and well within most exceedance calls boundaries. Except the danger lies in the exceedance above Vref and not necessarily above the planned approach speed.

A clarification is needed as I see it, and I wonder if exceedance calls on final by stabilisation minimum height would be better based on Vref rather than selected approach speed. Boeing offer no hints in this area - leaving it to operators to apply their own tolerances. And the problem arises further when the half-the-headwind component additive recommended by Boeing is required to be bled off approaching touch down. Depending how "approaching touch-down" is defined by Boeing (it is not) this could mean a steady reduction of additive commencing at 1000 ft for example. Free stream air flow starts around 2000 ft. Discussion appreciated

[ironbutt57]

The only possible hint from boeing is in the additives column of the "advisory" performance charts in the qrh regarding standard and non standard landing config's..where the additives to landing distance figures based on "kts above vref" are posted...anything above 20 kts sustained is unstable and if below 500ft call for go-around as landing distance figures adjusted for ref+20 cannot be guaranteed

[A37575]

Ironbutt57. Thanks for that pointer. My preference is towards exceedances based upon Vref - if and when a support call is needed. For various reasons the crew may elect to adjust the planned approach speed up or down, but Vref stays the same figure. It makes sense therefore to use Vref as the baseline. There are operators who mandate a support call at 500 ft agl on final. It may include a speed call-out and a rate of descent. For example "Speed Vref+8...Sink Rate 800 fpm." This is meaningful whereas a call of "Bug +2 based upon selected approach speed could be hovering on the danger area of Vref +20 limit where additives are large due wind gradient factors.

Overuns on landing are quite often the result of excessive threshold speed. In turn, excess speed is often due to wind additives (gradient and/or gust factors) that turned out in the end to be not needed. On long runways especially, excess speed over the threshold is accepted as normal because there is little danger of embarrassment. Some may regard this as careless flying and they are probably right.