Vref + X and Vtgt Question. DH8 pilots anyone?

[levashov]

Hey guys,

Im flying Dhc8 and our sop stating we must add a wind correction for Vref, at least 5kts in no Xwind, or half of Hwind value = after calculation we call that "Vtarget speed".
Keeping in mind That Bombardier Acft Flight Manual doesnt have any definition of Vtarget at all (or any Vref + X option, like Boeing does), is it a common in aviation around the world to add that wind correction speed for a reference speed? If does, what is the point of it? If we approach with Vref we still have a great magin to Vs, like 30%. Why would we need to raise that value? Why did Boeing rise it in FCTM?
Do you have this rule in your SOPs?

Appreciate your thouts, guys.

[Intrance]

On my type it's Vref + half of gust, up to Vref+10, no other correction. I am pretty sure this is intended to maintain that 30% margin to stall and to help in keeping the approach stabilized.

Most stabilized criteria (that I have known) include speed not to be less than Vref, adding a bit of margin will help in that. If you would aim to fly Vref in gusty conditions you would quite likely see the speed drop below Vref multiple times and would need to take action by adding thrust or pitching down, both increasing the risk of destabilizing the approach.

I'm not sure why you would add any correction in no crosswind conditions though, perhaps just operator related? Maybe also to reduce the chance at non-stabilized approaches by dropping below Vref?

[Tu.114]

We are generally invited to fly Va+10 for Flaps 15 or Va+5 (due to the increased tailstrike risk) for Flaps 35, both in non-icing conditions. In icing, it is Va-ICE without any increments.

Although an approach at Va is allowed under most circumstances and may be more desirable under some, the increase will add some energy to the aircraft, allowing for a smoother roundout and less risk of tailstrike. With Flaps 35, this is obviously a trade-in for an increased risk of a nose wheel landing.

Increments for wind speed are at pilots discretion. Va + 1/3 wind or gusts (whichever is higher), up to a maximum of 20kts increment and not exceeding Va-ICE is the general rule.

Of course, the LDA requirements need to be minded when increasing speeds. An increment of 10kts will increase required distance by 14%; 20kts will increase the LDR by 25% on top of all other factors. So speed increments are always subject to careful selection and very much dependent on the general situation.

[levashov]

Of course, the LDA requirements need to be minded when increasing speeds. An increment of 10kts will increase required distance by 14%; 20kts will increase the LDR by 25% on top of all other factors. So speed increments are always subject to careful selection and very much dependent on the general situation.

Thank you for your answer. What turboprop do you fly?
What confusing me most, is that if we increase Vref for more than 10kts, but less then 20, how is it agree with "stabilised approach" requirenments, when pilots should avoid any speed deviations of more than 10kts from Vapp? (Because Vref is the speed at 50ft before touchdown, thus we have to wash out that additional wind correction adjactives, and if so, losing those Vapp to Vref would bust stabilised approach).

[Tu.114]

I fly the DH8-400 and until the company phased them out used to fly the -300 as well.

With regards to approach stabilisation, here´s a summary of what our book says: Beside the normal requirements on ground track, glidepath, configuration, checklist status and power setting, speed is indeed a factor. But it is not Va (1,3 Vs) that is categorically required to be flown; our rules call for "the briefed approach speed" Vtgt derived from the rules mentioned in the previous post that needs to be achieved within the tolerances (+/-5 in this case) at the stabilization gate at 1000´AGL. This speed is then flown until flare.

The reasoning behind this is the following. If You fly an approach at a constant Vtgt and in wind conditions requiring a speed increment, which is typically a strong head- or (partial) crosswind, You will at some time enter the friction layer close to the ground where the wind speed will drop - a small windshear, if You will. Some runways show this characteristic more than others; in my operation, the 34 at VIE is often noted for its "hole" at ca. 200-300ft AGL. Carrying a bit of extra energy in there is not the worst of ideas. Also, looking at the ground speed in the approach (which is what determines LDR to a great part unlike TAS), a speed increment of another 10kts on top of the standard Va+10 (for flaps 15°) in a headwind of 10kts will leave You with exactly the same ground speed You would have in calm wind without the additional increment. On the other hand, in a strong tailwind (we have a supplement allowing operation in a tailwind of up to 20kts subject to several restrictions at above 10kts tailwind), a speed increment will be rather detrimental, as the "windshear" close to the ground will be performance increasing. In this situation, plain old Va without increments is often not the worst choice.

The DH8 offers quite a lot of options on approach that all have their influence on LDA and also aircraft handling. Flap setting, speed increments (procentual effects on LDR as mentioned before), Prop RPM (850rpm requires 6% more landing distance compared to 1020rpm), and not forgetting the deicing equipment that requires between 15 and 25 kts increment on the standard Va if selected on (of course, when this is selected, no further increments apply). It is most definitely not an easy aircraft to operate compared to a jet.