smartt1982
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Why Vref+5 On 737? WHat Is It On Your Aircraft?

Wed May 16, 2012 10:06 am

Hi,

In the FCTM for the 737 it states that the minimum command speed setting is VREF + 5 knots. Why is this? I have been told that we should bleed these 5 knots off over the threshold because your landing distance is based on touching down at VREF?

Is this the same for all Boeings/other aircraft makes, I have heard other aircraft like the BAE 146 approach @ VREF and downtown at less than VREF
 
KAUSpilot
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RE: Why Vref+5 On 737? WHat Is It On Your Aircraft?

Wed May 16, 2012 10:34 am

It's the same in both the EMB-145 and B744. Why? The airplanes were designed to fly the final approach at that speed. Through the course of a normal flare maneuver the 5 knots will usually be bled off just as the aircraft touches down. Flying for extended periods at Vref puts you too close to stick shaker in my opinion. Of course there are also additives for strong winds.
 
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derekf
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RE: Why Vref+5 On 737? WHat Is It On Your Aircraft?

Wed May 16, 2012 1:23 pm

The BAe146 flies approach at VREF+5 and should touch down at VREF-7.
Whatever.......
 
tdscanuck
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RE: Why Vref+5 On 737? WHat Is It On Your Aircraft?

Wed May 16, 2012 1:37 pm

Quoting smartt1982 (Thread starter):
In the FCTM for the 737 it states that the minimum command speed setting is VREF + 5 knots. Why is this?

Because the autorthrottle certified +/- 5 knots. It's the only way to have the authothrottle and the landing certification standards "get along" (the autothrottle can't put you below Vref).

Quoting smartt1982 (Thread starter):
I have been told that we should bleed these 5 knots off over the threshold because your landing distance is based on touching down at VREF?

You can, you don't have to. The flare will naturally shed speed and the throttles should be at idle at that point.

Quoting smartt1982 (Thread starter):
Is this the same for all Boeings/other aircraft makes

It's the same for all Puget Sound heritage Boeing's, I'm not sure about the others.

Tom.
 
BA777
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RE: Why Vref+5 On 737? WHat Is It On Your Aircraft?

Thu May 17, 2012 9:48 am

We have a correction for every 5 knots over VREF so add that to our landing distance calculations
 
smartt1982
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RE: Why Vref+5 On 737? WHat Is It On Your Aircraft?

Sun May 20, 2012 8:53 am

Quoting tdscanuck (Reply 3):
Because the autorthrottle certified +/- 5 knots. It's the only way to have the authothrottle and the landing certification standards "get along" (the autothrottle can't put you below Vref).

so because the autothrottle is +/- 5 knots you have to always set +5 so the autothrottle will never go below vref?
 
tdscanuck
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RE: Why Vref+5 On 737? WHat Is It On Your Aircraft?

Sun May 20, 2012 9:33 am

Quoting smartt1982 (Reply 5):
so because the autothrottle is +/- 5 knots you have to always set +5 so the autothrottle will never go below vref?

That's basically it, yes. It's more complex in the details (it always is) but that's the meat of it.

Vref is somewhat arbitrary...an alternative method would be to just bump Vref by 5 knots and not do the +5 increment but that would break a ton of commonality requirements.

Tom.
 
smartt1982
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RE: Why Vref+5 On 737? WHat Is It On Your Aircraft?

Sun May 20, 2012 9:42 am

Thanks Tom.

Where could one find out more detail behind it?

Cheers
Steve
 
tdscanuck
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RE: Why Vref+5 On 737? WHat Is It On Your Aircraft?

Mon May 21, 2012 4:02 am

Quoting smartt1982 (Reply 7):
Where could one find out more detail behind it?

A Boeing training pilot would be a good place, if you've got access to one (that's where I got my information). The Boeing Flight Crew Training Manual (737NG) says, for a stabilized approach, "the airplane should be at approach speed. Deviations of +10 knots to – 5 knots are acceptable if the airspeed is trending toward approach speed". Note that, coupled with Vref+5 approach speed, means that following standard procedure should keep you at or above Vref.

The speed portion of the FCTM also says, "When using the autothrottle, position command speed to VREF + 5 knots.
Sufficient wind and gust protection is available with the autothrottle connected because the autothrottle is designed to adjust thrust rapidly when the airspeed drops below command speed while reducing thrust slowly when the airspeed
exceeds command speed." That's basically a tacit admission that the autothrottle will hold you to at least VREF when you set commanded speed to VREF+5.

A different portion says, "When using the autothrottle, position command speed to VREF + 5 knots. Sufficient wind and gust protection is available with the autothrottle engaged that no further wind additives are needed."

Tom.
 
CosmicCruiser
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RE: Why Vref+5 On 737? WHat Is It On Your Aircraft?

Mon May 21, 2012 4:42 am

We do ref+5 regardless of A/T or not. The FMS defaults to it. EO is ref +10. we ad the wind factor regardless of direction, head, tail, x-wind. I was just thinking it was a "safety pad" so you wouldn't go below ref (1.3 stall) especially at heavy ldg wgts.

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