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sosuflyer

777 Vref 30 speeds always the same

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First, I just want to say thank you for an awesome simulation of the 777. It's been my favorite since I was a kid!

 

I have read parts of the FCOM and FCTM, but haven't come across an answer to this yet since I've been working a lot.

 

I was curious about the airplane's Vref speeds at different weights, so during cruise, I played around in the Approach page in the CDU, typing in different weights to see the corresponding speeds.  

In the -LR passenger, at MLW (491000lbs) I get Vref30 137, Vref25 146

When I experimented with extremely light weights, like 350,000lbs, I got Vref30 137 Vref25 137

 

Same thing in the Freighter, only with higher speeds at MLW since that is at 575,000lbs.

 

Seems the Vref30 is the same at 350k as it is at 491k. Just curious if this is normal behavior or not. 

 

I have not had time to fly approaches at both heavy and light weight yet to see how the plane responds, but that will be my first thing to do on my day off!

 

thanks in advance

 

 

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Glad you brought this up, because I too noticed that, but at first did not register it as being a bit odd. Hope you get a heads-up on this.

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Glad you brought this up, because I too noticed that, but at first did not register it as being a bit odd. Hope you get a heads-up on this.

 

yup, I'm just curious about it. If that's how the real thing is, then so be it. It'll just be a little boring flying the same approach speed almost all the time in the -LR pass.  The thing I loved about the MD-11 is you get a VREF35 range of 139-163 which was exciting. 

I guess if I want some speed action on the 777, I should keep flying the freighter at MLW Vref30 of 150 I believe

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I noticed this too. It seemed like the plane should work like that but now I'm not sure as I work through the Boeing manuals slowly and I did not see mention of this yet.

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If you look in the QRH Performance inflight 30.4 (page 560) you will see that at Sea Level the ref speed for F30 is 137 up to 220t and only then does it start to increase and with a MLW of 223T it would be 137kt for all ranges. At low weights we often use F25 to prevent at float during landing as the landing distance is only marginally increased at these low weights. For the 200F the increase in MLW will give you up to 149kt ref speeds.

 

So to sum it up, this is perfectly normal and in line with what we see flying the real 77L and 77F.

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If you look in the QRH Performance inflight 30.4 (page 560) you will see that at Sea Level the ref speed for F30 is 137 up to 220t and only then does it start to increase and with a MLW of 223T it would be 137kt for all ranges. At low weights we often use F25 to prevent at float during landing as the landing distance is only marginally increased at these low weights. For the 200F the increase in MLW will give you up to 149kt ref speeds.

 

So to sum it up, this is perfectly normal and in line with what we see flying the real 77L and 77F.

Aha! So that's where it is in the manuals. Very interesting stuff. I figured to use F25 for those lightweight landings where Vref is the same. Can I ask why the performance is this way? My guess would be that lower speeds must bring some instability due to something in the wing design.  

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It has all to do with controllability. Remember the the 200LR/F was developed from the original 200 with same fuselage and vertical fin. Now when you add engines with 45% more power you will have a potential much bigger thrust asymmetry in case of an engine failure hence to maintain a margin to Vmca Boeing added a Vref and V2 floor of 137kt. So if you at F25 have the same approach speed as on F30 the only real difference is the added drag of F30 which on landing is about 20 meters. If this floor had not been in place the real Vref speed for an empty 200F would be closer to 117kt resulting in a very low pitch angle (flying 20kt too fast really) and using F25 will help here as well as now we're only talking about 12kt too fast.

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So standard flap settings for landing when low weight is 25 degrees, ok :) cause I have been using 30 degrees flap

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If you look in the QRH Performance inflight 30.4 (page 560) you will see that at Sea Level the ref speed for F30 is 137 up to 220t and only then does it start to increase and with a MLW of 223T it would be 137kt for all ranges. At low weights we often use F25 to prevent at float during landing as the landing distance is only marginally increased at these low weights. For the 200F the increase in MLW will give you up to 149kt ref speeds.

 

So to sum it up, this is perfectly normal and in line with what we see flying the real 77L and 77F.

 

Come to think of it, I now remember that several news agencies were reporting that the intended landing speed for the ill-fated Asiana 777 at KSFO was 137kts. Thanks for the explanation!!

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i did a LEPA-EDDF trip today (short trip) with 170 instead of full 234 passengers and only with a 4000kgs total cargo, to notice if really the vref flaps 30 speed changed or not, because as you say was always 137kt. The vref on this approach was 136. I thought there was a issue, because it always gave 137 but is not a issue. If you load a small weight on the aircraft the vref will decrease. so i think this is normal, and the standard landing flap i think is 25 for this aircraft, and with 25 flap vref you will notice more differences on the vref speeds.

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