Vref?

[Guest]

Hi,Can someone please enlighten me on the concept of Vref(reference speed)? Why, for example, are speeds for approach flap settings given in the form, Vref+number? Is Vref only significant for takeoff and landing? How is Vref calculated for a particular aircraft?Thanks,Frank

[CactusDx 0]

Hi Frank,I'll start in reverse order since that may help clear up some things along the way. Aircraft performance standards are outlined in FAR part 25: Airworthiness Standards: Transport Category. It's some reeeally dry reading with a lot of math involved. :7How is Vref calculated for a particular aircraft? Basically Vref , or landing reference speed, is calculated based on 1.3 times the stalling speed of the aircraft in the landing configuration. This is/will be a variable number based on the weight of the aircraft and the desired landing configuration. Some examples of B737-300 numbers would be:Landing weight 100,000 Vref:Flaps 15 - 137Flaps 30 - 127Flaps 40 - 123 Landing weight 95,000Vref:Flaps 15 - 133Flaps 30 - 124Flaps 40 - 120Is Vref only significant for takeoff and landing? Vref is significant for landing. For takeoff your performance figures are in terms of V1, VR, and V2.V1 - Takeoff Decision Speed. The calculations for this speed can get pretty complex since it has to be derived in direct association to Vef, which is the speed at which the critical engine is assumed to fail. The V1 speed will also be directly affected by runway length for accelerate-stop distance calculations, and flap settings. The bottom line is that V1 is your last chance to decide whether you will reject a takeoff or fly. Any problems after V1 you're committed to fly, you don't have enough runway remaining to safely reject a takeoff.VR - Rotation Speed. Vr may not be less than V1, or 105 percent of Vmc, which is the minimum controllable airspeed with one engine inoperative. This is the speed where you can start to bring the nose up. V2 - Takeoff Safety Speed. This one gets pretty sticky as well. At this airspeed you have sufficient lift available to provide the specified climb gradient of 1.2% from the runway surface to 400 feet on a single engine if necessary. Why, for example, are speeds for approach flap settings given in the form, Vref+number? I've seen that type of information in the aircraft flight manuals. That, to me anyway, seems to be more of a blanket performance schedule issued by the airplane maker. Referring back to the Vref info above, it's a variable number. So Vref + 20 at 100,000 pounds will be a different airspeed than Vref + 20 at 95,000 lbs.Airlines will have speedbooks in the aircraft that will give you hard and fast numbers to abide by for each weight and flap configuration so there won't be any guessing going on. The dispatch release will also provide performance numbers to the flight crews.I hope this helped a bit! Again, FAR Part 25 has all the dry details you'd ever want.Mike CollierKPHX[table border=1 bgcolor=#eeeeee][tr][td rowspan=2] http://avsim.com/flightdeck/images/Radar_small.gif[/td][td] America West Airlines and proud to be a Beta Tester of[link:www.jdtllc.com]Radar Contact]The premiere ATC adventure add-on for FS

[Av-Fan 0]

Frank,The Vref+number is used for various reasons. One example is the Vref+number is used to define manuevering speeds at different gross weights and flap settings. For example, the DC-10-10 manual I've uploaded to Avsim has a Manuevering, Approach and Threshold Speed chart that is based on the Vref Chart immediately above it. (Please note that the Vref Chart shows speeds based on Flaps 50.)Using this chart we can see that the min manuevering speed with flaps 15 and slats extended with a gross weight of 360,000 lbs would be 174 kts (139 + 35). The same charts also show that if Flaps 35 is selected for landing with a gross weight of 280,000 lbs, Vref would be 132 (122 + 10) on approach with a threshold crossing speed of 127 (122 + 5) kts.Another example of the the Vref+number is for additions due to company or manufacturer procedures, icing and winds. If you look at my MD-82 performance manual you will see that the Minimum Approach Speed is equal to Vref + 5 with a maximum of Vref+20. This same chart shows that pilots are to add 1/2 the steady wind + the full gust value.All additions above Vref are designed to provide an adequate safety margin based on aircraft weight, flap settings and weather conditions.Hope this helps. :)BTW, you can find my manuals by using the Avsim File Library's Extended Search function. Just enter my last name in the Author field and you get a list of about 18 manuals.Matt Zagoren

[Donmo 6]

V ref or V speed is just an abbreviation for Velocity. Every action that needs to be initiated or will happen has an airspeed set as to when it should/will happen. These are always listed in the P.O.H. of an aircraft. Examples areVy = best rate of climbVx = best angle of climbVfe = maximum flap extention speed Va = maneuvering speedVno = normal structural cruise speedVne = never exceedAnd many, many more!Don

[barryward12345 0]

But how does this relate to our usage of Fsim? We all have many aircraft in our hangers - how do we calculate the various V speeds for each aircraft for each different flying weight and configuration . How do i even know what the weight is of the aircraft that I am intending to fly?Thanks Barry

[Donmo 6]

It may have been provided by the designer. If not, you can look in the config and see what is posted in it. Warning on that one. MANY of the items in the configs are wrong. You can do a search on the net too for airplane performance and such.. Get the real speeds. Not all a/s indicators are marked correctly for the aircraft they are installed in either. Don

[Guest]

Hello Mike and Matt,Thanks so much for your wonderfully informative replies!I am trying to get back into simming after an absence of over a year, and your posts both reminded me of things I had forgotten, and provided many fresh details. I'm looking forward to getting a new computer soon that will do justice to FS2K2, and hope the learning curve to flying(with all the amazing new addons I've been reading about in these forums) will not be too steep this time.Thanks again for your help,FrankPS, Mike, I do have the FAR/AIM handbook for 2000, so will check out Part 25 when I get home. F.

[Guest]

Hi Don,Thanks very much for the list of V speeds. I find it easier to remember those like Vne where the subscript is an abbreviation(never exceed, in this case), than in cases where it's just some letter of the alphabet(such as Va, in your list).I found Vref slightly obscure because of the way it's used in variable expressions(such as Vref+20), but as you will see, it has been comprehensively explained by Mike's and Matt's posts above.Thanks again,Frank