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ESzczesniak

Who can educate me on wet vs dry V-speeds.

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There have been a couple flights recently where I've forgotten to switch the runway condition on Takeoff Page 2 in the FMC from dry to wet until after the V-speeds were calculated. This allowed me to see how they all changed when selected wet and I've noticed that all V speeds (1, R and 2) all decreased when wet is selected.Now, I understand V1 decreasing. It is going to take long to stop in wet conditions and is obvious that you would need to initiate an RTO earlier.However, I always understand VR and in particular V2 to pertain to aerodynamic properties. Principally how heavy you are (i.e. how much lift you need) and how fast you need to be going to produce said lift. I haven't been able to see how the wheel/ground interface plays in to these (particularly V2). So....Can someone explain to me why VR and V2 depend on ground conditions? Specifically to either a wet or a dry runway?Eric Szczesniak

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Vr and V2 shouldn't be affected, at least on other types I know.Would be interested in hearing replys.Regards

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It' all have to do with the braking action on a wet or dry runwayGraz Jack Broens

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How does VR the minimum single engine climb out speed get affected by the wet stopping distance?

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Vr and V2 shouldn't be affected, at least on other types I know.Would be interested in hearing replys.Regards
Exactly. V1 should be the only effected speed when the runway is contaminated.

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It' all have to do with the braking action on a wet or dry runwayGraz Jack Broens
Or put another way :If Vr @ 15000lbs is 150knots, why would a wet runway change -Vr- to a slower speed.

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Or put another way :If Vr @ 15000lbs is 150knots, why would a wet runway change -Vr- to a slower speed.
I guess one way to put it is to keep the same accelerate/decelerate distance. You guys keep saying Vr changes. It does not.V1 (takeoff decision speed/reject or continue speed) is the only speed that changes to allow for the decreased braking action with a contaminated runway.

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Well I'm surprised I never picked on this and I guess it is why I wanted to hear some replys.I just did some calculations using the A330 FCOM and it seems that VR and V2 are affected by heavily contaminated runways, standard wet gave me the same v speeds.Still don't understand how VR is affected and I have been studying this for years.It would be good to get a definitive answer. Regards

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Well I'm surprised I never picked on this and I guess it is why I wanted to hear some replys.I just did some calculations using the A330 FCOM and it seems that VR and V2 are affected by heavily contaminated runways, standard wet gave me the same v speeds.Still don't understand how VR is affected and I have been studying this for years.It would be good to get a definitive answer.Regards
Now you've piqued my interest... I have trouble believing a contaiminated runway will have any effect what so ever on aerodynamic performance. Your instance must be a special case involving something we're not thinking of.V1/Vr are not effected in the 737NG FCOM I have here. They must be taking something into account special to the 330...

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V1 < Vr < V2.Also, Vr > Vmcg. Not sure if it's applicable in this case, but a potential reason.
So maybe Vmcg comes into play here? Interesting.

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Zach, I just checked with my old man, he has flown 737 classic, 767, a330 340 744F and now 777.The reason is due to the screen height being reduced, VR is based on single engine acceleration from v1 to VR, if v1 changes so do the other v speeds.Screen height (obstacle clearance)Is 35ft dry, 15ft wetSo in basic terms you have reached VR at a different point on the runway therefore affecting you climb and obstacle clearance speed v2To be honest I should have known this..Regards

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Zach, I just checked with my old man, he has flown 737 classic, 767, a330 340 744F and now 777.The reason is due to the screen height being reduced, VR is based on single engine acceleration from v1 to VR, if v1 changes so do the other v speeds.Screen height (obstacle clearance)Is 35ft dry, 15ft wetSo in basic terms you have reached VR at a different point on the runway therefore affecting you climb and obstacle clearance speed v2To be honest I should have known this..Regards
That makes sense. Thanks Rob!

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Zach, I just checked with my old man, he has flown 737 classic, 767, a330 340 744F and now 777.The reason is due to the screen height being reduced, VR is based on single engine acceleration from v1 to VR, if v1 changes so do the other v speeds.Screen height (obstacle clearance)Is 35ft dry, 15ft wetSo in basic terms you have reached VR at a different point on the runway therefore affecting you climb and obstacle clearance speed v2To be honest I should have known this..Regards
Ahha, learn something everyday, thanks!

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That 15ft wet is more of a JAA thing. We have that issue with the G550 since it's data was based on JAA certification that also includes reverse thrust in takeoff computations. So in my ops I have to verify that my takeoff ground run will ensure that I am atleast 35ft(FAA) by departure end on a wet/contaminated runway.

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Zach, I just checked with my old man, he has flown 737 classic, 767, a330 340 744F and now 777.The reason is due to the screen height being reduced, VR is based on single engine acceleration from v1 to VR, if v1 changes so do the other v speeds.Screen height (obstacle clearance)Is 35ft dry, 15ft wetSo in basic terms you have reached VR at a different point on the runway therefore affecting you climb and obstacle clearance speed v2To be honest I should have known this..Regards
WOW! Your dad flew all of those planes? What a carerr, kodus too him, I love hearing stories like that.

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Well, good to know about VR. That does make some sense. I'm still not sure I truly understand why V2 changes, but I can half &amp;@(&#036;* an attempted given the same explanation for VR.Eric Szczesniak

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FAA wise the rotate speed doesn't change with an RSC, atleast the types I've flown. On the DC-10, VR and V2 would change due to CG position. This is due to the aircraft being flight tested at 8% CG which is the max forward CG. As the CG moved aft, there were corrections at the base of the rotate and v2 charts. Basically what ever changes made to VR also applies to V2. VR ensures minimum control and is 105% of unstick speed. Reducing this number in theory puts you closer to minimum air control speed, which reduces your engine out safety margin for a given weight. RSC in FAA certed data increases your accelerate go distance. This distance puts you at 35ft. So if your accel go equals runway available, you will be at 35ft by departure end meeting the FAA 35ft restriction. If it exceeds runway available, you will reduce your weight which would equate to a lower VR in a round about way. FAA regs limit you to a 1/2 inch of wet snow, slush and standing water or 4 inches of dry snow.

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FAA wise the rotate speed doesn't change with an RSC, atleast the types I've flown. On the DC-10, VR and V2 would change due to CG position. This is due to the aircraft being flight tested at 8% CG which is the max forward CG. As the CG moved aft, there were corrections at the base of the rotate and v2 charts. Basically what ever changes made to VR also applies to V2. VR ensures minimum control and is 105% of unstick speed. Reducing this number in theory puts you closer to minimum air control speed, which reduces your engine out safety margin for a given weight. RSC in FAA certed data increases your accelerate go distance. This distance puts you at 35ft. So if your accel go equals runway available, you will be at 35ft by departure end meeting the FAA 35ft restriction. If it exceeds runway available, you will reduce your weight which would equate to a lower VR in a round about way. FAA regs limit you to a 1/2 inch of wet snow, slush and standing water or 4 inches of dry snow.
FAA regs currently don't address contaminated runway operations. There is some limited guidance available, and fortunately most airplane manufacturers provide advisory data for contaminated runway operations, but it isn't addressed yet in the regulations.

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FAA regs currently don't address contaminated runway operations. There is some limited guidance available, and fortunately most airplane manufacturers provide advisory data for contaminated runway operations, but it isn't addressed yet in the regulations.
what I'm getting at is that there is FAA accepted data that allows me to depart a contaminated runway safely. I state "FAA", to specify my governing body. Every jet type that I have flown has had a performance manual that included corrections in the charted data for RCR or RSC. That FAA limit I posted is actually a DC10 limit. That limit also applied to a military heavy that I flew. The charted data that I've used either corrected the critical field length, accel go distance or runway limiting weight charts. The difficult part is getting the airport to properly report a contaminated runway. The FAA gives some guidance on measuring and reporting but it's still vague at best. I'm up to 16 years of flying and I can only remember seeing a reported RSC twice and that was at a military base. If not reported and I'm suspicions, I either go with an RSC that is worse than current conditions/call my ops if i become weight limited. I attached some pics from a manual of one of the jets i currently fly.rsc1.jpgrsc2g.jpgrsc3.jpg

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FAA regs currently don't address contaminated runway operations.  There is some limited guidance available, and fortunately most airplane manufacturers provide advisory data for contaminated runway operations, but it isn't addressed yet in the regulations.
To my knowledge, they don't address it directly, which is true, but you know if an official shows up to talk to you about something happening, they'll cite the following:
Sec. 91.103Each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight. This information must include--(a) For a flight under IFR or a flight not in the vicinity of an airport, weather reports and forecasts, fuel requirements, alternatives available if the planned flight cannot be completed, and any known traffic delays of which the pilot in command has been advised by ATC;(b) For any flight, runway lengths at airports of intended use, and the following takeoff and landing distance information:(1) For civil aircraft for which an approved Airplane or Rotorcraft Flight Manual containing takeoff and landing distance data is required, the takeoff and landing distance data contained therein; and(2) For civil aircraft other than those specified in paragraph (b)(1) of this section, other reliable information appropriate to the aircraft, relating to aircraft performance under expected values of airport elevation and runway slope, aircraft gross weight, and wind and temperature.

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To my knowledge, they don't address it directly, which is true, but you know if an official shows up to talk to you about something happening, they'll cite the following:
And you know the FAA's motto "we're not happy until you're not happy"

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From 'Everything Explained for the Professional Pilot' by Richie Lengel, in the 'Formulas & Rules of Thumb' section:"Takeoff & Humidity; High Humidity conditions - add 10% to the computed takeoff distance and anticipate a reduced climb rate."

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On the Lear 45 that I fly, I also noticed that the the wet & contaminated runway VR and V2 differ from dry runway VR and V2 (see attached screen shots below).I’m going to take a WAG at the answer. On a wet or contaminated runway, V1 is established as close as possible to V1VMCG to minimize accelerate-stop distance. If the engine fails a VEF (critical engine failure speed), the pilot recognizes the engine failure at V1 speed but elects to continue the takeoff, then the aircraft must continue accelerating on the available engines (one in the case of the B737NG) to VR speed.VR speed is established meeting these requirements:

  1. Can’t be less than V1
  2. Must be at least 105% of VMCA
  3. Must be at least 105% of VMU (OEI) or 110% VMU (all-engines-operating)
  4. Must allow the attainment of V2 before reaching a height of 35 ft above the runway

It is the last requirement, attaining V2 prior to 35 ft, that may be(?) driving VR higher on wet and contaminated runways verses the dry runway.With the lower V1 speed, there is a significant gap between V1 and VR. Therefore, there is less residual acceleration remaining between V1 and VR speed. With less residual acceleration, the aircraft must accelerate a higher VR speed using one less engine to achieve required V2 speed at 35 ft. Even though, wet OEI takeoff distance (wet OEI accelerate go distance) ends when the aircraft reaches 15 ft., the scheduled V2 speed is still measured as the speed achieved at 35 ft. above the runway. It may be more efficient (i.e. shorter distance required) for the aircraft to achieve the required V2 speed at 35 ft if the aircraft is held on the ground and rotation initiated at a higher speed.As for the differences between V2 speed on a dry, wet, or contaminated runway, that could be the result of nothing more than onerous than the engineering that was used to create the data. For example, contaminated runway takeoff & landing data is not developed from actual flight testing, but rather, is computed using the dry distance and applying industry recognize engineering factors. That’s my guess at what's driving the higher speeds.Rich Boll

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Nice hit Rich. I did notice that at your lightest weight, that the V1 and rotate were the same on all three charts. Probably because you will hydroplane sooner at the lighter weight having a less effect on accel. Very interesting. Your wag brings up a thought. In the dc10, the VCEF was all ways V1 in normal situations. VCEF also ensured critical field length. At times we would do training flights at weights of 350,000 to 390,000 pounds. In this situation at very light weights and a wet or contaminated runway, VMCG would exceed charted VR. In this case, the rotate speed becomes VMCG and V1 and VR becomes the same. I would also have to verify my max allowable weight for runway available and conditions since I'm committing above VCEF. So i can see a situation on other aircraft that the VR may be raised if VMCG exceeds it on a contaminated runway.My company operates a few 737-700s and I took a look at the performance manual. The Vr for contaminated runways and dry runways are the same. It has tabbed data for contaminated runways. You enter the chart with dry/obstacle limited weight and intersect PA and slush/water depth. This gives you an amount to reduce from the original limiting weight. You also have to adjust your field length required for every 5c above or below the temp used to create the data. You use the adjusted length to acquire a VMCG limited weight. When done, you are good as long as you are below the contaminated and VMCG weight. If all is well, there is a correction grid for adjustment to the dry V1 based on the depth.Very nice thread, I haven't thought deeply of performance data since I was instructing and doing check flights on heavies.

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