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opherben

Question About VSpeeds

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Hello,it occured to me that the logic behind aircraft performance as published in its manual, and my understanding of aircraft performance are inconsistent, and am asking for anyone knowledgeable in this area to explain, here is the issue:1. According to the manual, V1 increases as TOGW increases. This contradicts my understanding that the runway distance for accelerate-stop would increase with increasing TOGW, thereby, for a finite runway, V1 should decrease with increasing TOGW.2. Exactly the same with altitude and ambient temperature.I would appreciate any input.

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i think you're just forgeting that runway length is a player too. as weight increases, everything increases- accel-stop, accel-go, brake energy and v1. making up numbers, i might be able to get a min weight 737 to v1 in 3000' and stop on a 5000' runway (2000' accel-stop distance), but a max weight takeoff might take 4000' to get to v1- i would probably not be able to stop before the end of the available 5000' runway. so i find a longer runway or get rid of some weight. in a way, it's not the runway length that determines v1, it's v1 that determines the required runway length.am i close?

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i get dizzy listening to the performance guru's too! i think the key might be that the speed data is based on the minimum runway you need for a given weight. on a std day, my a300 data shows v1 would be below vmcg for weights below 230,000. but the speed on that chart is only determined by wt vs temp/altitude- it doesn't know or care about the runway. so i don't start with speed, i start with a different chart that plots weight/temp vs runway. this chart may say that i can take off at a max wt of 330,000 lbs (long runway). but i only weigh 210,000 lbs. i look at the speed chart now for 210,000 and see that i have to use 121 kts for v1 because the "real" v1 is below vmcg. i think the speed chart is assuming i want to take off from a 3,000' strip, so the v1 is very low. i bet if i could find runway data for that 3,000' runway, it would say i can't weigh more than 200,000 lbs- the engineers want to give me more performance (by requiring me to be lighter) to make up for the fact that i'm using a higher than actual v1 (121 vs 118?).i think if we looked at the run-around charts from the afm performance section, where wt, speed, thrust (different a300 chart, btw), temp, etc., were on one chart it would be easier to see that all these values are interdependent. putting them in tabular form is easier for pilots to use but it muddies the relationships.anyway, if none of this makes sense then i'm certainly qualified to be a performance engineer ;)

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I gents,If I may add something to this interesting subject, it is my understanding that the first issue is to know if you are operating at high gross weight or not. If you are not operating at hight gross weight then a good solution is to play around between balanced field and minimum V1 and not to look at high take-off weight and the subsequent higher V1 and possible unbalanced fields. From what I know the balanced field technique gives the best optimisation for accelerate/stop, continued OEI take-off performance in view of getting the highest possible take-off weight. However balanced field technique has its limitation too.If Clearway and/or Stopway are available, and you are chasing maximum possible takeoff weight, then go ahead and use them. The problem is that any number of AFM documents, and the FMC, only look at the Balanced Field, or (even worse) advise that the AFM data is for an unbalanced field with the limiting weight being the lesser of the Accelerate-Stop or continued Takeoff cases.If, indeed, as you say, your 'typical' Takeoff weight is well below the maximum, then you should have an excess of performance available for both the Stop and Go cases. Even though you are below the limits, it serves well to maximise the safety margins, and in this respect taking precaution against the statistically most likely mishap would be well advised. Accelerate-Stop accident / incident numbere far out-weigh accidents / incidents for the continued Takeoff case and utilisation of the speed schedules offering the lowest possible V1 will statistically favour the worst case. Of course, such a speed schedule reduces margins for the continued Takeoff case, but statistically you are far less likely to have a problem here.It should be no mystery why the Accelerate-Stop manoeuvre is more fraught with danger than the continued OEI Takeoff if you analyse the in-built margins during certification. For the continued Takeoff, the margins are very generous (up to 50% excess) versus very slim margins for the Accelerate-Stop.It looks to me that I had a similar discussion on another forum in a previous life but I may be wrong.Michael

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Hello,you wrote,"it's not the runway length that determines v1, it's v1 that determines the required runway length."This is not the way commercial flight operations work. The situation is rather, that you fly a route, with permissible, optimally maximum permissible payload onboard, plus required fuel. Then sometime before taxi you get your takeoff runway with fit own fixed length, then you can compute V speeds. This order is the way to also enter data in the FMC. You first select a runway then get computed V speeds, and never vice versa.Next, since the departure runway has a given fixed length, in which you must prepare to accelerate to V1 then stop when circumstances warrant, how will increasing TOGW enable the aircraft to do so using higher refusal speed? I am assuming that V speeds are based on runway length, otherwise why do they erase when you change the departure runway?

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Sam,if I understand correctly, you are saying that an MD11 pilot may decide to stop the takeoff at V1 and plunge right into Boston Harbor, because it's V1 is higher than that for maximum available accelerate-stop distance.>Piqued my interest too . . . .and it

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The thing is gradually clearing up for me. FAR 25 definition basically stipulates that the V1 accelerate-stop distance be equal to the accelerate-go distance, which is 115% of both the total takeoff roll distance plus the air distance to clear a 35 foot high screen. This means that you do ditch in Boston Harbor.

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You wouldn't ditch in the harbor, because if your accelerate-go or accelerate-stop distances are longer than the runway, you have to pick a "better" runway or lower your weight.Weight, wind, and temperature determine two things:Balanced-Field (accelerate-stop and accelerate-go) distances which determine VspeedsRunway length requirements.They are all interrelated so it's not possible to say that V1 is determined by weight alone.It is possible to manually calculate this stuff. If I remember correctly:Calculate distances for standard day based on weight.Correct for non-standard temperature and pressure.Select a runway that is long enoughCorrect for wind and arrive at your final Vspeeds.The automated programs run the sequence for all runways.

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well, i guess we all have different methods for performance calculations. i've used your method in the past.the method i describe is used by a package carrier. you calculate the heaviest allowable take off wt based the most restrictive of the expected departure runway max wt data (wt vs temp for that rwy), climb gradient, landing climb limit, structural takeoff and landing weights and expected landing runway wt. you can do most of this in the cafeteria.then you find out after loading what the actual aircraft weight is. as long as actual is at or less than the most restrictive you calculated above you're ok. you then find the thrust and then find the speeds for actual wt. if the actual weight is less than the rwy limit wt and the climb gradient limit wt, then you can consider reduced thrust (assumed temp), to make use of the "excess performance" you have. you still use the speeds for the actual weight of the aircraft, not the wt used to calc the assumed temp. i'd guess you're pretty close to a balanced fld length using max assumed temp.the epr, takeoff limit wts, and speeds come from 3 separate tables. btw, the rwy takeoff limit weight data used above also accounts for obstacle clearance for that rwy, which can skew the numbers. i don't know how obstacles are accounted for elsewhere.

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Hopefully this will make some sense.Maximum Takeoff Weight is the most limiting of:-field length-climb capability -obstacle clearance -tire speed -brake energyField length required is determined by three other requirements. -Safely stop with the event(engine failure) below takeoff decision speed V1-Safely continue takeoff with the event(engine failure) above takeoff decision speed V1-Extra margin of runway for normal all engine takeoffThis is the FAR Balanced Field. Refer to the diagram (attachment) as you read the text below.The engine out takeoff distance decreases as V1 increases. This is because the airplane accelerates to a higher speed on full thrust before losing one engine.The accelerate/stop distance increases as V1 increases. This is due to the higher speed from which the airplane must be stopped and the extra runway used during the all engine acceleration.The actual field length must be at least as long as the minimum field length which is the point where the two distances are equal.Because the takeoff distance and the stopping distance are equal, this field length is called the Balanced Field Length.If the actual runway available is equal to the balanced field length, the only allowable takeoff decision speed, V1, is referred to as the Balanced V1.Good luck.Floyd http://forums.avsim.net/user_files/140468.jpg

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>Hopefully this will make some sense.>>Maximum Takeoff Weight is the most limiting of:>-field length>-climb capability >-obstacle clearance >-tire speed >-brake energyAdd one more item to this list:-maximum landing weight plus the planned enroute fuel burn.C McCarthy

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That sounds similar to the Boeing/MD11 performance book I've been playing with.First, the MD11 chart converts actual runway length into a corrected RW length by factoring in slope, wind, condition and engine bleed.Then, temperature and field elevation is applied to this corrected field length to get a Field Length Limited Weight (FLLW). This must be the max accelerate and stop weight for that runway? What do you think? Then, a 10 degree flap setting is attached to this weight. This now provides not just a weight at which the airplane can be stopped in the given field length (FLLW), but also describe this FLLW as a weight the airplane can actually lift-off the runway at a given flap setting. In this case, 10 degrees. I think the FLLW must be the "accelerate and stop" weight factor we been talking about, but then applying a flap setting also gives a maximum lift-off weight --- at that 10 degree flap setting. Here's an important point. My 'instructor' hammered the point that the airplane will lift-off more weight with a greater flap setting, but climbs better with a lesser flap setting. Both lift-off and second segment climb must happen with a single flap setting, so a compromise must be reached. Thus the need for step #3. Next, step #3. The chart is entered at a second point with temperature, field elevation and bleed to arrive at a Second Segment Weight Limit (SSWL) at 25 degrees flaps. This is the maximum weight that can be lifted with 25 degrees of flaps and still achieve a 2.7% climb gradient with one engine out and 25 degrees of flaps. This was described as the standard engine-out climb requirement. (Keep in mind that once you are beyond V1, this entire V-speed drill is based on having one engine shut down. Also, "Second Segment Climb" is 'gear-up' to the 'Transition Event,' normally acceleration altitude. ) Notice we now have two weight/flap setups. One for liftoff and one for Second Segment climb. Here comes the compromise.Finally, a weight/flap setting compromise must be calculated between the max weight a 10 degree flap setting can lift for the Field Length weight Limit (FLLW) and the max weight a 25 degree flap setting can lift for the second segment climb (SSWL). In "The way of the Chart", these lines curve and cross and finally 2 pieces of information are output. It's the compromise. The chart provides the maximum weight that will not exceed the Field Length weight Limit (accelerate and stop), AND allow a 2.7 degree/engine out climb gradient for second segment climb (accelerate and go). This provides a final, compromised Maximum Take-off Weight and the flap setting that will get you off the ground (at this weight) and let you climb with an engine out. This is the MD11's famous dial-a-flap setting. Woah. Is anyone still conscious?! Oh, but we're just warming up. Now, compare the TOW you want to fly with to the chart

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Thanks Tim,BTW wind correction is used only for tailwind, headwind advantage is not taken into V speed calculations.

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Thank you all for your detailed contributions.Now, why do I get the same V speeds for short and very long runways, everything else held constant?

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Sam,Great description! only one question at this stage: are you not in an unbalanced field situation the way you proceed?It is my understanding but correct me if I am wrong, that:If the take-off mass is the maximum that the runway and weather conditions allow, then there is only one value of V1. If the mass is less, then a range of V1 speeds is possible, and for each type of aircraft there is a way of calculating that range of speeds.A "balanced field" is a runway which had the "stop" distance equal to the "go" distance. The stop distance is the runway length plus any paved stopway. the go distance is the runway length plus any clearway.So, you don't unbalance the runway by changing the speeds or am I wrong?Michael

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>Thank you all for your detailed contributions.>Now, why do I get the same V speeds for short and very long>runways, everything else held constant?Possibly because the PMDG FMC doesn't take the selected runway length into account (I'm pretty sure it doesn't in the 737). For instance, you could select a flaps 1 takeoff at MDW in a fully loaded -800. You are NOT gonna get off the runway unless you've got a massive headwind (which as pointed out earlier shouldn't be taken into account).I believe the FMC in the PMDG only calculates the balanced-field vspeeds, and it's up to you to determine if the runway length is adequate.A program like Tomaflex or the virtual dispatcher one of the members was working on would take all the factors into account.Best wishes,

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A Balanced field is a minimum field length. You can not have one inch of runway less.But...Yes, you can unbalance V1 with excess runway available. If the actual runway available is longer than the balanced field length then there is a range of V1 available. Selecting a V1 other than the balanced V1 is called unbalancing.Unbalancing may be used to increase a Vmcg limited takeoff weight or a brake energy takeoff weight. Decreasing V1 can be used to reduce V1 below the maximum brake energy speed.there is a upper a lower bound on V1 speed. The upper bound on V1 is the rotation speed Vr, because no takeoff may be rejected after rotation. The lower bound on the V1 speed is V1mcg which corresponds to an engine failure at Vmcg(minimum control speed ground).Floyd

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Your method is the one used when pre-calculated data for the airport and runways is available.I believe my method is used when that data is not available.Your method is much quicker, and answers the question "how much stuff can I put in this airplane and still use the active runway"?My method answers the question "which runway (if any) will work for this already-loaded airplane?"Nowadays with electronic flight bags/performance computers one can work the problem either way depending on what the known values are.I remember watching a video once of some cargo company, and they were explaining to the client that they had to have their load aboard the aircraft no later than 8 AM or they couldn't lift it as they'd be temp limited after 10 AM.

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yeah, i've heard the temp problem is always a big issue in asia.

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