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  • V1 is or is not a LOCATION on the runway...

    ...where you make a go/no-go decision?

    I think this was thoroughly discussed on AD.com several years ago.

    "Scientifically/technically" V1 means "nothing" with respect to your runway location.

    OTOH, if the engines, aircraft, winds, performs normally, V1 is essentially a LOCATION on the runway where one can decide whether to continue the takeoff or abort.

    Anyway, what I recall was that the conclusion of this discussion was a pretty compelling dataset and argument that V1 is the way to make that call and not some 'mark' on the runway. (In fact, this would be both a location AND a speed milestone)

    (Please note liberal use of quotes where words and comments shoud be taken conceptually and not literally.)
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

  • #2
    V1 is always going to be a position on the runway - but that is not something that is specifically calculated or used...

    I'm sure we've had the discussion of what V1 actually is many times. But regardless, for a given weight, thrust and environmental conditions, you could always calculate a location on the runway where you will reach a certain speed.

    Would it be useful? Probably not. Although given the 'other' discussion around insufficient thrust for takeoff, I can't see why these days, given the computer technology available, you couldn't put a marker at a location along the runway, and have a computer calculate for your given conditions a minimum acceptable speed to pass that point. Interesting concept really.

    Comment


    • #3
      V1 is Velocity, hence the mnemonic and that it is expressed as km/h, kts, etc. It is not a position.

      That said, the position on the runway when you achieve the Vx's (V1, Vr, V2) is important obviously.

      Comment


      • #4
        Originally posted by MCM View Post
        V1 is always going to be a position on the runway - but that is not something that is specifically calculated or used...
        You had to say that...

        ...and use the word always.

        Yeah, Gabriel and I are not ATP's, but we will TELL YOU that you are very wrong. (the little dot is a period)

        You, who are an ATP...This is when these forums get weird.

        If something is amiss with the brakes and they are dragging....just a bit, you will arrive at V1 at a different position on the runway. If some crazy thing is a miss and engine 3 of 4 is only producing 75% thrust but the primary instruments look good...you will arrive at V1 at a different position on the runway.

        If everything is NORMAL, V1 is "a location"...if somethings amiss, it's not.

        Thus, the parlour-talking-ass-hat-private-pilot-MSFS-jocks have a valid question.

        Maybe the question is of minimal practical significance...however, we know that sometimes not everything is normal...so just saying that "it works out" is not good enough...it demands a half-decent Gabriel dissertation.

        As a last hope- I will ask if this is a word mincing/inference thing...When I read your sentence I see, "V1 is always a PARTICULAR position on the runway"....I am aware that you can play word games that the departure- end piano keys are also "A POSITION" where you can arrive at V1....hopefully not, but....

        ...and I do note where you finally say that a position and speed check could have some validity...so maybe no flame here, but a "holy crap say that again for me and more slowly".
        Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

        Comment


        • #5
          Originally posted by 3WE View Post
          "Scientifically/technically" V1 means "nothing" with respect to your runway location. Anyway, what I recall was that the conclusion of this discussion was a pretty compelling dataset and argument that V1 is the way to make that call and not some 'mark' on the runway. (In fact, this would be both a location AND a speed milestone
          Ok, so let's discuss this further using your logic. Your first statement is correct, and considering all of the other thing/bells/whistles and radio communications going on, the "nothing" part becomes important because calculating the point of V1 in relation to runway location is such a minuscule priority - that it rarely, if ever, is done.

          V1 is defined as;
          V1 means the maximum speed in the takeoff at which the pilot must take the first action (e.g., apply brakes, reduce thrust, deploy speed brakes) to stop the airplane within the accelerate-stop distance. V1 also means the minimum speed in the takeoff, following a failure of the critical engine at VEF, at which the pilot can continue the takeoff and achieve the required height above the takeoff surface within the takeoff distance.
          It is a term of Velocity - and so measured in speed, not a distance. You can use the measure to calculate distance, but considering that every take off, in the world, is going to require different imputs to arrive at said speed (weight, runway length, wind conditions, runway selection, weather, company policies, aircraft and engine tolerances), it would be rather maniacal to say "Ok, when we get to point X on the tarmac, you have to make a decision,". Back to speed, it's important because if something goes wrong, or perhaps there is a change as the takeoff is proceeding, the pilot can adjust, and use an actual measure (his speeds) to recalculate and take appropriate/time critical decisions rather than waiting to recalculate runway positions.

          Originally posted by 3WE View Post
          OTOH, if the engines, aircraft, winds, performs normally, V1 is essentially a LOCATION on the runway where one can decide whether to continue the takeoff or abort.
          Again, V1 is a speed, not a location or a distance. Theoretically, V1 happens at a location, but that location is solely determined by speed. The aircraft, theoretically should rotate after V2, and at V3 should retract flaps. Would you calculate those as well, in proximity to the runway, considering that each is dependent on speed?

          So, now I have a question - why would you want to locate the 'V1' location on a runway. If it is because using a visual cue - as in, "Hey, we are going to hit V1, by the time that you pass that taxiway," it would be foolish - because it would divert the pilot from his instruments, and in a issue, they are the only things that might save him.
          Whatever is necessary, is never unwise.

          Comment


          • #6
            So, now I have a question - why would you want to locate the 'V1' location on a runway. If it is because using a visual cue - as in, "Hey, we are going to hit V1, by the time that you pass that taxiway," it would be foolish - because it would divert the pilot from his instruments, and in a issue, they are the only things that might save him.
            See bold statements for the answer to your question.

            As stated by a professional pilot, "Due to wide variation in aircraft weight, a pilot cannot really determine if the acceleration is nominal or not"

            If your acceleration is slower than it should be, then you reach V1 further down the runway than you would otherwise

            ...and if you are somewhat further down the runway than you should be when you approach V1 and decide to abort, what keeps you from going off the end?

            The point here is that there isn't really a system to directly detect if your acceleration is normal or off (or dangerously off)...The V1 abort call (and the ability to stop in time) all depends on the assumption that the aircraft is accelerating normally.

            Maybe that's a ridiculously good and safe assumption...but saying "that's the way it's done" doesn't explain to me WHY it's a ridiculously safe assumption.

            Nor does one of two pilots cross checking speed versus a land mark strike me as a hugely dangerous activity- I do sort of hope they are watching the end of the runway and other items important to external situational awareness- and maybe worrying if the runway lights are turning yellow and they aren't at V1 yet...stopping may not be pretty if you get too far into the yellow lights...
            Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

            Comment


            • #7
              Read what AA 1818 wrote. V1 is ONLY a speed! It has absolutely nothing to do with a mark or place on the runway! It will change on almost every take-off depending on weight, wet or dry runway, barometric pressure and temperature. And by the way AA you rotate at Vr not V2.

              Comment


              • #8
                Argh I knew this was where we were going :P I have a feeling 3WE and myself are having a slightly different discussion to A1818 and Boeingbobby lol!

                If you have dragging brakes, lower thrust etc then your calculated V1 is not valid.

                That is the whole point!

                The speed of V1 in those circumstances is immaterial. In the 'stop case' you no longer have sufficient runway to stop, and in the 'go case' you don't have sufficient runway to safely accelerate to achieve... etc etc etc.

                For a valid V1, you can calculate a position on the runway at which you will achieve that speed. That is what the performance calculation achieves. Note here that I have said A position. It is not the same position every time, and will vary dramatically based on weight, thrust, ambient conditions.

                Not achieving the required acceleration on takeoff is actually a completely separate issue. Instead of looking at it in terms of V1, look at it in terms of "validity of takeoff performance". If you do not achieve the required acceleration, then you have invalid takeoff performance, regardless of if your V1 is purely a speed, or, heaven forbid, a location on the runway.

                So, and perhaps we're coming at this differently, I'll stand here and shout until I'm blue in the face - the position that you achieve V1 (given you meet your required takeoff performance) can be calculated as a position on the runway - however if the performance is invalid, then obviously V1 (or any location) is too.

                Now...

                ...and I do note where you finally say that a position and speed check could have some validity...so maybe no flame here, but a "holy crap say that again for me and more slowly".
                A position and speed check could be introduced to demonstrate, at a relatively low speed, that you are not meeting the expected performance on your takeoff. It is only practical now that we are using electronic calculations for flight data (such as EFB's). Would have been a PITA before that.

                The concept is: every runway has a marker. It could be where the PAPI is, it could be somewhere else. The location doesn't really matter, and can vary between airports.

                For a given takeoff calculation, the rate of acceleration can be calculated. This isn't that hard for an EFB. Obviously it depends on prevailing conditions. The EFB, armed with the knowledge of the location of the marker compared to the point that you commence your takeoff run, could calculate a minimum speed to pass the marker for your takeoff performance to still be valid. Obviously there would be buffers, different procedures etc... but you get the idea.

                Hypothetically, if you passed that marker slow, you could reject the takeoff at a still acceptably low speed, without getting anywhere near V1.

                Comment


                • #9
                  Turn blue all you want! I am willing to bet a years salary you will not find ANY performance figures or charts for runway distance equaling a V1 number. Dragging brakes, low thrust, who cares V1 is V1.

                  "A position and speed check could be introduced"

                  Ever hear the 80 knot call?

                  Comment


                  • #10
                    Originally posted by 3WE View Post
                    If something is amiss with the brakes and they are dragging....just a bit, you will arrive at V1 at a different position on the runway. If some crazy thing is a miss and engine 3 of 4 is only producing 75% thrust but the primary instruments look good...you will arrive at V1 at a different position on the runway.
                    Don't be so creative. Have your actual headwind slower than assumed for the take-off performance calculation and you'll reach V1 later, farther down the runway and at a higher groundspeed. Now stop from a faster groundspeed than expected and with less runway than expected remaining.

                    I think that the potential consequences are clear, but if not, think of an accelerate-stop distance (ASD) about 15% longer than expected for 10kts less of headwind (which can be the consequence of a slower wind or of the wind shifting to a more crosswind direction).

                    Even worse, the accelerometer method will pick a brake dragging a bit, less thrust than expected, or more weight than assumed, but it will not pick the above.

                    Originally posted by AA 1818
                    V1 is defined as;
                    V1 means the maximum speed in the takeoff at which the pilot must take the first action (e.g., apply brakes, reduce thrust, deploy speed brakes) to stop the airplane within the accelerate-stop distance. V1 also means the minimum speed in the takeoff, following a failure of the critical engine at VEF, at which the pilot can continue the takeoff and achieve the required height above the takeoff surface within the takeoff distance.
                    Source? Never mind. It's wrong anyway. Or, better, it's a myth.

                    V1 must be lower than maximum speed in the takeoff at which the pilot must take the first action (e.g., apply brakes, reduce thrust, deploy speed brakes) to stop the airplane within the accelerate-stop distance and must be higher than the minimum speed in the takeoff, following a failure of the critical engine at VEF, at which the pilot can continue the takeoff and achieve the required height above the takeoff surface within the takeoff distance.
                    Now it's better

                    First let's define (roughly, the FAR are a bit more complicated):

                    ASD (accelerate-stop distance): The distance to accelerate to a given speed and then stop. Note that the faster that given speed, the longer the ASD.
                    TOR (take-off run): The distance to lift-off with an engine failing at a given speed. Note that the faster that given speed, the shorter the TOR (you accelerate more time with all the engines and less time with an engine failed)
                    TOD (take-off distance): The distance to achive V2 at 35ft AGL with an engine failing at a given speed. Note that the faster that given speed, the shorter the TOR (you accelerate more time with all the engines and less time with an engine failed)

                    ASDA (ASD available): The runway length + the stopway.
                    TORA (TOR available): The runway length
                    TODA (TOD available): The runway length + the clearway.

                    V1 is any speed selected by the operator within the range that meets the following constrains:
                    ASD not longer than ASDA
                    TOR not longer than TORA
                    TOD not longer than TODA
                    (there are other constrains too, like V1 must be above Vmcg and must be not greater than Vr, which in turn must be such that Vlo is not greater than Vmbe, but let's forget about this by now).

                    It is often said that if you abort after V1 you won't be able to stop within the remaining runway and that if you continue a take-off with an engine failing before V1 you won't be able to complete the take off within the remaining runway. Neither is necessarily true. Attempt a take-off in a Cessna Mustang at Barajas and you'll see. You'll have a very wide range to select V1 (basically, between Vmcg and the Vr that will make Vlo=Vmbe) and neither extreme will put you in a boundary situation (from a runway length POV) situation if an engine fails at V1. It would be true of you are operating on a runway where TOD = TODA = ASD = ASDA (this is called the balanced field length). In this case there is only one possible value of V1.

                    There are several counter-intuitive things regarding V1.

                    For example, one might think that is very improbable and hence uncommon to be operating in a balanced field condition. However, that's what happens when the plane would be heavier than possible for the available runway length and the operator decided to leave some payload or fuel off board. They will unload only the minimum load that will make the runway available = the balanced field.

                    Quiz. For the same atmospheric conditions and the same weight, is it possible to get a V1 that is slower than the V1 of the balanced field? Answer, yes, with a longer runway (or even in some cases with shorter runway with a longer clearway). It is another myth that the V1 of the balanced field is the lowest one.

                    Quiz. A plane can't meet the required take-off performance with max thrust. The dispatcher says: Let's lower the thrust. Now the plane is legal to take off. Possible? Yes. Google "derated take-off".

                    --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                    --- Defend what you say with arguments, not by imposing your credentials ---

                    Comment


                    • #11
                      Originally posted by BoeingBobby View Post
                      "A position and speed check could be introduced"

                      Ever hear the 80 knot call?
                      What do you check at the 80 knots call except... well, 80 knots?

                      "Position and speed" meant "Position vs speed".

                      --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                      --- Defend what you say with arguments, not by imposing your credentials ---

                      Comment


                      • #12
                        who cares V1 is V1
                        Erm I care. A LOT. So do the boys from EK.

                        Of what possible interest is V1 to anyone except an accident investigator if V1 is predicated on a set of conditions that don't exist? What use is V1 if you've accelerated so slowly that by the time you reach V1 you no longer have room to stop, or room to continue the takeoff engine out?

                        Operationally, yes, of course V1 is a speed, and not described as a position on a runway. I've never said that any operator ever uses any other means to define V1 for operations. What I've said is that the position of the aircraft on the runway at V1 CAN be calculated. In fact it inherently must be, otherwise you could not work out if the aircraft can be stopped/can continue! Of course, that is all contained in the calculations behind the scenes that as pilots we're lucky never to have to see.

                        Ever hear the 80 knot call?
                        Yes, funnily enough I've made and heard the 80kt call many times.

                        At 80kts you are checking for thrust to be as desired (assuming you have selected the correct desired thrust in the first place...), and airspeed indicators are correct. Above that speed you're now only stopping for major failures.

                        The 80kt call is NOT a position call. It could be, with some extra software in the EFB, but it is not.

                        One of the reasons to stop the takeoff below 80kts is slow acceleration. But how do you know if you've got correct acceleration? The older Boeing guys will say "approximately 80kts at the fat markers". Fine on most occasions and if you're departing from the full length of a runway. But with the ever increasing range of intersection and derated takeoffs available, those old "confidence checks" are a lot less useful. Aircraft are now regularly pushed to the end of the runway, and operated over a wide variety of speeds and thrusts. Going from, in this case, MEL - DXB is going to have a very different acceleration if you depart off RWY 27, compared to RWY 34. If an aircraft normally crosses the 1000ft markers at 80kts on a 34 takeoff, if he crosses 1000ft markers at 80kts on 27 at the same weight and environmental conditions he is in a world of hurt.

                        My suggestion is a simple one - you calculate the speed that the correct acceleration would have you passing a fixed marker. Its purely a way to identify if you are accelerating as per your calculated performance. If you're not, then you're not meeting the predicated takeoff performance.

                        Going back to the original comment/statement - V1 as a SPEED is most clearly the correct way to manage it operationally. I'd never advocate to turn V1 into a position on the runway. It would be complex, confusing, and add no value. But technically it could be done. The position would just vary on every takeoff. The argument that you couldn't use a position because the position of V1 would change if you had different acceleration is a redundant one, because that position and a valid V1 speed are directly related.

                        If a position on a runway became inaccurate as a go/stop marker because of something like dragging brakes, then similarly a speed V1 as a go/stop marker is similarly inaccurate and unusable.

                        Comment


                        • #13
                          Sorry 3WE and MCM.

                          Why on Earth do you want to do a speed vs position check to verify the actual acceleration matches the one of the take-off performance calculations when the plane is loaded with sensors?

                          You can get acceleration from the GPS, the ASI, the DME, or even the... ACCELEROMETERS!!!! Every FDR and QAR nowadays records longitudinal acceleration among a googolplex* of other parameters. Other than just a bureaucratic record, the longitudinal acceleration is actually used by the airplane's systems: The INS/IRS uses it to calculate speed and position, the autobrakes uses it to add the necessary amount of brake input to match the deceleration of the selected autobrakes setting (unless it's MAX or RTO).

                          If youn want to check the acceleration, why don't you check the accelertation and that's it? Oh, I forgot. Because while that important piece of data is recorded in several recorders to be able to check it in case of an accident or mishaps and used by various airplane systems to actually operate the plane, it is nowhere displayed to the pilot (actually, I think it is there somewhere if you navigate through the apropiate page of the FMS or MFD).

                          Anybody said AoA?

                          * = 10^10^100

                          --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                          --- Defend what you say with arguments, not by imposing your credentials ---

                          Comment


                          • #14
                            Originally posted by MCM View Post
                            If a position on a runway became inaccurate as a go/stop marker because of something like dragging brakes, then similarly a speed V1 as a go/stop marker is similarly inaccurate and unusable.
                            Ok, so here's a point - I understand where you are coming from now. I get it - V1 is only important if the take off roll is a safe one. I get that.

                            Consider something though - we begin the take-off run, and we calculate the V1 speed to be (just for sh*ts and giggles) to be 155, with my VR being called at 170. You calculate (or a comp. does) that the V1 is 5000 feet down the runway, with VR at 6000 feet. I understand what you are saying - that if by the 5000 feet mark, you don't see me at V1, we might not have enough room to make it to V1, or worse - not enough space to stop. All valid points - but in reality, other systems are monitoring the same thing - including both members of the cockpit crew - who have access to speed and acceleration. It's hard for me to believe that there are conditions in which the pilot simply cannot tell that he/she was not accelerating, or that something was amiss and that there were no indicators from the cockpit systems.

                            I would say that it is ultimately a good check that pilots can preform on their own, and I believe that younger pilots should at least be able to understand the logic behind the issue - but ultimately it is something that we have made redundant with the current cockpit processes in place. Don't get me wrong - if used correctly only once, it could save lives - but it seems more of a side-thought, and exercise, rather than a standard practice in that - if something were going on, and that take off roll were compromised, I would rather have other information at my disposal, and alot sooner, telling me that something is wrong (brakes applied, or inadequate thrust, or engine failure), and the systems are there to tell me that as soon as they happen, thus making me make that decision as soon as the issue happens - rather than at an arbitrary point. I agree with the 80 knot call statement - it's meant to kinda do just that - check on the progress of the roll out, at a much lower speed that most V1s.
                            Whatever is necessary, is never unwise.

                            Comment


                            • #15
                              Originally posted by Gabriel View Post
                              Source? Never mind. It's wrong anyway. Or, better, it's a myth.
                              http://www.ecfr.gov/cgi-bin/text-idx...dno=14;cc=ecfr

                              From the FAA, not so made up.

                              I like the way that you re-worded it though, because it does seem a little more comprehensible in that way.
                              Whatever is necessary, is never unwise.

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