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Posts posted by PerfManJ

  1. Not sure about the C-130, but the C-5 had an parking weight limit. The issue was tire deflection which could cause flat spots or cracked walls. If it was parked overnight above the weight limit, the main gear tires would have to be replaced...that's 24 tires!

  2. The Y-axis shows "Total Distance" which is the distance from brake release to stop (accel-stop line) or continue to liftoff (accel-go line). It is not speed vs. the distance along the runway. The scissor chart shows the accel-go and accel-stop distances over a range of VEFs rather than just one so that VCEF can be determined.

    For example, if you enter the chart from the X-axis at 100 knots, the distance associated with the accel-go line is the total distance to accel to 100 knots, fail engine, and continue to liftoff. The distance associated with the accel-stop line is the total distance to accel to 100 knots, fail engine, then stop.

    You may be confusing the scissor chart with the illustrations at the beginning of the Takeoff Section in SMP 777 which show speed vs. distance along the runway for the different CFL scenarios.

  3. I came across these two articles recently regarding the use of iPads by DOD and specifically USAF. It sounds like USAF (at least AMC) is moving forward with plans to replace paper pubs and they are now cleared with use on DOD networks. AMC is planning to buy up to 18,000 devices and I had read something previously about interest from AFSOC as well.

    Any updates on how the transition is going in the C-130 community?



  4. No, the graph is correct as shown. If an engine failure (VEF) occurs at low speed, then the distance to stop is shorter than the distance to continue to liftoff. The extreme case would be an engine failure at brake release: the stopping distance would be basically zero, and the distance to continue to liftoff would basically be the 3-engine ferry takeoff distance. As VEF increases, the distance to accelerate to VEF increases and aircraft kinetic energy increases (KE = 1/2 (mass) (velocity)^2) therefore the stopping distance increases. In other words, the faster you go, the longer it takes to stop.

  5. @eddychewingum: I modified the scissor chart to show the nominal effect of headwind on accel-go and accel-stop distances. Again, the scissor chart is simplified (accel lines are not really linear, and curvature is different between accel-stop and accel-go) but it illustrates the basic point that wind shifts both lines with differing magnitudes. Because of this, the balance point (accel-stop = accel-go, CFL, VCEF) shifts on both axes. The VREF wind correction (purple line) only accounts for the change in the accel-stop line, while the VCEF correction (orange line) accounts for both stop and go (re-balanced CFL). So, you can use wind-corrected CFL and the VREF correction or uncorrected CFL and the VCEF correction and the result should be very similar.

    It sounds like you are using the two different methods properly, and hopefully this figure helps clarify what is going on. The wind corrections are not perfect, but I would expect either method to give you the same speed within a few knots. Try out several examples and see if the difference is ever more than 2-3 knots.

    I don't have access to a 1-1 or 777 right now so I can't copy the text. I'm pretty sure the 1-1 does not explain the proper use of the VCEF wind correction either. In fact, I think some later editions of the 1-1 removed the separate VCEF wind correction altogether.

    I don't recall how the nosewheel steering corrections are shown in 777. Is there a correction on the CFL chart and on the VCEF/VREF chart? Does 777 have separate balanced and unbalanced field length charts or just a single CFL chart?


  6. "CFL and VCEF are measures of takeoff performance that are independent of runway length and cover both sides of the takeoff problem (accel-go and accel-stop)" This may be the case, but without a runway length taken as part of the equation then the CFL/Vcef actually become useless. This seems to imply that if you have 3,000' available and CFL is 4,000' then you can till use Vcef just because of takeoff performance. Maybe this is just a simplistic way of looking at it.

    Runway length is still important because you want to ensure that CFL is equal to or less than runway available. Only knowing VREF doesn't tell you whether the takeoff can be completed within the runway. If CFL > runway available, then VREF < VCEF and a scenario is created where you are committed to takeoff with an engine failure above VREF, but you will not have sufficient runway to either stop or continue the takeoff (VREF < VEF < VCEF). The minimum criteria for a normal takeoff is CFL = runway available (VCEF = VREF) to prevent that scenario. Knowing the relationship between VREF and VCEF allows the AC to more adequately assess the risk of a given takeoff.

  7. Gentlemen, and perhaps lady's

    Thanks for all the info and explanations. I think in the end the concept is all the same. Something surprises me in the last post it is said that if we use the wind correction in the CFL chart we don't use in the Vcef chart? Why is that? In the SMP777 it is not said not to do that.


    Correct, though I could have explained it better. SMP 777 is pretty similar to the USAF 1-1, so the correct use of the VCEF wind correction is probably not explained in the text.

    To compute VCEF, the VCEF/VREF chart is entered with CFL, gross weight, and takeoff factor (I think that's everything). Next you apply corrections. Since the VCEF wind correction also accounts for the change in CFL due to wind (hence to lower slope), the non-wind corrected CFL should be used to enter the chart. If a wind-corrected CFL is used in the VCEF chart, use the VREF wind correction since the distance (CFL) has already been corrected. Using either method should yield the same VCEF value.

    In summary, to compute VCEF: Use VCEF wind correction with non-wind-corrected CFL.

    Use VREF wind correction with wind-corrected CFL.

    Always use Runway Length and VREF wind correction when computing VREF.

  8. Glad I could help! I think that the scissor chart is a good supplement to the explanatory CFL figures at the beginning of the Takeoff section and gives a slightly different perspective. One thing I didn't make clear in my original response was how to use the separate VCEF wind correction which I don't think is very clear in the 1-1: you must enter the VREF/VCEF chart with a CFL that has NOT been corrected for wind. The separate VCEF wind correction accounts for the re-balance of CFL so if the VREF/VCEF chart is entered with a CFL that has already been corrected for wind, the effect will be double-booked.

    But you are correct that the chart I included is a simplification. In reality, both the accel-go and accel-stop curves are nonlinear and wind (among other things) affect the curves differently. The dominant force during acceleration is engine thrust, which changes with airspeed, while the dominant force during deceleration is braking, which is essentially constant. Also, the simple chart assumes a balanced condition, where VCEF is not being limited. With the E and H models, VMCG plays a big role and can set VCEF when VCEF < VMCG. This is called an unbalanced condition since CFL is being set by accel-stop distance alone. There are also other factors like maximum brake energy limits, power transition limits, etc.

    Accel check time is there to ensure that adequate engine power and thrust is available during takeoff to meet the scheduled performance. When wind is present, the reported values should always be used in the accel check time chart to get an accurate speed and time.

    I think the big takeaway from this discussion is that VREF and VCEF are both important. In my experience, a lot of pilots only care about VREF because they don't really understand what CFL and VCEF tells them. VREF only addresses half of the problem: it tells you the maximum speed when you can stop within the runway. It doesn't tell you anything about the distance that would be required to continue the takeoff from that speed. CFL and VCEF are measures of takeoff performance that are independent of runway length and cover both sides of the takeoff problem (accel-go and accel-stop).

  9. I’m a little late to the party, but with my experience working on the H and J performance manuals (1-1), I hope I can address the original questions. First, some definitions:

    Refusal Speed (VREF) – During a takeoff ground run, the speed at which an engine failure occurs and the aircraft can stop within the remaining runway.

    Critical Engine Failure Speed (VCEF) – During a takeoff ground run, the speed at which an engine failure occurs and the aircraft can stop or continue to liftoff in the same distance (Critical Field Length)

    VREF is the maximum speed at which takeoff can be aborted safely. Abort below VREF and the aircraft will stop before the end of the runway. Abort above VREF, and the aircraft will depart the end of the runway (and maybe start a brake fire if the maximum energy is exceeded). Nosewheel steering affects VMCG which is irrelevant for VREF since the calculation only considers the distance required to accelerate on all engines and then abort (accel-stop). A takeoff can be aborted below VMCG since the power on the remaining engines is reduced to idle or reverse and the thrust asymmetry that drives VMCG is virtually eliminated.

    On the other hand, CFL and VCEF must also take into account the distance required to accelerate on all engines, lose 1 engine, and continue the takeoff at full power (accel-go). In this scenario, VMCG becomes the lower limit on VCEF since departure from the side of the runway is likely if an engine failure occurs below VMCG and takeoff power is maintained on the remaining engines. The WITHOUT NOSEWHEEL STEERING correction is there is adjust the lower limit of VCEF to account for the associated increase in VMCG.

    Unless limited by VMCG or VROT, CFL and VCEF are based on a balanced condition where accel-stop and accel-go distance are equal. I find that a “scissor chart†is the best way to explain this relationship. After brake release, the accel-stop distance increases as the engine failure speed (VEF) increases and the accel-go distance decreases. At a certain point, accel-stop = accel-go. The distance and speed associated with that crossover point are CFL and VCEF. When wind is thrown into the mix, it affects both the accel-stop and accel-go distances but the curves are not affected equally. Accel-stop is equivalent to Refusal Distance, so the effect of wind on VREF is determined using the runway length (not affected by wind) and the wind-adjusted accel-stop curve. For the effect of wind on VCEF, the accel-go curve must also be adjusted, which moves the crossover point (CFL and VCEF) along both axes. In other words, the addition of wind results in a new, re-balanced CFL and associated VCEF. Since the CFL is not held constant, like Runway Length in the VREF calculation, the effect of wind on VCEF is different, resulting in a separate wind correction.

    The intricacies of CFL and VCEF can be difficult to grasp, but I hope the “scissor chart†helps to illustrate the relationship between accel-go and accel-stop. It’s all a balancing†act! (I couldn’t resist the pun.)


  10. Sorry, I should have been more clear: I was referring to the Argentinian bomber configuration, which has the bomb rack mounted at the inboard hardpoint. Alternatively, the Harvest HAWK has Hellfires mounted on the outboard pylon. Hellfires are a pretty light load compared the 6 bomb cluster on the Argentinian C-130.

  11. The wings are already designed to carry pylons and external tanks at that hard point, which weigh about 9,000 lb each when full. And the external tanks act to reduce the wing-root bending moment during flight by counteracting wing lift. The drag load from the bombs is probably different, and I'm not sure of the effect of the response from bomb release. But one could probably configure for bombs occasionally without a significant increase in wing fatigue.

  12. Never heard of the Argentinian C-130 bomber, but it makes sense as a bomb truck. I guess the latest version of the C-130 bomber would be the USMC Harvest HAWK configuration on their KC-130Js. 4 Hellfires and EO/IR targeting pod on the left external tank.


  13. US Herk is absolutely correct: switching to iPads is about saving money for USAF. In addition to saving money on printing and distributing paper pubs, removing all of the manuals from the aircraft saves weight and fuel. The civil aviation world has been using EFBs of some form for years for both of those reasons. But USAF also has to invest in an IT infrastructure to manage and distribute epubs.

    iPads are powerful tools and have a lot of potential. But like with any tool, you have to know how to use it. It is still crucial that the pilots, flight engineers, and navigators know how use the flight planning applications and understand the data that they produce. For example, a full understanding of the data presented in the -1-1 is necessary to accurately compute TOLD. I'm sure there are a lot of FEs out there who would love to have a application to compute TOLD, but first you have to know how to run the charts and complete a TOLD card. In the J, there is no FE and it is the pilots' responsibility to plan the entire flight, including TOLD. There are onboard systems that do much of the work for them, so I doubt they ever have to crack open their -1-1.

    I think that USAF using iPads is a move in the right direction as it allows an enormous amount of information to be at your fingertips, and it is easily searchable and up to date. There is also the potential for iPads to host applications that automate weight and balance, TOLD, and other flight planning. However, training still needs to focus on the foundational knowledge required to understand the calculations, not just on how to punch in the numbers...otherwise the iPad is just a 'black box.'

  14. Hey everyone, I'm new to the forums, but hope I can add some constructive content!

    To any active crew members out there:

    I've been reading a lot lately about USAF testing out iPads as electronic flight bags (EFB) for C-130 crews. It sounds like they are being used to replace paper manuals, checklists, and charts initially. Has anyone been issued an iPad? Any word on future plans for them, such as weight and balance, performance calculations, or other flight planning?


  15. External tanks are not included in in the baseline configuration for slick Js. The KC-130J and new HC/MC-130J models come equipped with external tanks and air-refueling pods.

    All J models can be equipped with external tanks, but most operators choose not to use them. The improved fuel efficiency of the J model eliminates the need for the extra fuel capacity of the external tanks for most missions. Removing the external tanks reduces the aircraft empty weight, which translates to either increased payload capacity or fuel savings, and reduces airframe drag, which also yields fuel savings.

    On the other hand, the external tanks relieve bending at the wing-root when in flight. This reduces wing fatigue over time, but I'm not sure of the long-term impact (there are many factors in determining wing life). As with most things in aviation, its a trade-off.

  16. Nope, there are no LC-130J's out there...yet. Agree with P3_Super_Bee that it was probably an LC-130H with NP2000 props from the NYANG. Although, the Australian Antarctic Division (AAD) is looking for some new wheel/ski-equipped aircraft for Antarctic operations; the RFI is here.

    The NP2000 flight test program at Edwards was completed in 2011 and showed significant increases in the takeoff thrust. The test airplane was a C-130H3 from Cheyenne (WYANG). As far as I know, the only units with the NP2000 props are ANG: 153rd AW in Cheyenne, which only has 1 shipset, and the 109th AW in NY, which operates LC-130H models. The main driver for the NP2000 test was the LC-130 mission, which requires additional takeoff thrust to achieve takeoff speed on skis.

  17. I worked on the test program and the fuel efficiency savings of the Series 3.5 upgrade are pretty significant. There is also potential to increase takeoff power under certain conditions, but that will probably be addressed in the future. Another component of the Series 3.5 upgrade is the addition of the UTAS (formerly Hamilton-Sundstrand) Electronic Propeller Control System (EPCS) which improves maintainability and reliability over of the mechanical system. I'm not sure if the reliability numbers quoted by Rolls-Royce account for the EPCS.

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