Jump to content
Aero Precision provides OEM part support for military aircraft operators across more than 20 aircraft
Sign in to follow this  
Guest

Performance refusal-critical failure speed

Recommended Posts

Guest

Hello guys

I am new in C-130 family and i would like to ask why in performance (about t-56-a15 engine C-130H) table with title “REFUSAL SPEED AND CRITICAL ENGINE FAILURE SPEED†we have 2 correction grid for wind (one for refusal and one for critical failure speed)?

Also, why we have correction grid for “WITHOUT NOSEWHEEL STEERING†only for critical failure speed and not for refusal?

I know that refusal speed based on runway available but how the wind influence in different form these two speeds.

Thanks in advance for your time

George

Share this post


Link to post
Share on other sites

Well for refusal speed, you dont correct for nosewheel steering.

When you are looking at refusal speed you are simply looking for how fast you can go, then stop without going off the end of the runway.

Whether you go off the side of the runway or not doesn't figure into that calculation, simply based on the end of the runway.

As to the other things you ask I will have to pass on the answers there, I don't have a 1-1 anymore so I cant answer those questions without possibly screwing it up:rolleyes:

Dan

Share this post


Link to post
Share on other sites

If you look at the CFL chart which is used to compute needed length of runway there is a correction for wind. This correction will make your CFL shorter which is better.

The wind correction for Refusal speed is a single therefore more drastic correction value on the chart. (look at the angles of the two corrections).

Share this post


Link to post
Share on other sites

Easy answer... in the "accounting for wind" section it says to NOT use winds to make a mission.... YES I wrote it that way... only if you need every bit of performance... any headwind will only benifit you.

ALWAYS correct for a tailwind.

Dan said "When you are looking at refusal speed you are simply looking for how fast you can go, then stop without going off the end of the runway."

Refusal Speed. Based on the runway available, defined as the maximum speed to which the aircraft can accelerate with engines at takeoff power and then STOP the remainder of the runway available.

Critical Engine Failure Speed is the speed to which the aircraft can accelerate, lose an engine, and then either continue the takeoff with the remaining engines, or stop in the same total runway distance. (NOSE WHEEL STEARING is required to continue the takeoff at lower speeds to maintain centerline due to the reduced airflow over the rudder)

If the NWS is inop the correction to VCEF should bring you up to VMCG (if you compute without NWS) which is the speed you can maintain centerline after engine failure. You need to know this if you are going to continue the takeoff.

Share this post


Link to post
Share on other sites

I am writing a paper on this question. The influence of wind on Refusal or Vcef. For refusal speed it is in fact very simple. It is only a correction from Indicated airspeed to groundspeed. In fact the point Refusal drawn on a runway is always the same point for a given gross weight. In fact it is a distance not a speed. So for every not headwind it will increase refusal with one knot. Offcourse this is only when we use 100% wind what we never do. Just theoretically this is correct. For the effect of wind on Vcef it is different. Headwind helps us for takeoff so CFL will decrease which makes that Vcef increases.

Share this post


Link to post
Share on other sites

"In fact it is a distance not a speed."

This is not exactly true...

The calculated "Refusal Speed" can be converted to a "distance" and this is actually discussed in the 1.1 Acceleration check time section.

Share this post


Link to post
Share on other sites

I like where this is going.

Yes, for a given weight, all else being equal, there will always be a set distance required to stop from any designated speed. This is due to limitations of brake energy and on the civilian side, the charts are even labeled as such.

The problem we solve with refusal speed is we determine what the acceleration side of the equation can be to still fit in with the stopping side.

What varies is the acceleration side. The stopping side is pretty fixed for a given weight and speed - how quickly can you get to that speed is what varies the most.

Winds just put extra numbers in each of these equations.

I never fully trusted the acceleration check times. I think they're padded far too heavily on the acceleration side and additionally, don't compensate for higher density altitudes very well. We were always taught to use the highest speed we could to get a more accurate time, but all that does is cut into your margin even more. Far too many times I looked up when the Nav called "time" and asked myself the question, "could I stop from here if I had to?" and more often than not, the answer was, "I don't think so"...this seemed to be far worse at high DA.

Share this post


Link to post
Share on other sites

I never fully trusted the acceleration check times. I think they're padded far too heavily on the acceleration side and additionally, don't compensate for higher density altitudes very well. We were always taught to use the highest speed we could to get a more accurate time, but all that does is cut into your margin even more. Far too many times I looked up when the Nav called "time" and asked myself the question, "could I stop from here if I had to?" and more often than not, the answer was, "I don't think so"...this seemed to be far worse at high DA.

I have been told to use lower speeds because you can identify the problem (failure to accelerate) earlier and stop easier. We're usually going faster than charted because most engines do better than 95%. I wonder though, if you were torque limited in cold weather if the check time would be more accurate because you can't use more power than the chart assumes. Another variable is the 3 second delay built into the 1-1 refusal speed, and at 100 knots you'll burn up an extra 500' of runway in that time, but that's supposedly included in the calculation.

Share this post


Link to post
Share on other sites

@NATOP1 it is indeed not very clear but Vr is only the speed at witch we can stop on the remaining runway. The acceleration part I understand but still it does not impact the performance I would think. If for a theoretical reason we use 100% wind factor The point Vr will always be the same for a giver gross weight. Would you think wind impacts acceleration that much? In the end this comparison is made in the Vcef vs Vr and acceleration is implemented in Vcef calculations. Correct me if I am wrong. I am tryin to make a briefing on this to finally clear this up because this question is asked a lot in our squadron. If you want I can send you a draft of it. Can you maybe forward me that section of your 1.1? We use SMP777 so I don't have that part.

Share this post


Link to post
Share on other sites

I have been told to use lower speeds because you can identify the problem (failure to accelerate) earlier and stop easier.

I always briefed my FE to do just that when it really mattered...

We're usually going faster than charted because most engines do better than 95%.

This only compounds the problem on one side. In theory, you will reach your speed sooner and thus have more distance to stop. But with an abnormal acceleration, in some circumstances can find yourself further down the runway and unable to stop. It defies initial review (two trains leave the station, one is slower, at the end of a given time, which one is farther down the track). The problem lies in the fact the acceleration curve has been factored into distance remaining - and that's just not quite right.

I wonder though, if you were torque limited in cold weather if the check time would be more accurate because you can't use more power than the chart assumes. Another variable is the 3 second delay built into the 1-1 refusal speed, and at 100 knots you'll burn up an extra 500' of runway in that time, but that's supposedly included in the calculation.

I've experienced the same in Norway when very torque limited. I didn't trust those numbers at all for the acceleration check time.

And the 2-3 second delay is reality. As much as we sit here in the comfort of our air conditioned easy chairs and talk about how quickly we can identify something, I watch experienced crews in the sim eat up valuable concrete in abort scenarios trying to figure things out. When it's happening to you in real time, it's amazing how much our brain speeds up and distorts the passage of time...makes us think we're lightning quick, when, more likely, we're average. ;)

@NATOP1 it is indeed not very clear but Vr is only the speed at witch we can stop on the remaining runway.

The challenge is, how did you get to that speed? You had to accelerate there. That acceleration eats a theoretical distance of your available concrete up. Acceleration is not linear, it is a curve. The faster you go, the faster you go faster. ;) So too is stopping, although not as sharp a curve. The slower you go, the slower you can go slower. ;) But your point is correct - for a given speed/weight, Vr is, effectively, a distance. We simply represent it as a speed primarily because that's what we've got in the cockpit to measure it. Because it is a speed, measured on an airspeed indicator, wind affects the reading of it. Wind also affects the actual performance of the plane - there is a lot of surface area to deal with. That's why the headwind and tailwind corrections are in the books.

The acceleration part I understand but still it does not impact the performance I would think. If for a theoretical reason we use 100% wind factor The point Vr will always be the same for a giver gross weight. Would you think wind impacts acceleration that much?

No, the SPEED Vr remains the same. And because you're measuring it with an airspeed indicator, wind affects it. And because of the huge surface area, wind affects the actual performance of the plane. So to answer the last question in this selected quote: Yes.

In the end this comparison is made in the Vcef vs Vr and acceleration is implemented in Vcef calculations. Correct me if I am wrong. I am tryin to make a briefing on this to finally clear this up because this question is asked a lot in our squadron. If you want I can send you a draft of it. Can you maybe forward me that section of your 1.1? We use SMP777 so I don't have that part.

What question is asked a lot?

Vcef solves two problems simultaneously (is there a speed where I can lose an engine and either stop or go) and Vr only solves one problem (how fast can I go and still stop). I'm over-simplifying, I know, I'm not trying to provide the checkride definition here, merely simplify for illustrative purposes.

Sometimes, there are "two" Vrs - the one that's posted on the card (which is often limited by takeoff speed) and the actual one. I always asked my actual Vr - especially when heavy. As for why, that's a different discussion on non-standard ops... ;)

Based on what you've posted thus far, I think you're oversimplifying Vr.

Share this post


Link to post
Share on other sites

@NATOP1 it is indeed not very clear but Vr is only the speed at witch we can stop on the remaining runway. The acceleration part I understand but still it does not impact the performance I would think. If for a theoretical reason we use 100% wind factor The point Vr will always be the same for a giver gross weight. Would you think wind impacts acceleration that much? In the end this comparison is made in the Vcef vs Vr and acceleration is implemented in Vcef calculations. Correct me if I am wrong. I am tryin to make a briefing on this to finally clear this up because this question is asked a lot in our squadron. If you want I can send you a draft of it. Can you maybe forward me that section of your 1.1? We use SMP777 so I don't have that part.

Ok we need to separate a few items/ideas then we can sort this out.

(CFL/Vcef vs Vr and acceleration)

CFL is the minimum runway required to operate safely.

Vcef is the speed at which we can continue to take off or stop on CFL.

CFL and Vcef are a planning tools; they allow us to look at a situation and determine if we can operate safely from a particular runway with atmospheric conditions taken into account.

Vr is the maximum speed we can go on the runway and stop on the remaining runway. In a NO wind calculation Vr and Vcef are equal if the runway required is equal to the runway available; in reality the wind value and the reliance of the wind to stop vice continue the take off ensures that your Vr is always higher than Vcef.

Acceleration check time is a way to ensure the aircraft is performing to a standard to allow the crew adequate time to identify and react to a lack of performance prior to Vr (or stop point on the runway) Vr can be converted into a distance as previously discussed.

We can calculate this time/speed any day, any takeoff however it has been deemed only necessary during short(er) field operations.

If the Acceleration check time is not achieved we will be beyond our refusal point on the runway when we reach Vr; or below Vr at that point on the runway.

We use speed due to the inability to physically mark the runway.

Would you think wind impacts acceleration that much? Yes the charts are designed to account for the full stopping affect of the wind and a smaller portion of the wind for a continued take off.

I would like to see your paper you can PM me on this site…

Share this post


Link to post
Share on other sites

I always briefed my FE to do just that when it really mattered....

It always matters; you can do an acceleration check time calculation for any takeoff; you are just giving yourself more runway to stop on by detecting a problem sooner (or at all) and at a lower airspeed.

This only compounds the problem on one side. In theory, you will reach your speed sooner and thus have more distance to stop. But with an abnormal acceleration, in some circumstances can find yourself further down the runway and unable to stop. It defies initial review (two trains leave the station, one is slower, at the end of a given time, which one is farther down the track). The problem lies in the fact the acceleration curve has been factored into distance remaining - and that's just not quite right..

Not sure what you mean by this statement... I think you have a few ideas in one statement.

If you had the same aircraft (weight) on the same runway under the same conditions the aircraft with the 100% engine would reach a speed (Vr) "faster" (sooner) than a 95% engine aircraft which would result in more runway remaining to stop on. Also the take off factor would be higher in the 95% engine resulting in a longer runway requirement, therefore you could induce a safe margin by calculating at 95% even if you had 100% engines.

If you encountered "an abnormal acceleration" you would not end up in a position where you could not stop IF you computed the acceleration check time. You should detect this "abnormal acceleration" during the acceleration check time no matter which aircraft you were on 95 or 100% engines.

I've experienced the same in Norway when very torque limited. I didn't trust those numbers at all for the acceleration check time.

And the 2-3 second delay is reality. As much as we sit here in the comfort of our air conditioned easy chairs and talk about how quickly we can identify something, I watch experienced crews in the sim eat up valuable concrete in abort scenarios trying to figure things out. When it's happening to you in real time, it's amazing how much our brain speeds up and distorts the passage of time...makes us think we're lightning quick, when, more likely, we're average. ;).

3 knots for acceleration check time not seconds

(3 Knot tolerance for airspeed)

So why do you not trust your numbers? It is a power vs weight/drag calculation.

Tq of 19.6 vs a 150K Aircraft at 50% Flaps. X amount of power will accelerate X weight and X drag to X speed within X amount of seconds...

Sometimes, there are "two" Vrs - the one that's posted on the card (which is often limited by takeoff speed) and the actual one. I always asked my actual Vr - especially when heavy. As for why, that's a different discussion on non-standard ops... ;).

So if you have a malfunction after takoff you may elect to stay on the runway? For??? load shift? Fuselage Fire? Please do tell....

Share this post


Link to post
Share on other sites

It always matters; you can do an acceleration check time calculation for any takeoff; you are just giving yourself more runway to stop on by detecting a problem sooner (or at all) and at a lower airspeed.

Training vs real world. Simulated vs actual. AF books told us to calculate the highest check speed/time so it would be the most accurate - normally 10kts below the rounded down GO speed (whether that was T/O or Refusal). My concern is this has always reduced the margin for error, so I would ask my FE to crunch 20kits lower when the numbers were "tight".

Not sure what you mean by this statement... I think you have a few ideas in one statement.

If you had the same aircraft (weight) on the same runway under the same conditions the aircraft with the 100% engine would reach a speed (Vr) "faster" (sooner) than a 95% engine aircraft which would result in more runway remaining to stop on. Also the take off factor would be higher in the 95% engine resulting in a longer runway requirement, therefore you could induce a safe margin by calculating at 95% even if you had 100% engines.

The challenge is in the acceleration curve. The faster you go, the faster you go faster.

If you encountered "an abnormal acceleration" you would not end up in a position where you could not stop IF you computed the acceleration check time. You should detect this "abnormal acceleration" during the acceleration check time no matter which aircraft you were on 95 or 100% engines.

Once again the challenge is the acceleration curve. The ability to stop is based 100% on accelerating to that speed normally. This determines a distance remaining once you reach that point. Since a normal acceleration will achieve that speed sooner, you have more runway remaining than if you reach that speed later. This really isn't the "two trains leave the station" kind of problem it appears on the surface. And it's all because acceleration is a non-linear curve.

ADDED: This acceleration curve is why we usually "blow through" the accel check speed. In my last plane, we rarely missed it by anything less than 10-20 knots, forget the 3 knot tolerance. This goes beyond 95% vs 100% engines by a wide margin...and we rarely had 100% engines. My brothers on the Talon I side of the house had to have 97% engines for their TF and I've discussed this concept with them as well - they had similar experiences - "beating" the accel time speed by wide margins. So there's a pad in there that's padded on the wrong side of the equation if you ask me...

I really won't argue this with you too much. It's too counter-intuitive and goes so far against our training it is difficult for people to grasp/believe. I finally had a physics guy/pilot reach my same conclusion, but it almost cost him an ulcer by the time I convinced him... ;) This is a 'light bulb' kind of thing - the worst part is it's incredibly difficult to convey as it seems so counter-intuitive. I'll just ask you to keep an open mind, don't completely discount what I'm saying, and just look up when the nav calls "time" and ask yourself if you could stop if you had to. I believe things are padded too much on the acceleration side and these relationships are skewed worse in high DA situations.

3 knots for acceleration check time not seconds

(3 Knot tolerance for airspeed)

Yes, but there is a reaction time built into the computation (I believe it's 2 seconds). This is what the OP was referring to and hence my reference.

So why do you not trust your numbers? It is a power vs weight/drag calculation.

Tq of 19.6 vs a 150K Aircraft at 50% Flaps. X amount of power will accelerate X weight and X drag to X speed within X amount of seconds...

Because acceleration isn't linear, it is an exponential curve. That, and at least one no-kidding, real-world, I missed my accel/time check, but no way in H**L I could stop meant I had to go. Fortunately, our T/O numbers are padded as well...even max effort T/O. Previous experience in another AF had given me the ability to make a different decision based on more than just theory and numbers...and we all lived to tell the tale...with some margin. This event precipitated my scrutiny of accel time check numbers.

So if you have a malfunction after takoff you may elect to stay on the runway? For??? load shift? Fuselage Fire? Please do tell....

If I can continue past "go" and still stop, why would I take the airplane into the air if I don't have to?

The concept of V1 vs VR is a jet thing. Jets are more stop limited than go limited. Herks are more go limited than stop limited. In other words, I can land places I can't take off from (the J-model is the other way around and I'd be curious to crunch their numbers - I'll bet the traditional thinking works far better for them). So I'm usually able to stop.

So, long runway, heavy plane, I will leave it on the runway until 5-kts below nosewheel speed (if I can get there, or whatever my real refusal is). Knowing that I can still lose an engine after "go" and still stop comfortably.

Speed is life. Altitude is only life insurance. A sick plane on the ground with room to stop is far better than a sick plane wallowing just out of ground effect.

I've had the occasion to operate at extreme gross weights operataionally (well over 180K), conducted take offs at extreme low speeds (low 80 knot region), climb terrain at charted 1.3 Vstall in test profiles, fly deep stall profiles (test), and operated 3-engine in many regimes. There are margins and pads built into every aspect of our performance book. Most of them are very positively in our favor and a very few aren't that accurate (recent Vmca changes prove this, but the ongoing Vmcg issue also illustrates this). Fortunately, we don't spend much time in the margins.

I didn't teach anything contrary to our books. I always trained A/Cs to follow the performance books. Instructors, I challenged to understand more of what was going on. I offered my experiences for them to think on. I told them I could not tell them to ignore the books, but to think about things, use their own experiences, and always have a plan-B.

I also told them my secret was that I always planned to fail up until I succeeded. In other words, I assumed I'd abort every takeoff until I left the ground. I assumed I'd "no drop" on every drop until the LM called, "load clear". I assumed I'd go around out of every single landing until I touched down and was decelerating. And so on. It's not a failure mindset, but a mindset to survive. Those "two second" reaction times they always talk about are real...even when you're prepared. The tough part is making the decision, what you did after that was canned - or at leas thought through by preparing. Folks who didn't do it that way, often ate more time than they needed to. Sometimes, those fractions of a second are important.

Good judgment comes from experience and experience? Well, a lot of that comes from bad judgement. - that's one of my favorite quotes (attributed to many people), but some experience comes from bad luck too - that counts as well - if you survive.

Always learn - even if it's what not to do. But in that case, don't learn too much! ;)

EDITED (added a short paragraph above and the following) - I will concede that my old airplane may have had worse numbers. And I certainly don't mean to imply that all C-130s always perform the way my experience was. There were corrections and factors into the speeds we had in the cockpit (KCAS vs KIAS) and I was never confident those always translated correctly into our -1-1. However, if there were factor-induced differences, it only magnified an existing problem, it didn't fabricate one that didn't exist.

Share this post


Link to post
Share on other sites

I always liked flying with a pilot that was more on the ball than just the average stick jockey. I could give him the numbers, etc., but an informed pilot took those numbers and knew what he could and could not do with them. The pilot knows how the plane "feels" to him, I did not other than what I could see, hear, and somewhat feel. Luckily I never really had one of the short field, hi-pucker factor take-offs. Sure some short field stuff at Alaska sites, but we were never really that heavy or with high temps/PA.

Only had to take off a few times really heavy but those were from long runways, Hickam, Kelly, even EDF, etc.

Share this post


Link to post
Share on other sites

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.)

[ATTACH=CONFIG]3415[/ATTACH]

Share this post


Link to post
Share on other sites

That's an excellent explanation, thanks for that. I wish I'd seen that scissor chart when I was first starting out!

My only comment is that this is how I learned this stuff early on (without the chart) and I think we could do better by also explaining that neither acceleration nor deceleration are linear when we talk about more advanced concepts. The relationships don't change, and the chart is 100% valid to explain the relationships, but when used in absence of true curves or explained that way, it doesn't explain everything. In fairness, when we're first learning these concepts, it's probably better to keep it simple, but there still needs to be some explanation that acceleration is not linear. And this skews our understanding of following concepts like accel time checks.

To be clear, my point doesn't affect this aspect of TOLD at all, and to address the OP's question, this is probably as far as we need to delve into it. My point about acceleration curves only affects the accel time check which was brought up during the discussion.

Share this post


Link to post
Share on other sites

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).

Share this post


Link to post
Share on other sites

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.

E

Share this post


Link to post
Share on other sites

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).

It seems to me that the best way to apply these numbers is CFL/Vcef first. It would seem that if the runway available meets or exceeds CFL then Vref may not come into play since you can accelerate to Vcef then stop or go, then Vcef=Vref. It would also seem that the only time Vref comes into play is when CFL is greater than runway available. Then Vref is calculated and used, as well as speed/time check, refusal distance, etc.

This comes from an old FE's memory so take it with a grain of salt. I haven't seen the charts in 30 years, so....... To me this is the simplest way to explain it.

"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.

Share this post


Link to post
Share on other sites

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).

One thing we don't have in the Vcef / Vref discussion is Vtakeoff. Vto will always exceed Vcef, but Vref may exceed Vto. Since peacetime ops always has Vto >= Vmca, if Vref > Vto, I can always stay on the runway beyond Vto up until Vref. If I lose an engine beyond Vto, but below Vref, I can either takeoff or stop in the remaining runway. I wish I could draw a pretty chart, it'd be a lot easier to explain.

The point is, our charts give us the minimums, but do not do a lot of explaining about what happens beyond these minimums and what other factors can help or hurt us.

Share this post


Link to post
Share on other sites

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.

E

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.

Share this post


Link to post
Share on other sites

"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.

Share this post


Link to post
Share on other sites

@ PerfManJ: Ok let me try to repeat this and tell me if I am correct: To find the correct CFL ( during mission accomplishment off course ) I use the wind correction as said in the 777 ( 50 % HW and 150 % TW ). To find the Vcef I use the uncorrected CFL with the Vcef wind correction grid on Vcef chart OR the corrected CFL with the Vr wind correction grid.

You say that it should be + or - the same but I still have a few kts difference. Also in theory if we use 100 % wind just for trying out it should be the same but the difference is even higher. I can see what you say and I believe you because I found an example that is done this way. The only thing is that I would like to know why and I want to prove it because that is what I will try to do in my briefing which will be given on a training day to the whole squadron.. so I can expect some questions;)

As I said before the correction grid for Vr for me is only + or - the wind to see the exact refusal distance. IAS to GS.

Can you copy paste that part of the 1-1 for me?

Also if you use only one correction to avoid double correction is that also for the nose wheel correction?

thx

Share this post


Link to post
Share on other sites

@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?

[ATTACH=CONFIG]3430[/ATTACH]

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Sign in to follow this  

×
×
  • Create New...