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CFL BALANCED OR UNBALANCED???


VIPER1616
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Somebody could help me to understand why I have to check the CFL Balanced and CFL Unbalanced charts and use the higher value. ( assuming CFL is < than runway available). I am a little bit confused trying to understand the concept.

Thanks

CARLOS LANDINEZ

COLOMBIAN AIR FORCE

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Our version of these charts are a little different but...

Question is how much longer 500' or 2000' or more?

You only know if your true CFL is more than runway avalible if you compare BAL to UNBAL as the charted distance varies.

Say takeoff factor is 1.0

Gross Weight 150,000

All chart correction inputs equal...

Your avalible Runway is 4400'

Balanced distance required 4000'

Unbalanced distance required 4500' (0 degree temp dev chart input)

After you compare you use the "Longest runway requirement" so your aircraft can perform to the Critical Field Length definition.

Takeoff or stop in runway avalible.

Critical field length is the... (minimum) runway distance required to accelerate on all engines, experience an engine failure, and then to either continue the take-off or stop.

Balanced field length

When the distance required to continue the take-off is equal to the distance required to stop.

This is ok as you can takeoff or stop on the runway you have avalible.

Unbalanced field length

This would not be ok because you cannot stop.

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YES I UNDERSTAND THE IDEA BUT I CAN NOT UNDERSTAND WHY WITH THE SAME CONDITIONS AN AIRPLANE IN ONE CHART NEEDS 4000 FT (BALANCED) TO ACELERATED TO VCFE AND STOP IN THAT DISTANCE(4000 ft) ON THE OTHER HAND (unbalanced) THE SAME PLANE WOULD NEED 4500 ft TO ACELERATED AND STOP. IT IS SOMETHING ABOUT THE SPEEDS THEY USE.????

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Seems like AF only terms, we do not use them. We have rec (RA more than CFL) min rec RA equal to CFL and not rec (RA less than CFL).

I think the terms are used as a decision making process to determine/define who (qualificaton level) can takeoff from any given field/op condition.

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YES I UNDERSTAND THE IDEA BUT I CAN NOT UNDERSTAND WHY WITH THE SAME CONDITIONS AN AIRPLANE IN ONE CHART NEEDS 4000 FT (BALANCED) TO ACELERATED TO VCFE AND STOP IN THAT DISTANCE(4000 ft) ON THE OTHER HAND (unbalanced) THE SAME PLANE WOULD NEED 4500 ft TO ACELERATED AND STOP. IT IS SOMETHING ABOUT THE SPEEDS THEY USE.????

VIPER,

Here is the easiest way to explain the difference between the two. All the previous definitions are correct and I'll try to explain the difference.

BALANCED CRITICAL FIELD:

Balanced is determined using Vcef (Critical engine failure #1 eng). What this means is that I'm on takeoff roll and and at a predetermined speed (calculated through the refusal (Vr) speed chart) you lose the most critical engine and you can either continue the takeoff or stop in the same distance to/from the decision speed (Vr) (balanced less than actual runway length)

UNBALANCED CRITICAL FIELD:

This is where some engineers put their heads together and thought, well that's all and well, but what if I don't lose my engine at Vcef what speed can I be at and still be able to stop past Vcef?? Well here is what they came up with. You have Vmcg (ground minimum control speed) and Vrot (5 Kias below Vto). This is what Unbalanced Critical Field is based off of, the greater of the two. More times than not you will have Vrot being the determining factor due to the runway's being longer and RCR of 23. What this means in english is in theory if I lose an engine before Takeoff (i.e. Vrot) I will be traveling at a much faster speed than Vcef, therefore, it will take me longer to stop (basically Unbalanced is more of a stop distance than a go), so basically this is why you have two critical field lengths. Also, you will be closer to the Vr speed, so this is why they call it unbalanced. You have a shorter distance until you reach Vr, but a longer distance to stop after this point (make sense??). These guys are trying to cover every basis.

I hope this answers your question. I think the easiest way is to plug your numbers in a line graph. You also have to remember that most TOLD information is based off of worse case scenario stuff. Let me know if I helped with this because it took me a little bit to get my brain wrapped around it too.

Dave

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This is where some engineers put their heads together and thought, well that's all and well, but what if I don't lose my engine at Vcef what speed can I be at and still be able to stop past Vcef??

Vr would be the easy answer...

Vcef is a planning tool

(how much (minimum) runway do I need)

BRAKE RELEASE-----------------CEFS(SPEED BASED ON CFL)-----------------------TO or STOP (ON CFL; NOT RA)

Vr is a "Real world" Limit.

(I have extra runway for my conditions so I can go faster and still stop)

BRAKE RELEASE-----------------VR(SPEED BASED ON RA)-----------------------------STOP

All the extra is a bunch of ________... IMHO

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This is how a normal line would look on a lot of runways we use:

Balanced CFL:

BL-------------------Vcef------------/Vto----------------------Vr

Unbalanced CFL:

BL-------------------Vmcg-----------Vrot-----/Vto-------------Vr

It's a lot of killed brain cells over stuff like this!

Dave

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That Vrot is a bunch of crap foisted on us by 141 engineers...

If you do that in a Herk, you fly. When you're ready to fly, you rotate in a Herk. Unlike in a Jet, where you have to set takeoff attitude and fly off the ground, the Herk is flying as soon as you rotate. It's a load of crap and needs to be removed from our books.

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That Vrot is a bunch of crap foisted on us by 141 engineers...

If you do that in a Herk, you fly. When you're ready to fly, you rotate in a Herk. Unlike in a Jet, where you have to set takeoff attitude and fly off the ground, the Herk is flying as soon as you rotate. It's a load of crap and needs to be removed from our books.

Agree 100%!

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That Vrot is a bunch of crap foisted on us by 141 engineers...

If you do that in a Herk, you fly. When you're ready to fly, you rotate in a Herk. Unlike in a Jet, where you have to set takeoff attitude and fly off the ground, the Herk is flying as soon as you rotate. It's a load of crap and needs to be removed from our books.

See what happened when they made us all C's instead of A's! Of course that was back in the day before the current AFSC designation change.

Back in '79 when I went through the FE Performance School (out of the 16 TATS at LRF) as part of the C-130 FE certification for the school you could almost read the the writing on the wall. Thinking back on it maybe I should have taken the job offer to instruct there. At least that way there would have been a Herc FE giving some insight into the program. But that would have taken me out of the flight deck and relegated me to going to Dyess once a month or so for currency.

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  • 1 year later...

A balanced field takeoff is a condition where the accelerate-stop distance required (ASDR) is equal to the takeoff distance required (TODR) for the aircraft weight, engine thrust, aircraft configuration and runway condition. For a given aircraft weight, engine thrust, aircraft configuration and runway condition the shortest runway length that complies with safety regulations is the balanced field length.

The unbalanced field length is a condition where the (ASDR) is less than the the takeoff distance required (TODR), thus if you notice in the chart, the Vcef is reduced to be equal minimum control speed on ground (Vmcg) , thus ; in the case where the runway is shorter than CFL, the rotation speed Vro = Vcef = Vmcg.

So it's a matter of relation between the distance and speeds, as distance increases, speed increases to the max of take off speed ( not more than takeoff speed) as the aircraft is supposed to be airborne after takeoff speed.

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That Vrot is a bunch of crap foisted on us by 141 engineers...

If you do that in a Herk, you fly. When you're ready to fly, you rotate in a Herk. Unlike in a Jet, where you have to set takeoff attitude and fly off the ground, the Herk is flying as soon as you rotate. It's a load of crap and needs to be removed from our books.

Never used it in the gunship we go at takeoff!

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@C-130 pilot: The definitions you posted are accurate in the civil world, but the military rules are a bit different, especially on the Herk. The biggest difference is that the "continued takeoff" distance (accel-go) is computed from brake release to liftoff, not to 35 ft (civil) or 50 ft (mil).

Balanced and unbalanced field lengths are determined by comparing accel-go (distance to accel to a given speed, lose the critical engine, then continue to liftoff) and accel-stop (distance to accel to a given speed, lose the critical engine, and stop). The "balanced" condition occurs when accel-go = accel-stop and the "unbalanced" condition occurs when the engine failure speed is constrained by some other limit speed, such as VMCG or VROT.

For the Herk, CFL is the greater of balanced and unbalanced field length. Balanced field length is defined as the distance required to accelerate to VCEF on all engines, experience a critical engine failure (#1) and continue to liftoff or stop IN THE SAME DISTANCE. This "balanced" condition occurs when accel-go = accel-stop and VCEF is unconstrained. Unbalanced field length is the distance required to accelerate to VCEF on all engines, experience a critical engine failure (#1) and continue to liftoff or stop. This "unbalanced" condition occurs when VCEF must be increased due to another limit speed, such as VMCG or VROT. See attached "scissor chart" for a visual explanation and similar thread (http://www.c-130hercules.net/showthread.php?t=1731)

@Ronc: Do you rotate at charted VTO? If so, you may exceed your charted takeoff distances. According to the 1-1, rotation should occur at VTO - 5 knots.

[ATTACH=CONFIG]3503[/ATTACH]

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For me rotating 5 knots before TO speed is neither here nor there. Five knots at those speeds, meh. By the time the nose wheel gets up you're at TO speed.

As for max effort, because of the conditions for a max effort, you'd want all the speed you can get before lifting off. Don't want to be wallowing around anymore than you may already be at ME TO speed.

But it's been 27 years since I strapped my ass into a Herc so a lot of brain cells have forfeited their selves for the greater good.

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@Ronc: Do you rotate at charted VTO? If so, you may exceed your charted takeoff distances. According to the 1-1, rotation should occur at VTO - 5 knots.

For a normal takeoff it does say to start rotating 5 knots before takeoff speed in the USAF book.

That whole concept is a jet-ism. In a Herk, if you rotate at Vto-5, you're flying at Vto-5. Yes, you will continue to accelerate (assuming you don't over-rotate) and go through Vto likely as the mains come off, but you're flying before you're supposed to. This is the challenge when trying to put a one-size-fits-all definition into books...

The Herk will fly much, much slower than even Vmeto...buyer beware.

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That whole concept is a jet-ism. In a Herk, if you rotate at Vto-5, you're flying at Vto-5. Yes, you will continue to accelerate (assuming you don't over-rotate) and go through Vto likely as the mains come off, but you're flying before you're supposed to. This is the challenge when trying to put a one-size-fits-all definition into books...

The Herk will fly much, much slower than even Vmeto...buyer beware.

Not if you add the five knots to the distance.

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Not if you add the five knots to the distance.

As long as you are correcting CFL and takeoff distances for a rotation speed increase of 5 knots, then you're good to go.

That whole concept is a jet-ism. In a Herk, if you rotate at Vto-5, you're flying at Vto-5. Yes, you will continue to accelerate (assuming you don't over-rotate) and go through Vto likely as the mains come off, but you're flying before you're supposed to. This is the challenge when trying to put a one-size-fits-all definition into books...

The definition of Vto is the speed when the aircraft becomes airborne (i.e. when the mains come off) which is the basis for the performance manual ground distances (brake release to liftoff). The Herk has a lot of powered lift, but it still has to be rotated to increase the angle-of-attack and achieve takeoff attitude to fly. Raising the nosewheel takes time, which adds speed and distance between rotation and liftoff. At 100 KGS, it only takes 1 second to eat up nearly 200 ft of runway.

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