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View Full Version : Help with Critical Field Length


midas
12-17-2018, 09:47 AM
Hello experienced aviators! Can someone kindly guide me to understand why the crtical field length required decreases when you perform a reduced power take-off? Any C130 crew around, pls?


Prettywhacked1
12-17-2018, 10:03 AM
with a reduced power takeoff, takeoff decision speed (lowest of refusal speed, critical engine failure speed, and rotation speed, but no less than ground minimum control speed) typically decreases in a balanced field as critical engine failure speed decreases as a function of being able to lose an engine at a lower power setting and stop or continue (accelerate stop distance decreases).

I’d also recommend FAR/AIM for a very detailed discussion of these definitions.

V1 is the maximum speed at which the rejected takeoff maneuver can be initiated and the airplane stopped within the re- maining field length under the conditions and procedures defined in the FAR’s. It is the latest point in the takeoff roll where a stop can be initiated.
Second, with respect to the “Go” criteria, VI is also the earliest point from which an engine out takeoff can be continued and the airplane attain a height of 35 feet at the end of the runway.

joepilot
12-17-2018, 10:22 AM
I Would bet that this reduced critical field length only occurs at light weights.

V1 and VR cannot be less than VMC. If you are using reduced power, VMC will be reduced to a lower number, which may allow a reduced critical field length. At higher gross weights, where VMC will always be less than Vstop or Vgo, I am confident that the critical field length is longer at reduced power.

Joe


Twin Wasp
12-17-2018, 05:59 PM
To be more correct, V1 can not be less than VMCg and Vr can not be less than VMCa. As joepilot pointed out, with reduced thrust comes a lower set of VMC speeds. While the slower acceleration to the reduced V speeds may or may not take the same distance as the faster full thrust acceleration to the higher speeds the reduced stopping distance from a lower speed abort can lead to shorter runway required numbers.

TAHOEJ8
12-17-2018, 10:46 PM
I Would bet that this reduced critical field length only occurs at light weights.

V1 and VR cannot be less than VMC. If you are using reduced power, VMC will be reduced to a lower number, which may allow a reduced critical field length. At higher gross weights, where VMC will always be less than Vstop or Vgo, I am confident that the critical field length is longer at reduced power.

Joe

I just ran the numbers for the C-130 and there were no weights that I saw the CFL at 970 TIT below the 1010 TIT numbers. However, they did get closer and closer the lighter I went. I did not go below the lowest practical takeoff weights.


Midas, if you want to play with the numbers, you can download a C-130 told calculator on the app store. The one with clouds is what the FE's on the 130 unofficially use.

galaxy flyer
12-18-2018, 08:20 AM
At least in the C-5, we used full rated power (TRT) for Vmcg and Vmca calculations because TRT was always available.

GF

cougar
12-18-2018, 10:48 PM
Airlines generally use the term balanced field length, which is the same as critical field length, such that accel-stop and accel-go are the same distance. For Boeing aircraft, only if the balanced field length is less than the runway distance available (TORA, TODA or ASDA) can a reduction in thrust (or unbalancing the field), be considered. In other words, reducing thrust only increases the balanced field length (critical field length). However, this statement is only true for a dry runway.

On wet or contaminated runways, a derated thrust provides a lower VMCG, thus a shorter accel-stop distance. This means for certain contaminated runway conditions, using a derated thrust allows for a greater TOGW than full thrust, and may be the only valid performance solution available. As such, for a given weight, the derated thrust balanced field length is less than the full thrust balanced field length for certain contaminated runway conditions.

Note VMCG is not weight specific. For a 747-400, standard day, VMCG for full thrust is 126 knots, while TO2 (15% reduction) VMCG is 117 knots.

C130 with props may have very different performance characteristics of which I'm unfamiliar.

galaxy flyer
12-19-2018, 07:24 AM
Note VMCG is not weight specific. For a 747-400, standard day, VMCG for full thrust is 126 knots, while TO2 (15% reduction) VMCG is 117 knots.

True and not so true. The FAA defines Vmcg as the most aft CofG and lightest weight, worst case, but doesn’t apply crosswind. Actual Vmcg will be much higher when crosswind is applied. In the 747, it’s about 1.3 knots increase per knot of crosswind. It’s about 1 for 1 in the C-5. If you ever saw the two parked nose-to-nose, you’d see why the difference. I can’t say for the 747 but the Vmcg chart in the C-5 1-1 has about a 20 knot increase for a wet runway.

GF

cougar
12-19-2018, 11:45 AM
There is no adjustment for crosswind in determining VMCG per Boeing performance methods. VMCG is a function of pressure altitude, OAT, and the rated thrust selected. There are however crosswind limit guidelines based on the runway surface contaminant.

galaxy flyer
12-19-2018, 11:57 AM
Boeing in their class manual, Jet Transport Performance Methods, page 17-15,

It’s worth mentioning here that the VMCG constraints shown above do not include consideration of a crosswind. However, a crosswind from the direction of the failed engine would significantly increase the minimum control speed. In the illustration above, a crosswind from the right would be adverse, because it would cause the airplane to “weathervane” – that is, turn its nose toward the direction from which the wind is coming. A crosswind coming from the side opposite the engine failure would be favorable.

Yes, but think of how an upwind engine loss would affect your yaw moments. If you are applying downwind rudder to counteract the weathervaning in the crosswind, you’ll have less rudder to counter the engine swing. FAR 25 used to use a 7 knot crosswind, then dropped it as a certification standard.

Yes, no one accounts for crosswind effect on Vmcg, but it’s there even if it would be a real bad day that you had a strong crosswind and an upwind engine loss near Vmcg. But, any take-off scheduled where V1 is limited by Vmcg should be pause for thought.

GF

cougar
12-20-2018, 01:01 AM
I don't disagree with you that crosswind has an effect on VMCG. All I'm stating is there is no correction in the AFM for crosswind on VMCG. It is simply a function of OAT, PA and thrust in the performance chart. As such, there is no adjustment for VMCG when AeroData computes our takeoff performance. Why there isn't a correction, I don't know.

galaxy flyer
12-20-2018, 06:29 AM
My understanding is first, not computing the crosswind effect means sveryone’s take-off numbers look better, both in the marketing brochures and in the computations. Second, the FAA was convinced by the manufacturers that the chances any engine failure near a minimum speed scheduled take-off (V1 or Vref near Vmcg) AND having a crosswind strong enough to be a factor AND on the upwind was pretty low. Which then brings up the question why do we spend so much sim time doing V1 cuts when statistically they are low probability hazard?

GF

PurpleToolBox
12-21-2018, 11:49 PM
At least in the C-5, we used full rated power (TRT) for Vmcg and Vmca calculations because TRT was always available.

GF


So the C-5 never does a reduced thrust takeoff?

galaxy flyer
12-22-2018, 06:49 AM
So the C-5 never does a reduced thrust takeoff?

No, we used reduced most of the time, but just like the FAA rules, Vmcg and Vmca were based on full rated power, which is why full thrust is available on a reduced thrust take-off.


GF

sourdough44
12-27-2018, 04:37 AM
Oh for the old days when things were simple. That is, push those throttles right to the stops, no matter the conditions.

PurpleToolBox
01-01-2019, 01:13 AM
Which then brings up the question why do we spend so much sim time doing V1 cuts when statistically they are low probability hazard? GF

Because even though the risk is low, the probability of kill if the maneuver is performed poorly is extremely high.

On the 777 yesterday, we had driving rain and crosswinds for takeoff. The software computed a 20% reduction in thrust (TO2) but did not allow for a reduction by the assumed temperature method. My mind started thinking about this thread since we had a significant split between V1 and VR with a stop margin of less than 500 feet. I am glad FDX calculates stop margins.

That is better than previous company. There the only data points we were given for takeoff was power setting, V1, V2, VR and maximum crosswind. Smoke and mirrors to me.

Adlerdriver
01-01-2019, 04:56 AM
No, we used reduced most of the time, but just like the FAA rules, Vmcg and Vmca were based on full rated power, which is why full thrust is available on a reduced thrust take-off.


GF
When we use derated (10% or 20%) power as determined by our performance software by itself or in combination with an assumed temperature method (reduced thrust takeoff), Vmcg and Vmca are computed using maximum derated thrust. So, in those situations (such as the one Purpletoolbox described above), we don’t have assurance of full control authority should we select full rated thrust. The flight manual cautions that a thrust increase during a derated takeoff following an engine failure could result in loss of control. In fact, Boeing recommends not advancing the throttles during an engine out when using the combined derate and assumed temp because the derated max limit the Vmcg and Vmca is based on is not displayed.

tomgoodman
01-01-2019, 06:46 AM
Sneaky sim IPs would give a V1 cut, wait until you are all trimmed up but have not yet retarded the throttle, then suddenly restore the engine. Nobody expected that, because it’s a checkride and an engine-out approach is always next, right? Flailing & wailing could ensue when the “bad” engine came roaring back at full power. :D

galaxy flyer
01-01-2019, 07:21 AM
When we use derated (10% or 20%) power as determined by our performance software by itself or in combination with an assumed temperature method (reduced thrust takeoff), Vmcg and Vmca are computed using maximum derated thrust. So, in those situations (such as the one Purpletoolbox described above), we don’t have assurance of full control authority should we select full rated thrust. The flight manual cautions that a thrust increase during a derated takeoff following an engine failure could result in loss of control. In fact, Boeing recommends not advancing the throttles during an engine out when using the combined derate and assumed temp because the derated max limit the Vmcg and Vmca is based on is not displayed.

No disagreement here. By “full rated” it means at the selected rating, not the maximum without the derate.


GF

cougar
01-01-2019, 10:18 AM
On the 777 yesterday, we had driving rain and crosswinds for takeoff. The software computed a 20% reduction in thrust (TO2) but did not allow for a reduction by the assumed temperature method. My mind started thinking about this thread since we had a significant split between V1 and VR with a stop margin of less than 500 feet. I am glad FDX calculates stop margins.

The stop margin of <500' is likely an example of Derated thrust (TO2) being the only viable solution for takeoff as the TO or TO1 VMCG V1 would result in an Accel-Stop beyond the runway available. Assumed Temperature Method is not allowed for contaminated runways, but is available if the runway is wet.

In my view, on dry or wet runways, TO should be used with an assumed temperature. There is diminishing returns on engine wear below approx 20% thrust reduction. This would allow for full thrust (firewall) if needed throughout the takeoff. Also, assumed temperature is inherently conservative. Due to the differences in TAS, the accel-stop and accel-go distances will be less than what is calculated. Further, the most fuel efficient takeoff profile is maximum thrust, full climb thrust to altitude. At some given thrust setting there is an ideal intersection of fuel economy and engine wear.

TO1 and TO2 should be reserved for contaminated runways for the reasons addressed previously.