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?

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.

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

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.

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.

At least in the C-5, we used full rated power (TRT) for Vmcg and Vmca calculations because TRT was always available.

GF

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.

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

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.

Boeing in their class manual, Jet Transport Performance Methods, page 17-15,

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