Agh, its annoying to read something that is completely incorrect, so I’m back…..
Ssg, I suggest you educate yourself by reading FAA AC25-13 and FAA AMC 25-13. The Boeing training course presentation entitled Reduced Thrust and the Airbus “getting to grips with flex thrust”. You will then see that your statement of:
[quote] The engineers don't take it into account because it doesn't exist as a benefit.....so using your logic...the slower we go (flex acceleration) the more lift with a given time/distance?[\quote] Is incorrect. But I will stress that the comparison is between a Take Off at a specific OAT compared to Assumed OAT, it isn’t a comparison between Full Rating and Flex.
For this we are using a B777 at a weight of 235,000 kgs (That’s 518,086 lbs SSG..) S.L. Airport with 11,500 feet runway, no obstacles, Fixed Derate of 10%. That’s about average for us to do a 6 hour sector under our normal conditions.
What Pugilistic Animus was trying to point out to you is the difference between taking off at with OAT 50C, compared to OAT 25C assumed to 50C.
OAT 50C
Takeoff Distances
All Engine Takeoff Distance = 9680 FEET
All Engine Takeoff Run = 9045 FEET
One Engine Inoperative Takeoff Distance = 10055 FEET
One Engine Inoperative Takeoff Run = 8994 FEET
Accelerate-Stop Distance = 10055 FEET (1445 feet runway available)
OAT 25C assumed to 50C.
Takeoff Distances
All Engine Takeoff Distance = 9552 FEET
All Engine Takeoff Run = 8922 FEET
One Engine Inoperative Takeoff Distance = 9943 FEET
One Engine Inoperative Takeoff Run = 8897 FEET
Accelerate-Stop Distance = 9943 FEET (1557 feet runway
You can see that there is a 100 feet difference in the accelerate stop.
If we were going to use Maximum Thrust all of the time, the figures would be:
OAT 25C
Takeoff Distances
All Engine Takeoff Distance = 6298 FEET
All Engine Takeoff Run = 5783 FEET
One Engine Inoperative Takeoff Distance = 6930 FEET
One Engine Inoperative Takeoff Run = 6244 FEET
Accelerate-Stop Distance = 6930 FEET
So Accel Stop = 6,930 feet whilst with DER1/Assumed it would be 9,943 feet. It’s this difference in runway length that we are trading against reducing the takeoff thrust. But remember we still have 1,557 feet of runway left.
Even with full 10% fixed Derate and 25% Assumed Temperature Derate, we get this:
Takeoff Distances
All Engine Takeoff Distance = 10764 FEET
All Engine Takeoff Run = 10088 FEET
One Engine Inoperative Takeoff Distance = 11050 FEET
One Engine Inoperative Takeoff Run = 9857 FEET
Accelerate-Stop Distance = 11050 FEET
So even with 35% of a thrust reduction, we can still operate a 6 hour sector, and if in the event of an engine failure at VEF, we could continue the takeoff or stop with 450 feet of runway remaining.
So as you can see, we are never scraping over the fence on the way to immediate doom.
Finally to expand on what Pugilistic Animus was trying to explain to you, this comes from Boeing, I don’t know the aircraft type. It should be split into 3 columns, so it might be easier for you to paste it into excel due to formatting restrictions in Pprune.
Details / OAT40C / OAT15C Assumed 40C
EPR / 1.376 / 1.376
V1 (IAS/TAS) / (147/153) / (147/147)
VR (IAS/TAS) / (155/162) / (155/155)
V2 (IAS/TAS) / (162/169) / (162/162)
Thrust @ V1 /30960/31210
Thrust @ VR / 30610/30880
Thrust @ V2 / 30300/30570
So the inherent benefits that PA was taking about are shown in the speeds IAS vs TAS and the available thrust.
Using reduced thrust reduces the jet engines internal operating pressures and temperatures, which results in:[list][*] Reduced Stress and wear on the engine.[*] Reduced costs on parts and maintenance[*] Increased Engine Life[*] Increased Reliability[*] Improved operational safety and efficiency.
These words are taken straight from Mr Boeing….
That should keep you busy for a while….
Mutt (No time to edit)