Pack2,
Can anyone explain why more fuel is used when flex thrust takeoffs are used ? -
IT DEPENDS.
Like some, I'm not sure if you're addressing the Takeoff in isolation, Takeoff and Climb, or Sector Fuel.
Any discussion relating to fuel saving or penalty in making comparisons between optional thrust settings must consider optimum Thrust Specific Fuel Consumption (TSFC), with the optimum typically ocurring in the vicinity of Climb Thrust, around about 93%. Any more or less than optimum TSFC RPM and you burn more fuel per unit of thrust produced. It must also be borne in mind at all times, that the primary goal in using Thrust Reduction for Takeoff (and Climb) is economics, i.e. increased engine life at the lesser expense of time and fuel. Safety, is of course, a huge factor as Reduced Thrust is much less likely to precipitate engine failure.
Takeoff - Full Takeoff thrust is at an engine speed somewhat above optimum TSFC, thus a Full Thrust Takeoff is "fuel expensive". At the lowest typical Flex / Reduced Thrust, particularly if the Flex is applied to a lower RATING, the RPM is well below the optimum, and is again "fuel expensive". If fuel saving during the Takeoff was a consideration (It is NOT), the Takeoff would be best conducted at a modest Thrust reduction from Maximum, where the N1 was about 93%. As I've said on previous occasions on this same topic, you have to be a bit of a nit picker to find it. Normally, Takeoff thrust to a nominal 1500' is for only about 2 to 2.5 minutes, such a short period of time that any 'off optimum' TSFC would barely be noticeable. If you're an operator that usually uses modest thrust reductions (medium / long haul) you MAY notice a small improvement in fuel used. If, however, you are typically operating Very Long Haul (Full Takeoff thrust) or short haul (large Thrust reductions) you may well notice a small fuel penalty.
Climb - As optimum TSFC is typically close to Climb Thrust, any Climb reduced thrust will suffer fuel penalties. This is where fuel penalty becomes noticeable, because the 'off optimum' TSFC is applied for a MUCH longer time. As an example, the B777 with RR Trent has 3 Takeoff thrust ratings, TO, TO-1, and TO-2, with 3 associated Climb thrust ratings, namely CLB, CLB-1, and CLB-2. Climb at CLB-1 or CLB-2 costs additional fuel, but saves on engine maintenance. Above 10,000 feet, CLB-1 and CLB-2 revert to full Climb thrust (we can take the pain no longer).
Having said that, the climb speed chosen for climb is probably of far greater significance. The fastest way to 'kill' your sector fuel is to climb at a too low speed. Many jurisdictions do not have the 250 KIAS limit below 10,000 feet, and upon passing 5,000 feet (my company's 250 KIAS limit) immediately accelerate to normal climb speed, about 320 KIAS. (I can sense
Canuck Birdstrike about to swoop). At the first Cruise Waypoint, I regularly note Fuel is UP by about 500 Kg, whereas it is Zero gain if the 250 KIAS is maintained to 10,000 feet. Sector fuel is all about fuel consumed per mile, NOT the lowest fuel flow, or the fuel consumed for the climb itself without considering the distance covered.
Optimum Sector Fuel, will require a climb a little faster than the best Rate of Climb speed. Rate of Climb depends upon excess Power (NOT Thrust), and, in the vicinity of the greatest delta between Power Required and Power Available, the PA and
PR curves have only slight convergance until Mcrit is reached. The upshot of this is that, FOR THE JET, a change of climb speed of (typically) up to 20 to 30 knots has only a small impact on rate of Climb, whilst the ground covered for the same Fuel burned will vary almost directly with the speed. Typical figures for the B777 are Best Rate of Climb = 300 KIAS, Best Econ Climb = 320 KIAS. At 320 KIAS, Rate of Climb decreases by only 2%, whereas ground distance covered increases by about 7%. If the B777 were to climb at Best Rate speed instead of 320 KIAS, it would reach TOC less than 1 minute earlier, and some 25 miles sooner. Now, it will have to cruise those 25 miles to the point it would have reached TOC at 320 KIAS, and have burned more fuel to that point (a little saved on the climb, a lot more wasted for the incremental cruise).
If you want to put the windy statement above into simpler terms, Climb is the most sustained high fuel flow regime of our flight, so, if you're going to suffer a high fuel flow, cover as much ground as possible during the climb. Tolerate a very small ROC penalty, and cover a lot more ground for the fuel used.
Regards,
Old Smokey