February 10, 2017, ©. Leeham Co: We now continue our journey how an airline engine is operated during a typical mission.
Last week we explained basics for engine control and Take-Off flat-rating. We now continue with Climb, Cruise and Max Continuous ratings and why these are important.
We also touch on de-rating and Cost Index and how these affect how the engine runs.
Last week we explained that engines are flat-rated for Take-Off. The aircraft and the pilot can then rely on getting the same maximum Take-Off (TO) thrust up to the “Kink point” temperature, which for most engines is 15°C over ISA, i.e., 30°C.
For lower rated engines in a family, like the CFM56-5B4 (a 27klbf version of the 32klbf -5B3), there are more margins in the turbines. Therefore the “Kink point” can be higher; it’s 45°C for the -5B4. Conversely, for the highest rated engine in a series, especially “thrust bumped” ones (i.e., stretched in retrospect for some reason), the Kink can be lower, e.g., 25°C.
There is a time limit for how long the TO thrust can used by the pilots, normally five minutes. This is enough to reach the “Thrust reduction” altitude after take-off, normally 1,500ft above the airport.
In case of an engine failure during take-off, this time can extend to 10 minutes. This is enough for climb on the remaining engine(s) to an altitude where excess fuel can be dumped, followed by a landing.
As we showed in previous Corners, the take-off is the most stressing part of the mission. You only need maximum TO thrust for a heavy aircraft taking off from a short runway. When there are margins, either in TO weight or runway length, the pilot will program the engines to deliver less than the maximum rated thrust at TO throttle setting.
This is called de-rating or thrust reducing the engines, dependent on which method is used. It’s can done by setting a lower thrust level in the FMS (Flight Management Computer, called de-rating) and/or artificially raising the outside temperature the Engine Control Computer use when calculating the thrust (Temp Flex thrust reduction).
The last aircraft I piloted, the Bombardier CS300, had both settings. The TO thrust derate set the engine between the approved ratings (e.g., if the aircraft had a 23.3klbf engine and there was a 21klbf version, I could set that level as the mission only needed 21klbf TO thrust). Then, in addition, I could set the OAT (Outside Air Temperature) to 35°C while real temperature was 20°C. This gave the engine a further thrust reduction during take-off.
The difference is that derate limits the thrust for the take-off. All speeds are adjusted to this new maximum thrust level. When reducing the thrust further with FLEX temperature the speeds are not changed, one can still command maximum thrust by moving the throttles to the max position (called TOGA which stands for Take-Off and Go Around).
This is done if for instance an engine goes inoperative. The take-off speeds (all speeds below V2, the safety speed) are therefore adapted to a single engine flight after liftoff with one engine at TOGA thrust level. A TOGA thrust creates more asymmetric thrust than an engine at FLEX setting and therefore requires a higher speed to counter the yawing moment with the rudder.
When reaching the thrust reduction altitude, the throttles are moved back to Climb thrust rating. This is below an engine limit called Max Climb thrust, which is a turbine temperature profile the airframe and engine OEMs agreed on.
The step down in turbine temperatures guarantees the engines can stay on wing the projected time between overhauls. At the same time, the thrust enables the aircraft to climb to a good initial Flight Level (typically FL330-FL350 for an Airbus A320 with the CFM56).
The cruise rating is once again agreed between airframe and engine OEM. It shall enable the aircraft to keep its different cruise Mach numbers (Maximum range, Normal and High speed cruise) at the Flight Levels that are optimal for the aircraft (typically between FL330 and FL410 for an A320).
The cruise rating is not stressing the engine thermally. The long times at cruise power rather affects the erosion and deposit deterioration of the engine.
The engines are also derated during climb and cruise, this time by the set Cost Index in the FMS. The Cost Index (normally between 0 to 99) controls how hard the aircraft is flown against its limits. A low Cost Index saves fuel and the engines by flying slower (less drag), but the lower speed will increase the time in air. Costs which are time-dependent will rise (crew cost, time based maintenance costs, aircraft utilization), whereas fuel and stress dependent maintenance costs will reduce.
These trades are all captured in the Cost Index program in the FMS and the airlines will have Standard Operational Procedure (SOP) which prescribe at what Cost Index different missions should be flown.
When an aircraft get trouble with an engine at cruise, the remaining engine(s) Max Continuous rating will decide at what minimum Flight Level the flight can be continued. This is important if the route is over mountainous terrain and will decide which routing an airliner can take (the pilots must always plan for losing an engine).
The Max Continuous rating is the highest thrust the engine can run at for an undetermined time without damage. The rating is therefore higher than all other ratings except the Take-Off rating.
When an engine is certified, the authorities are only monitoring the Take-Off and Max Continuous ratings. Both have a flat rated thrust level at sea level in the Certification documentation with “Kink point” temperatures. Maximum climb and cruise thrusts are agreed thrust ratings between engine and airframe OEMs. They are not certification relevant.