Bjorn’s Corner: Aircraft engines in operation, Part 4

By Bjorn Fehrm

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.

Figure 1. Principal picture of a direct drive turbofan. Source: GasTurb.

We also touch on de-rating and Cost Index and how these affect how the engine runs.

Take-Off thrust rating

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.

Derated take-off

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.

Climb rating

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).

Cruise rating

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.

Cost index

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.

Max Continuous thrust

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.

Certification ratings

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.

9 Comments on “Bjorn’s Corner: Aircraft engines in operation, Part 4

  1. You write the engine is artificially derated during take off. Is that not a potentially safety risk when you give up a margin that would be available in case of an emergency?

    • Hi Matth,

      the derate that you enter in the FMS contains margins. The calculation of what derate to use both for take-off and climb/cruise is set by the aircraft take-off weight calculation together with the OAT + altitude of the airport and available runway length and any obstacles present.

      The preparation for flight is a rigorous process where all these values are set based on the aircraft’s manuals certified data. Establishing of this data is part of all the test which is done during the 2,500h flight test to certify an airframe/engine combination.

  2. In a contemporary design why does the system not simply allow the pilot to input the aircraft weight, auto detect runway length using GPS and then self-determine the TO thrust? Fudging by pretending the air temperature is different seems archaic.

    • As Matt pointed out the two derates are a bit different. Derate changes all speeds at TO, FLEX doesn’t. You could have the FADEC and FMS do all these calculations and settings but it’s in the end the Captain who is 100% responsible for all the settings. OEMs and Certification authorities are very careful with changes which affects this responsibility.

      You have a procedure that works today with responsibilities clearly defined and wetted by time. You don’t change that easily.

      • Thanks Bjorn. Do you feel there is much (or even any) pressure to change from the current system, or is it likely to satisfy all for the forseeable future?

  3. Hi Bjorn,

    Thanks for this very interesting series!

    Can I please suggest to more clearly distinguish in the terminology between

    -“derate” (changing the engine rating for takeoff/climb, meaning that restoring full thrust is prohibited for the takeoff) and

    -“reduced thrust” (by assuming a higher temperature, meaning that restoring full thrust is permitted when required).

    I am fully aware that in normal day-to-day operational speak, both methods are dubbed a derate, but if specifically discussing engine operation, I feel this is an important point to consider.

    Thanks very much & best regards,

    • Thanks Matt,

      I should have pointed it out. I changed the text a bit to reflect that.

  4. £4billion loss rumoured for Rolls Royce. As Bjorn has written about already (behind the paywall, so I can’t read it),the engine’s are all sold at a significant loss and the profits come from servicing.I can easily see a lot of RR A380s,A340s and B777s being scrapped early particularly if engine maintenance costs are uncompetitive. How will they ever get their money back? Add in plenty of technical problems and compensation plus massive bribery fines.

  5. It seems to be generally accepted that an aicrafts engines can have a lower EDR (Engine Deterioration Rate) through derating. Some airlines will opt for the derated A350-900 R. In what way is the 900R derated. Im wondering what combination of cruise, take off and max continuous thrust etc. would be derated for the 900 R.

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