Let's restrict this question to airliners with 2 jet engines.

For the remaining of the question, I'll use the term flex to designate reduced take off thrust as I'm more familiar with airbus's systems. Flex will be the thrust selected when thrust levers are in the flex detent and climb thrust the one selected when lever are on the "climb" detent without autothrottle engaged. I assume other manufacturers have an equivalent way to easily select those values.

Both flex and climb thrust are lower than TOGA thrust. In good condition (long runway, good weather, almost empty aircraft,...), how low can flex thrust be? Can it be lower than climb thrust?

In this case, what is the rational for accelerating from flex to climb thrust given flex is made to increase engine lifespan by reducing thrust?

  • $\begingroup$ Are you specifically asking about Airbus? FLEX thrust is an Airbus term, Boeing calls this Assumed Temperature Method. $\endgroup$ – Bianfable Feb 19 '20 at 8:32
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    $\begingroup$ @Bianfable I'm not asking for a specific aircraft. I should have speak of reduced take off thrust but flex was more convenient to write (this is a really bad excuse) (edited accordingly) $\endgroup$ – Manu H Feb 19 '20 at 8:35
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    $\begingroup$ Post-edit (at least) this is a very clear and well written question. $\endgroup$ – FreeMan Feb 19 '20 at 15:28

Yes. On the CF-34 family, in theory, a maxed-out flex setting can be in the mid 80s N1 whereas climb thrust is normally high 80s/low 90s, and you could see thrust rise when setting climb if an extreme flex setup (max Assumed Temperature) (or in the case of the non-fadec engines like the 3B1, the climb bug appears at a higher value than the flex take-off thrust bug when you select "climb" thrust in the FMS)

The reason is to do with what you might call the mission priority of the two settings, and where the actual benefit in flex lies.

With flex, the priority is distance and gradient for obstacle clearance. The flex setting is the lowest setting that meets the takeoff and initial departure performance requirement for accelerate/stop and initial climb gradient for that runway and all up weight. The benefit of using the lowest setting is only partly from lower temperatures; an even bigger factor is erosion from silica particles (outdoor dust) which is most severe below 1000 ft. The dustier the environment, the larger the flex benefit. (In the Middle East, CRJ operators were wearing out engines after only 5-7000 hours because they were doing max thrust takeoffs all the time and had to be cajoled into using flex.)

Climb thrust has a time priority, from an operational balance-of-factors perspective. You want to get to altitude in an optimized time-related profile (as soon as possible taking various things into account), and an optimized time-related profile can require more thrust than the minimum required to meet take-off performance, in cases where a very large flex allowance is available allowing minimal take-off thrust. Setting climb thrust that is a little higher than the flex take-off setting has little negative impact on engine life because you are now high enough that the erosion impact is negligible so there's little engine life benefit to a slower climb to altitude and you might as well optimize climb thrust to get there faster.

  • $\begingroup$ It sounds weird. Given the distance criteria can be met with the climb setting, why not use it from the beginning? $\endgroup$ – Manu H Feb 21 '20 at 7:30
  • $\begingroup$ I didn't think to include the biggest factor in flex usage which is physical erosion of the engine. I added some bits that may help clarify things more. $\endgroup$ – John K Feb 21 '20 at 14:00
  • $\begingroup$ Is the Middle East remark an anecdote? As of the 2018 census, literally only 8 CRJs (all variants) were active in the ME (it's never been popular there unlike in NA). Plus, no ops in this day and age would need to be "cajoled" to use flex. $\endgroup$ – ymb1 Feb 21 '20 at 14:57
  • $\begingroup$ Direct experience. I used to be on the program. When a Libyan and and Iraqi operator started running 900s about 10 years ago or so they were complaining pretty bitterly about how fast their engines were wearing out. Their capts generally declined to use flex and their airlines didn't require them to. GE and BA tech support had to convince them they had to use it if they want their engines to last anything like they expected in the fine sand environment. $\endgroup$ – John K Feb 21 '20 at 18:56

Can Flex thrust be lower than climb thrust?

For Airbus, no, it cannot. Because as you've noted, there's no rationale for it.

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Flex allows there to be various settings for Takeoff power. Climb thrust is less than Takeoff power (Takeoff is usually time-limited whereas Climb is not), but there can be several settings for Climb thrust to ensure that today's Climb is less than today's Takeoff.

Boeings use Climb, Climb1 (-10%) and Climb2 (-20%) and the FMC picks the suitable Climb power to go with today's Takeoff power. So a full derate will end up with Climb2, whereas a partial derate may need allow Climb1.

  • $\begingroup$ Does it mean that the lowest achievable thrust for reduced takeoff thrust is climb2 on Boeing's aircraft? $\endgroup$ – Manu H Feb 19 '20 at 10:32
  • $\begingroup$ @ManuH No, you can reduce takeoff thrust by up to 25% from the rated thrust (legal maximum) by combining a thrust derating with a high assumed temperature. $\endgroup$ – Bianfable Feb 19 '20 at 12:00
  • $\begingroup$ @Bianfable thus take off thrust (max -25%) is lower than climb thrust (climb2, max -20%). Am I correct? $\endgroup$ – Manu H Feb 19 '20 at 13:03
  • $\begingroup$ @ManuH I am sure that the lowest reduced TO thrust (-25% of rated thrust) is lower than the full climb thrust because I have actually experienced a thrust increase in a 737 when going to climb thrust. I am however not sure if Climb2 (-20% of full climb, not of rated thrust) would be even lower. I couldn't find anything in the manuals, so I didn't answer. $\endgroup$ – Bianfable Feb 19 '20 at 13:06
  • $\begingroup$ @Bianfable your last comment do answer my original question. Now I fail to see the rationale of using climb thrust greater than reduced take off thrust. Why not using climb thrust from the beginning? $\endgroup$ – Manu H Feb 19 '20 at 13:27

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