B738 increased thrust after liftoff?

[Beirut pilot]

Please help an observant youngster here...

Flew with AB with their 738s some days ago and noticed something i really never experienced before... Say about 30 seconds after liftoff the engine power increased notable... It did the same on the outbound leg too...

Why? I thought that T/O thrust never could get below CLB?

Thank you,

BP

[ImbracableCrunk]

I think CLB is automatically selected to be less than TO, not the other way 'round.

[Capt Claret]

On the b717 when Using Flex takeoff powers, if one flexes over about 50 degrees C, then takeoff power will be less than climb power, so once the aircraft passes through the acceleration altitude, power is increased to climb power setting.

I agree with you, if feels and sounds weird. :8

[misd-agin]

With long runways and light weights it's not uncommon to have takeoff power to be less than climb power. When climb power is selected the power advances(increases).

[safelife]

AB actually use "derate 2" for take off, which is less than climb thrust.
Flex thrust normally cannot be less than clb thrust, at least for all the aircraft I flew.

[Beirut pilot]

Okey, interesting. Thanks for your replies.

The captain told us during the climb that our TOW was 55T...Runway length 4000m...

So yeah, the ''light weight'' could explain it.

[AerocatS2A]

The captain told us during the climb that our TOW was 55T...Runway length 4000m...

That is an odd thing for the Captain to tell the passengers. :suspect:

[aviatorhi]

Maybe it's common sense kicking in but why on earth anyone would set do a takeoff at a power setting which is less than that required for climb is beyond me. :ugh:

[Wizofoz]

' Cause it's not actually what is "Required" for climb, it's just what is programmed into the EEC or FADEC.

[Capt Claret]

Maybe it's common sense kicking in but why on earth anyone would set do a takeoff at a power setting which is less than that required for climb is beyond me.

Why?

The company produces manufacturer endorsed performance figures, that allow a certain level of flex (assumed temperature) power (haven't used de-rate so can't comment) which meet the takeoff performance requirements, for the given runway, atmosphere and gross weight.

[Piltdown Man]

...and on an Embraer when using Take Off 2 or 3. Someone did some difficult sums some time ago and calculated that using the highest possible climb power would save us a fortune in fuel. And according to recent feedback, the sums were right.

PM

[captplaystation]

Current company uses min (determined by EFB ) take-off thrust possible, often DeRate 2 (22k from a 27k engine) & max assumed temp , + improved climb, so take-off N1 = something close to high idle :rolleyes:
However. . .this is purely to preserve engine life & apparently power setting after airborne has no detrimental effect on eng life. So given the most economical in fuel burn is to use full climb thrust (if continous climb expected) to shorten time in climb , it goes like this. . . . . . .

Take off at derate 2/55c or whatever . . 800' full climb thrust :cool: indeed feels odd, but that is what they want us to do.

[WallyWumpus]

B 737-800. CLB thrust is frequently more than T/O thrust.

[de facto]

Current company uses min (determined by EFB ) take-off thrust possible, often DeRate 2 (22k from a 27k engine) & max assumed temp , + improved climb, so take-off N1 = something close to high idle

Is your airline making any money?:E

[Denti]

AB uses user selected full climb thrust if a unrestricted climb is likely. Independent of take off reductions full climb thrust is therefore the normal thing and that is indeed nearly always a thrust increase at climb thrust reduction altitude (1000' AGL). Nowadays it feels weird to experience a real climb thrust reduction, for example when deadheading on the airbus fleet.

[misd-agin]

Lots of airplanes have reduced climb power also. CLB 1 or CLB 2 are both less than regular CLB power. CLB 2 is in the range of 10-15% less than CLB power.

CLB power uses less fuel getting to altitude but the tradeoff is a higher EGT.

And the a/c that use the lowest power of the multiple engine 'choices' along with flex/derated power takeoff with a power setting that's about the same as cruise power. :eek:

[Denti]

That is true for the 738 as well, however the pilots can always user select the full climb thrust regardless. And that is exactly what AB, and apparently cptplaysations outfit as well, do. According to Boeing and CFM it has no negative impact on engine life and saves some fuel which accumulates to several million Euro a year quite fast.

[FlyingStone]

Current company uses min (determined by EFB ) take-off thrust possible, often DeRate 2 (22k from a 27k engine) & max assumed temp , + improved climb, so take-off N1 = something close to high idle

You use de-rate + assumed temperature with improved climb to save engines on the NG? We use A/C OFF or improved climb only when we can't get out with A/C Auto - not to save money by using less than full takeoff thrust(Classic).

[Penworth]

I fly the 737-800. We always derate and use an assumed temperature if possible and this automatically selects a reduced climb 1 or climb 2 thrust. Unlike other contributors however, my airline wants us to leave the reduced climb thrust in as apparently it does save engine life (at the expense of increased fuel burn) and with our lease agreement, increased engine time on wing saves far more money than the increased fuel burn costs.

[plain-plane]

Use of reduced thrust:

On T/O: Reducing T/O thrust, significantly reduces engine wear, keeps greater EGT margins long term- keeps the specific engine operating more efficient.

On climb: CFM/Boeing say fuel savings, climbing at full thrust outweighs long term engine wear.
-----

CLB = full trust for the climb

CLB1 = an approx 10% derate of thrust (3% N1), (gradually increasing up to full thrust at approx 12000' i think)

CLB2 = an approx 20% derate of thrust (6% N1), (gradually increasing up to full thrust at approx 15000´ i think)

Also the aircraft "should" be preprogrammed so that Climb thrust does not exceed T/O thrust unless selected on CDU N1 page.


So to save the company money, reduce thrust as much as possible for T/O and climb at full thrust once above acceleration height if you are expecting unrestricted climb to above 12-15000'

[PENKO]

So what is the use of flex TO when you will get a higher N1 once safely away from the runway?

[aerobat77]

it may be as easy as ATC requesting to expedite climb due to traffic separation.

in real you have at initial climbout other things to observe than making an expertise out of an expedite.

cheers

[RTO]

So what is the use of flex TO when you will get a higher N1 once safely away from the runway?
At this stage you save fuel by climbing faster, for a slight tradeoff in higher egt.

[aviatorhi]

@CaptClaret,

You're going to be pushing the throttles up anyway when you get to climb, why not set power at or above climb in the first place... worst case scenario you have better runway and initial climb performance. Which doesn't sound terrible to me at all.

[Capt Claret]

aviatorhi,

Nup. Press AUTOFLIGHT button once satisfied engines accelerating at similar rate early in the takeoff run (generally before 30 kias). Auto throttle system sets flexed take off thrust, then at the appropriate altitude sets climb power.

No push pull at all.

If Mr Douglas/Boeing/Airbus says reduced thrust take offs save on engine life and therefore dollars, who am I to say to the Co, "stuff that, I've got to used climb power at some point so I'll use it for takeoff, and be damned that I don't have performance data to back me up"?

[aviatorhi]

On the equipment I fly I never use an assumed temp over 38 and that results in never requiring a power increase for climb power.

Now if the bean counters have the professional outfits by the proverbial b***s to the point where the Captain can't make that call I'm happy to not be at a place like that.

[de facto]

On the equipment I fly I never use an assumed temp over 38

I guess the tropical island where you work from is quite a cool one then:E

[Capt Claret]

On the equipment I fly I never use an assumed temp over 38 and that results in never requiring a power increase for climb power.

Often I don't use an assumed temp over 38 either. But have to contend with actual temperatures up to 48.

I have the option of making the call should I want but why? What's so wrong with increasing power to climb power? The performance figures say it can be done. If it goes to mud one can increase to ref power if required. Why second guess the performance figures, just because you "feel" it's better. Using the same "feeling" we should make every takeoff a full power one, from the start of the runway, rather than flex at all, or consider an intersection takeoff.

[aviatorhi]

Just because something can be done doesn't mean it makes sense to do it.

[barit1]

The engine guys are likely to propose this:

The transient (taxi idle to TO) is a major parts-life consumer. Using the highest FLEX temp (lowest EGT) minimizes this transient, and that's a good thing for engine life.

Once the engine has been running at TO for a minute or so, it's pretty well warmed up, and thus a push up to CLB is a minor EGT transient; and if that push saves trip fuel, with negligible wear & tear, then that's a good thing.

In other words, breaking the whole EGT picture down, splitting the big transient into two parts is an optimized operating cycle from a hardware standpoint.

(Even though the average pilot finds this counterintuitive...)

[Denti]

Indeed, that is pretty much what those CFM guys told us. Even cruise thrust at optimum level is higher than fully reduced take off thrust nowadays. Which is basically a second warm up period.

[plain-plane]

@aviatorhi: Just because something can be done doesn't mean it makes sense to do it.


Very good point, but here is an even better point: The Engine experts say it is a good idea, and they have the facts to back it up...

BTW(common sense will also back this up, be nice to your engines and they will be nice to you; last longer and use less fuel- that should keep both you and the bean counters happy)