B738 increased thrust after liftoff?

[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

[Capt Claret]

Originally Posted by 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.

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)