Question for Flyboy: thrust increase after take-off

[Gabriel]

Hi Flyboy, I have a question...

A couple of months a go did a "business tour" in which I took 7 flights in 4 days, all of them with Southwest, so all 737s (but I suspect the same would happen with the A320).

In ALL of these flights, a couple of minutes after take off the thrust was increased, at about the time the airplane configuration became clean.

Now, I think I understand how that happens: Runways are so much longer than needed for a 737 that they allow for such a reduced take-off thrust that it ends up being below the climb thrust. So when it is time to "reduce" the thrust from take-off thrust to climb thrust, that "reductions" ends up being an increase.

My question is... What is the point of reducing the take-of thrust to the point that it is below the climb thrust? Why not use the climb thrust as a lower bound for the reduced take-off thrust? Or, if there are real benefits in reducing the take-off thrust so much, why not to keep climbing with that reduced thrust (at least to some point, let's say 10K ft)?

[flyboy2548m]

You surprise me, Your Engineerly Amazingness! An engine that's at a lower thrust setting produces less what? Hint: said "what" is much less of a factor at higher altitudes (such acceleration height and up).

I will tell you the situation you described does NOT happen on the 320, at least not that I've ever seen. It did happen occasionally on the E170/-175.

[Gabriel]

You surprise me, Your Engineerly Amazingness! An engine that's at a lower thrust setting produces less what? Hint: said "what" is much less of a factor at higher altitudes (such acceleration height and up).

I will tell you the situation you described does NOT happen on the 320, at least not that I've ever seen. It did happen occasionally on the E170/-175.

Tear and wear? Fuel consumption and CO2 emissions? Noise?
Nah, I don't think that the engine will produce much more of these things at 0ft AGL than at 1500 or 3000 ft AGL (at the same thrust setting, that is)

Interesting that I was wrong about the A320. I wonder why. Maybe the A320 does have this policy of not using a reduced take-off trust that is under the climb thrust? How often does it happen that the thrust does not reduce when it is switched from FLX to CL?

[elaw]

Unless I'm missing something, "noise" would fit the profile. You take off with as little thrust as you can safely get by with, in order to keep the whiners who bought cheap houses near the airport from complaining about the noise. Then when you've put some distance between yourself and their ears, you throttle up.

[Not_Karl]

An engine that's at a lower thrust setting produces less what? Hint: said "what" is much less of a factor at higher altitudes

Suction. Sucking birds is much less of a total air disaster factor at higher altitudes.

I will tell you the situation you described does NOT happen on the 320, at least not that I've ever seen.

That's why they suck geese on take-off and have to land on water.

You're welcome.

[flyboy2548m]

Unless I'm missing something, "noise" would fit the profile.

Indeed.

[3WE]

When we level off at cruise, it gets a lot more quiet in 32A on a 737-MaxLav...the noise thing is very real.

[Gabriel]

Thank you. Wonder why the A320 doesn't follow the same "rule" then. Is it intrinsically less noisy than the 737-NG?

[flyboy2548m]

Thank you. Wonder why the A320 doesn't follow the same "rule" then. Is it intrinsically less noisy than the 737-NG?

I don't know that it's necessarily a 320 vs 737 "rule". It could be that our performance vendor does not calculate a flex temp high enough to where FLX falls below CLB, whereas WN's does. In any event, the E170/175 is quieter than either the 737 or the 320, but, as I have said, I have seen that situation on that airplane at times.

I suppose this is part of a greater debate of what flex is, how it works, and what it does, which is a debate I won't get into lest it give me a major headache. I will leave that to you and Tweedledumb.

[3WE]

320 vs 737 flex temp high FLX CLB, WN E170/175 737 320, debate of what flex is, how it works, and what it does, debate major headache.

Thank you.

I will ass-ume that you are not unnecessarily over stressing your engines in any significant way and will continue to focus on drooling on the window, when the flaps are set, and trying to deal with that God-Awful pull up angle on takeoff that reminds me of practicing power on stalls in a 172 at 3000 feet, as opposed to 300 feet where YOU do it.

[elaw]

...that God-Awful pull up angle on takeoff that reminds me of practicing power on stalls in a 172 at 3000 feet, as opposed to 300 feet where YOU do it.

Ah, you're just jealous. Toss a couple of CFM56 engines on your 172 and you could do the same thing!

[3WE]

...that God-Awful pull up angle on takeoff that reminds me of practicing power on stalls in a 172 at 3000 feet, as opposed to 300 feet where YOU do it.

Ah, you're just jealous. Toss a couple of CFM56 engines on your 172 and you could do the same thing!

No!

It's a foffie thing: What if one of the engines exploded and I was below Vmc? The 172 rudder isn't THAT huge and it's so easy to envision stall, spin, crash, burn, and did died.

HOWEVER, what I WOULD do is to ignore noise abatement procedures, can you imagine the NEW sound of a 172 taking off?

Actually, instead of CFM56's about a couple of JT8-Ds?

[elaw]

That's a tough one. The JT-8's would fit under the wing better (thus avoiding any mounting weirdness that might tempt the manufacturer to install a Multiple Crash Achieving System to compensate) and have that cool "retro sound". OTOH the 56's tend to explode less.

[3WE]

That's a tough one. The JT-8's would fit under the wing better (thus avoiding any mounting weirdness that might tempt the manufacturer to install a Multiple Crash Achieving System to compensate) and have that cool "retro sound". OTOH the 56's tend to explode less.

Good thoughts.

If we keep the aeroengineers and computer systems out of it, I'll feel safer.

[Gabriel]

aeroengineers out, I'll feel safer.

Hey!