why does this happen before taxi to takeoff?

[B767]

Originally Posted by Maximillian_Jenius

Anyone ever notice that before flight on EVERY 737 that I've ever been on. Or viewed pushing back from the gate that all the control surfaces move before flaps and slats are extended.

Now to save someone some typing I'm aware that pilots check flight surfaces prior to flight. But I'm talking EVERY 737 I have seen, all control surfaces are moved. Then immediately afterwards the flaps/slats retract to take off position.

Other planes I have observed or been on typically do a control test while taxiing but after slats/flaps are already retracted.

They retract the flaps? Do they extend them too far and then retract them for t/o or was it just mistyped?

[kellwolf]

Originally Posted by Cessnaflyer

Then if you use less bleed air then you wouldn't need as high a thrust setting for takeoff. Does it not work this way?

APU. That's what it's for. If you need the thrust (which rarely do you need that much for takeoff), then you just takeoff with the APU running. I'll leave the APU on for takeoff on a hot day just to keep the cabin a bit cooler. There is no using "more" or "less" bleed air. You can't set the PACKs to take out a certain amount of bleed air. They get what the engines give, and that's it. They don't make the engines work any harder, and they don't burn more fuel. If you wanna keep it cool, keep the APU on. It doesn't burn that much gas anyway. Having the PACKs on doesn't really affect your t/o thrust setting. The engines are there to make the airplane go. The bleed air extracted for environmental reasons is just a bonus by-product.

[Maximillian_Jenius]

Originally Posted by B767

They retract the flaps? Do they extend them too far and then retract them for t/o or was it just mistyped?

Yes, a mistype. I meant extend!

[CameronF]

Originally Posted by Cessnaflyer

Then if you use less bleed air then you wouldn't need as high a thrust setting for takeoff. Does it not work this way?

I would agree as well.

[Cessnaflyer]

Originally Posted by kellwolf

APU. That's what it's for. If you need the thrust (which rarely do you need that much for takeoff), then you just takeoff with the APU running. I'll leave the APU on for takeoff on a hot day just to keep the cabin a bit cooler. There is no using "more" or "less" bleed air. You can't set the PACKs to take out a certain amount of bleed air. They get what the engines give, and that's it. They don't make the engines work any harder, and they don't burn more fuel. If you wanna keep it cool, keep the APU on. It doesn't burn that much gas anyway. Having the PACKs on doesn't really affect your t/o thrust setting. The engines are there to make the airplane go. The bleed air extracted for environmental reasons is just a bonus by-product.

But either way it will use slightly more fuel and that is what Ryan air is trying to save on. Any little bit helps.

[FlyboyZR1]

Originally Posted by Maximillian_Jenius

Other planes I have observed or been on typically do a control test while taxiing but after slats/flaps are already retracted.

So is this a pre taxi checklist or something?

It's all part of the taxi checklist specific to the company and airplane. On the CRJ after the flaps are extended we check the primary flight controls (spoilerons included). However if it's a first flight of the day the ground and flight spoilers are also checked.