why does this happen before taxi to takeoff?

[kellwolf]

I would think that on a sunny day the air conditioning packs would have to work extra hard and thus use fuel for running them.

I don't think the PACKs burn any more fuel when it's hot than when it's comfortable outside. It's all off of bleed air from the engines. You don't spool the engines up to cool things off any more. Sure, it'll make the air blow out of the nozzles faster, but it also makes the plane move faster.....

 

[mrivc211]

Depends on the plane. On ours, the PACKs go off since the bleed air is used to start the engines. As far as the lighting, I don't think it gets turned off unless the FA turns them off.

Yup.

 

[Cessnaflyer]

I don't think the PACKs burn any more fuel when it's hot than when it's comfortable outside. It's all off of bleed air from the engines. You don't spool the engines up to cool things off any more. Sure, it'll make the air blow out of the nozzles faster, but it also makes the plane move faster.....

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?

 

[Baradium]

I don't know how you fancy big jet guys do it, but in the 1900 we shut off the environmental part of our bleeds for takeoff.

 

[Maximilian_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.

So is this a pre taxi checklist or something?

 

[B767]

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]

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.

 

[Maximilian_Jenius]

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]

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]

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]

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.