Flight Controls Check with Flaps Extended
 

Why do we extended Flaps and then do the flight control check on the B737, before taxi?

Is it just to standardise the SOP or is there a technical reason behind it?

Isn't it flight controls before flaps? This puts less demand on the hydraulic system at the same time as well

One can always hear in the cabin if the pilot has done the flap extension before the flight control check as you can hear the hydraulic motors extending the flaps vary in pitch as the available hydraulic flow varies during the flight control check.

It could be something to do with time saving. Boeing say you can perform the flight controls check once the "Green Light" on the flaps has illuminated. If you perform the control checks first then select 1,5,15, or even 25 you could be there for another 1-2 minutes.

The following is from our SOP.

Flap lever .................................................... Set takeoff flaps ..... F/O

Verify that the LE FLAPS EXT green light is illuminated.

Flight controls ............................................................ Check...... CPT

Thanks Aviator & 900ER...... both your inputs makes sense.

What if there is some sort of mechanical failure that happens when the flaps are extended that ends up resulting in the controls being affected? Best to find this out during the control check instead of at or immediately after rotation.

I think Boeing standard procedure is flight controls check first then select flaps which is what we do, although some people ignore the SOP, select flaps then do the flight controls check while they are running.

On the NG I don't think there will be a big issue with either way to be honest, you may find classic variants don't cope so well but I may be wrong.

...if you paid attention in your type rating...

If the flaps are up, the rudder is limited in travel. You need flaps extended to check full travel. Simples..

If the flaps are up, the rudder is limited in travel. You need flaps extended to check full travel. Simples..

I don't know, but does that also affect the rudder pedal displacement or only the rudder itself?

As far as I am aware the rudder movement is not restricted on the ground by the flap position.. maybe RVF750 can provide a ref for this?
Maximum rudder panel deflection is approximately +/-15 degrees on the ground, reducing to around +/-8 degrees at a typical cruise altitude.
When rudder centering units are fitted they don't test with flaps down.As always keen to learn..ref please

I did pay attention during my type rating and nothing was taught regarding that. Aside from that it's company policy for us to configure that way so Boeing has no objection.

Elevator neutral position changes with flap ext. Don't have a reference, sorry, but you can feel the CC move when flaps travel. Perhaps relevant?

No, it's only limited above 1000ft AGL on takeoff and below 700ft AGL on landing, unless N1 difference between engines is at least 45%.

I very much doubt it.

That's on the Classic. The NG has a flow limiter which gradually reduces hydraulic pressure above 135kts.

The reason that flying controls are checked after the green light is to minimise hydraulic load on System B.