Speed Brake ARM when landing 737 NG
 

Does anyone know the reason to arm the speed brake before landing IF the 737NG will deplot it anyway based on the sensor pressure done to the right main landing gear?

Not sure about the NG, but on the Classic with the speedbrake lever not in the ARM position, the ground spoilers only deploy when you select reverse thrust.

right but I'm refering specifically to the NG version.

My theory is that the speed brake is put in ARM position as a contingecy only in the case of the sensors in the right main gear fails.

Mmm let's see..

From what I understand.. ANY strut will deploy Flight spoilers & Right strut will deploy the ground spoilers. And that's a reason you get the occasional smooth landing on crosswinds :}

In case you didn't forget to arm the speed-brake, that's what you need to deploy them on landing:
-Lever in ARMED & Green light on
-Less than 10 feet RA
-Landing gear compressed
-IDLE thrust levers
-Wheel spin up of more than 60kts

In case you forgot to ARM the speed-brake:}, the automatic speed-brake will come up to help you when:
-IDLE thrust levers
-Wheel spin up of more than 60kts
AND
-thrust levers positioned for reverse thrust

For the LDR calculations, Boeing assumes that Reverse Thrust to interlock will be selected 2 seconds after touchdown. Which means that, using the speed-brakes based on the "Forgot to ARM" scenario (& assuming you are fast on that Reverse) it gives you a delay of 2 seconds compared to the normal "Speed-brake armed" scenario.

Now, looking further into those 2 seconds.. Boeing wants an expeditious application of those speed-brakes not only because they are more effective on high speeds; but they also dissipate the lift on the wings and in turn it puts the plane down firmly on their wheels so the brakes can do their job.

Not to forget the Manual Speed-brake correction on dispatch which reduces the field length limited weight and the extra landing distance required on the inflight calculations.:8

Hope that helps :ok:

Braking effectiveness is reduced as much as 60% without speedbrake deployment.

Rapid deployment of speedbrakes is vital and it's main job is to reduce lift. The aircraft can very quickly become airborne again if speedbrakes do not deploy on main gear touchdown. Auto deployment can also be inhibited by touching down with thrust on which also has the same effect of increasing the risk of a bounce.

You do not want to train/rely on the application of reverse to raise the speedbrakes, as pulling the reversers commits you to a landing and they can also be deployed below 10' RA.