737 Speedbrakes

[RGPILOT]

Does anybody know if there is any restrictions to use speedbrakes in b737 classics in an intermediate position between DOWN DETENT and FLIGHT DETENT ???

[CaptainSandL]

Yes, you are advised not to use intermediate speedbrake (ie all or nothing) because it will increase your roll rate more than full speedbrake.

[LEM]

This happened recently in the sim: while going back to departure airfield in emergency, the Captain passed the control to the FO.

The FO deployed half speed brakes because he felt he needed so.

The Capt. took control again, just before intercepting the LOC.
He hadn't noticed the speedbrakes were partially extended.

As a result, they flew the whole LOC- circling approach with the speedbrakes half extended!

ROFL

Had they been fully extended, that would have been noticed.

On more modern jets, a master caution is triggered if you advance the thrust levers with speedbrakes extended.

[I-2021]

that's why a nice thing to do when using speedbrakes is to keep one hand always on the lever exept when you must use it for another action (call the FA for a cofee or stuff). :ok:
:8

[alexban]

there is a recommendation to use only flight detent position for the speedbrakes.
The reason,as i've heard, is this: the spoiler mixer is calibrated for two speedbrakes positions -retracted and flight detent. If you use them at a different position while in straight flight no problem will arise.However,if you do this in a turn,there is a chance for a wrong deployment of the spoilers during turn (to assist the ailerons) ,calculated by the spoiler mixer with an uncalibrated speedbrakes position. This can result in an sudden ,higher than normal,roll.
It happened to me once,not so funny,and since then, I use only flight detent position,and I try to extend and retract the speedbrakes only during straight flight:}

[LEM]

Hi alex.
David Davies has explained this behaviour very well in his "Handling the big jets".

That's not a calibration thing, as you've heard, but the very design of the system.

There are two kind of spoilers: defferential and not.

On the first type, the one we have I presume, when the spoilers are half extended, if you enter a (right) turn the panels on the (right) wing will extend more, and the panels on the (left) wing will retract, thus causing a high roll rate.

I've experienced this as well flying manually, the rate was quite high.
You can try again anytime.

On the NON differential type, the spoilers on the (left) wing will stay where they are, and you won't notice any increase in roll rate.

This could be used to your advantage while flying an approach with degraded flight controls, with one channel blocked and one of the pilots operating only one aileron, for example.
You could keep the spoilers extended a little bit to increase a poor roll rate capability.

Cheers, LEM

[jetsome727]

LEM, your analysis is absolutely correct. The theory applies to any large jet with differential spoilers.

[md-100]

Is there any restriction or limitation with the speedbreak and flaps deployment?

[I-2021]

md-100,

check this thread. Anyway, speedbrakes should not be deployed with flaps greater than 15.

[CaptainSandL]

Hi LEM,

The 737 does have differential spoilers, I have just taken this series of photos to demonstrate.

I-2021 I think the limitation is no speedbrake with flaps greater than 5.

S&L - Still learning about the 737 !



http://www.b737.org.uk/diff_spoilers.jpg

The above series of photographs (737-300) show how the flight spoilers move with various combinations of aileron and speed brake. With speedbrake down, the spoiler simply rises on the down-going wing with aileron. With speedbrake applied, not only do the spoilers on the down-going wing rise but also the spoilers on the up-going wing fall. Notice that even with full speedbrake applied the spoilers still rise on the downgoing wing.

This property of the spoilers on both wings to respond to roll inputs is known as differential spoilers. It only occurs when speedbrake is used which is why the roll rate is increased when speedbrake is used. Boeing recommend that speedbrake is not used below 1000 feet for this reason.

NB In the bottom two photographs the speedbrake lever was only at the flight detent position but because the aircraft was on the ground the ground spoilers deployed. This is why if you have any sort of technical problem that might be due to a faulty air-ground sensor eg QRH "Gear Lever Will Not Move Up After Takeoff" you must not use speedbrake in case the ground spoilers deploy in-flight. You can see from the series of photographs just how much extra drag ground spoilers will give over flight spoilers.

[LEM]

Notice that even with full speedbrake applied the spoilers still rise on the downgoing wing
That's quite interesting, didn't know about that.

Your great pictures eventually shed light over a little known aspect of the 737.
Thank you for your effort!

Ps: The Training manual states on page 4.17 : "The use of speedbrakes with flaps extended should be avoided, if possible.
With flaps 15 or greater, the speedbrakes should be retracted."

Cheers, LEM

[CaptainSandL]

Thanks LEM.

You are correct it is flap 15 in the FCTM, I was remembering a previous company SOP.

S&L