Spoiler operation

[flyburg]

Hi all,

I have a question about the B737-800 FCOM text below. I have forwarded the question to our tech department but unfortunately they did not know they reason either so hoping to get some input here.

the operation of speedbrakes upon landing: the conditions mentioned below state that a wheel spin up is required and any gear strut compression for the flight spoilers and right main gear compression for the ground spoilers! However, when no wheel spin up is detected upon touchdown the flight spoilers still deploy on any gear compression and the ground spoilers deploy on right main gear compression!

So, what’s the difference between the two situations. Why is the wheel spin up required? As I read it, weather a wheel spin up is detected or not, the spoilers still deploy under the same conditions! Flight spoilers upon any gear strut compression, ground spoilers on right gear compression!

looking forward to some insight and thanks in advance!



Ground Operation
During landing, the auto speed brake system operates when these conditions
occur:
• SPEED BRAKE lever is in the ARMED position
• SPEED BRAKE ARMED light is illuminated
• radio altitude is less than 10 feet
• landing gear strut compresses on touchdown
Note: Compression of any landing gear strut enables the flight spoilers to deploy.
Compression of the right main landing gear strut enables the ground
spoilers to deploy.
• both thrust levers are retarded to IDLE
• main landing gear wheels spin up (more than 60 kts).
The SPEED BRAKE lever automatically moves to the UP position and the
spoilers deploy.
If a wheel spin-up signal is not detected, when the air/ground system senses
ground mode (any gear strut compresses) the SPEED BRAKE lever moves to the
UP position and flight spoiler panels deploy automatically. When the right main
landing gear strut compresses, a mechanical linkage opens the ground spoiler
interlock valve and the ground spoilers deploy.

[sonicbum]

Suggestion : insert the aircraft type in the subject otherwise You might not get many views on that topic. I believe You are referring to the B737.

Originally Posted by flyburg

Why is the wheel spin up required? As I read it, weather a wheel spin up is detected or not, the spoilers still deploy under the same conditions! Flight spoilers upon any gear strut compression, ground spoilers on right gear compression!

My shot : if a strut compression is not detected for any reason the wheel spin up logic will trigger the flight spoilers deployment as all other conditions are met thus avoiding a situation of no spoilers deployment at all.

[B2N2]

Without getting into detail it’s a fail safe system.
One sensor not operating correctly doesn’t prevent spoiler activation.
So it’s an either/or system.

* Remember also, technical manuals are not excempt from spelling and syntax errors, printing errors and just plain old fashioned Engrish.

[flyburg]

Originally Posted by sonicbum

Suggestion : insert the aircraft type in the subject otherwise You might not get many views on that topic. I believe You are referring to the B737.

Duh, corrected!

[flyburg]

Originally Posted by B2N2

Without getting into detail it’s a fail safe system.
One sensor not operating correctly doesn’t prevent spoiler activation.
So it’s an either/or system.

* Remember also, technical manuals are not excempt from spelling and syntax errors, printing errors and just plain old fashioned Engrish.

understand, but weather you get a wheel spin up or not, the system depends on gear strut extension!

[pattern_is_full]

I think it is a two-phase operation. Ideally, once you get strut compression, then the spoilers only go up once you ALSO get wheel spinup to 60 knots. Compression enables them, wheel spin then deploys them, as a slight planned delay to avoid a hard nose drop (or trouble lowering the nose at all - not sure of 737 flight dynamics with ground-spoiler deployment.) In other words, normally, both strut and spin conditions must be met.

However, if there is a wet/contaminated runway with hydroplaning, and no or slow wheel spinup (and when one really, really needs the spoilers right now for drag and maximum WoW) then the strut compression alone will deploy as well as enable the spoilers once the lack of wheel spin is noted by the system "brain."

The fact the FCOM very carefully uses two different terms (enable, and deploy) means the writer is not using them interchangably. Enable means "capable of deloying" - Deploy means, well - "actually deployed." Armed, enabled, deployed - three different stages of operation.

Strut compression is necessary, but not sufficient - unless/until the system logic figures out the wheels are skidding and not spinning.

[vapilot2004]

There are two things happening, the movement of the SPEED BRAKE LEVER and the actuation of the panels, which (SYSTEM A) are split into two groups, GROUND and FLIGHT.

When you have wheel spin up on any two main wheels, the lever moves UP and the FLIGHT panels extend. Once there is compression on the RIGHT MAIN GEAR squat switch, the GROUND panels extend. If there is no wheel spin up detected, the system will delay deployment until one of the squat switches is compressed.

The advantage is an earlier deployment of the boards during landing, as the wheels will generally spin up before the squat switches activate.

The bypass of the wheel spin up is a belt and braces approach, enabling two protections: allowing spoilers to extend even with an anti-skid failure which processes the wheel spin inputs, and preventing the GROUND spoilers from deploying while airborne.