View Full Version : speed brake arm 757
lenstrad
7th May 2004, 18:32
does anyone knows why in a 757 the speed brake deploys only after the nose wheel of the airplane is in the ground
Probably because it doesn't!
It deploys on truck tilt sensor. :ok:
Perhaps the incident you saw, the Auto Speedbrake was not working for one reason or another... In that case, the PNF would have deployed it manually, and by the time he did that, the nose wheel was on the ground....
As Dogma says, when it's armed, it works by using the 'weight on wheels' principal....
:)
Bealzebub
8th May 2004, 13:28
I am assuming you mean the Auto speedbrake function, as the speedbrakes deploy in the air as well as on the ground.
When Auto speedbrake is armed the lever is moved to the up position and all the spoiler panels (including ground spoiler panels 4 & 8 ) deploy, when 2 conditions are met. The thrust levers must be at idle and the main truck sensors detect a no tilt condition (aircraft on the ground). It doesn't use the weight on wheels principal ( whatever that is) The nose air/ground sensing system uses strut compression sensors ( probably what the poster alludes to) but in any event that is not the source for the auto speedbrake system. The main gear trucks utilize tilt sensors activated as the trucks level out on touchdown and it is these that the auto speedbrake system reads. For that reason the nose doesn't need to be on the ground for auto speedbrake deployment.
As an aside the use of thrust reversers on the ground ( the only time they can be used normally) will cause the auto speedbrake system to deploy and the lever to move to the up position whether the system is armed or not.
In the answer given above a failure in the auto speedbrake system would still enable the speedbrakes to be deployed manually on landing, although this is usually done by the PF as application tends not to be as smooth as the automatic system. Of course different airlines may operate different practice in this regard.