50% speedbrake? (737)

[mmrassi]

Hi,

I noticed those 737s which are equipped with winglets has the 50% mark on the speedbrake lever.

Can anyone explain me the reason why this mark in over there?

Thanks

[Trent 972]

Boeing Aero No.17

Retrofit of the 737-800 commercial airplane includes a load alleviation system to obtain full use of the speed brakes to the in-flight detent position during typical airline operations. The system, which is installed in the flight deck aisle stand, arms at heavy weights and high speeds at extreme portions of the flight envelope. When armed, the system actuates the in-flight speed-brake handles and retracts them to 50 percent.

[Fullblast]

http://www.737ng.co.uk/B_NG-Flight_Controls.pdf

Some more info, page 18.

[president]

From Boeing.com:
The mid- to outboard portion of the wing was designed for speed-brakes-up maneuver loads of 2.5 g. Loads in this area can be lowered by reducing the in-flight speed-brake angle. The reduction in the acceptable speed-brake angle depended on airplane utilization by the operators: The angle was reduced by 50 percent for the BBJ; the 737-800 commercial airplane required full use of the speed brakes to the in-flight detent position for emergency descent certification requirements. For 737-800 retrofits, a load alleviation system was developed to reduce the speed-brake angle automatically at heavy weights and high speeds for critical design load conditions. For airplanes in production, a strengthened wing allows for full speed-brake capability to be retained. Figure 10, which shows the net load reduction from changing the toe angle and reducing the speed-brake angle, depicts how structural changes to the wing were minimized.

[mmrassi]

Thanks for the info,

I saw this on a 500 series plane (equipped with winglets). According to what I read on the replies, can we say that because of having a winglet and a low drag characteristics, we have to use load alleviation system to avoid high load in high gross weight/speed conditions. I mean IF we have a 737-800 which has not winglet so we don't need this system anymore also.

[porch monkey]

Simply depends upon whether the airframe was built for winglets or whether they are an aftermarket fitment. One has the structural strengthening, one won't. If it doesn't, then the load alleviation feature will be active.