Hi, Can't find it in my FCOM but in what conditions will the speedbrakes deploy on a RTO? I know one of them are "reverse thrust" but any other conditions and where are they listed?

 

I rejected my takeoff below 80kts but speedbrakes did not deploy even though i used reverse thrust.

 

Cheers!

I rejected my takeoff below 80kts but speedbrakes did not deploy even though i used reverse thrust.

 

I think it's above 60, with reverse thrust. Either way, it should be part of the memory item actions.

Yeh manually raise the speed brakes before intiating reverse thrust. But it got me thinking what are the conditions

The FCOM reference is in Vol 2, page 9.20.18. As Kyle said, 60 knots.

 

RTO auto braking occurs above 90 knots with thrust levers at idle, it's possible that this speed has been used for the speedbrake deployment as well by mistake. Worth rechecking.

 

edit, speedbrakes deploy if rejected above 60 knots for me.

Thanks guys got it

Not that it matters much below 80 KIAS, they are there to kill the lift generated by the wings, but there is hardly any at that speed.

Not that it matters much below 80 KIAS, they are there to kill the lift generated by the wings, but there is hardly any at that speed.

They also create a lot of drag, though at 60 knots that will only be a quarter of what it is as 120 knots.

Kevin, is there an aerodynamics formula (seems awfully close to the inverse square law of light) to calculate that ?

 

I remember that the speedbrakes--called spoilers back then--were very small on the Boeing 707-320b, considering the size of the wing and weight of the aircraft, I would dare say that the 737-800 probably has more area per wing. They were strategically placed over the wing and in line with the flaps, where the wing camber was greater when they were lowered to any setting. Back then there was only talk that the main function on the ground was to transfer the weight of the aircraft to the wheels for more effective braking--the effect as a brake mechanism negligible below 80 kts. I have noticed that more emphasis has been placed of speed brakes surface in recent model aircraft.

Kevin, is there an aerodynamics formula (seems awfully close to the inverse square law of light) to calculate that ?

 

 

It is a function of the square of the velocity of the fluid (or air) as well as several other factors: https://en.wikipedia.org/wiki/Drag_equation

 

Google is your friend.

Kevin, is there an aerodynamics formula (seems awfully close to the inverse square law of light) to calculate that ?

 

I remember that the speedbrakes--called spoilers back then--were very small on the Boeing 707-320b, considering the size of the wing and weight of the aircraft, I would dare say that the 737-800 probably has more area per wing. They were strategically placed over the wing and in line with the flaps, where the wing camber was greater when they were lowered to any setting. Back then there was only talk that the main function on the ground was to transfer the weight of the aircraft to the wheels for more effective braking--the effect as a brake mechanism negligible below 80 kts. I have noticed that more emphasis has been placed of speed brakes surface in recent model aircraft.

It's a square law, not an inverse square law. All other things being equal aerodynamic forces increase with the square of speed.

 

Spoilers are the individual panels. "The speedbrake" is those spoilers being moved symetrically. the main reason to deploy them on touchdown is indeed to put weight on the wheels by destroying lift. But on ground the spoilers are deflected to a much greater angle than they are in flight and so they also create even more drag than they do in flight,

Ground spoilers make a significant contribution to stopping the aircraft on landing. You can test this for yourself by not arming the speedbrake before landing and not using reverse thrust or brakes. Then try the same landing with the speedbrake armed. Without the speedbrake you won't lose much speed. With speedbrake deployed you decelerate quite rapidly.

They can actually increase the downforce. Think of the vectors and the Newtonian effect of having that air deflected upwards. More weight on wheels = more brake effectiveness = greater stopping ability.

We did an excersise in the sim where we did three rejected takeoffs.  The first was normal with spoilers and reverse thrust, the second with spoilers only and the third with reverse only.  Not using reverse only added 200-300 feet to the distance used, not using spoilers added around 1200-1300 feet.  It really drove home the importance of making sure the spoilers get deployed promptly.

JoeDiamond, at what airspeed did you reject the takeoffs in these excercises, was it just before V1 ?

JoeDiamond, at what airspeed did you reject the takeoffs in these excercises, was it just before V1 ?

 

I don't recall the exact speeds but it was at max weight for runway 20R at KSNA using flaps 25 with the reject right at V1.

Joe diamond, after a full stop after an abort before V1 do you set the parking brake (captain) and the FO sets flaps 40 when stopped? My SOPs call for this!