Hello all,

As SLF with a mere PPL and background in aviation safety, I have a question out of simple curiosity for the technically informed a.net group.

From time to time in my experience as an airline passenger in multiple types, I notice the following scenario: On the arrival or in the approach phase, with thrust apparently at idle (AFAICT), we’re descending with speedbrakes extended. Then we level off, evidently to reduce speed, with the speedbrakes remaining extended, and I can feel the g’s associated with this transition from descent to level flight. The speedbrakes remain extended throughout this maneuver. Within maybe ~30-45 seconds, after reducing speed,* we continue the descent with the speedbrakes’ position unchanged. Of course, being in row 5 or 35, I can’t state definitively the speedbrakes’ position relative to their flight detent, but they’re visibly extended.

But for purposes of this question let’s assume that they ARE at the flight detent or very close to it. It seems to me that a level-off that exerts 1.x g’s on the airplane—coupled with the downward force exerted on the wings by the speedbrakes—creates a significant load on the wings and supporting structures. I think about it, in crude terms, like doing a push-up with an extra heavy object on my back.

So I guess I’ve got a couple of questions. (1) Do any pilots out there apply a technique of retracting speedbrakes before a level-off, even if you’re leveling off to reduce speed, in the interest of minimizing stress on the airframe? (2) More to the point, are the wings and other airframe structures designed to take the load of transitioning from descent to level flight with speedbrakes extended?

Or is my analysis flawed? Am I overestimating the speedbrakes’ downward force on the wings? (I remember that, on the MD-80 series, when the speedbrakes popped up even a little, there was a visible downward little “jerk”.)

Many thanks for your input! It’s a question that has bugged me for years.

*As a pilot I know not to trust my inner sense of deceleration to know we’re reducing speed. FlightAware and FR24 help tremendously in reconstructing vertical profiles and corresponding speeds.

If you need to slow upon reaching the altitude, leave the brakes extended until about 10-15 knots above the desired speed. The spoilers don’t create downforce, they just reduce lift, in that sense reduce stress. The wing will stall earlier with boards up. The weight supported by the wing doesn’t change and +1.xG isn’t significant in a wing designed for 2.5G in service and ultimate factor of 1.5 times that.

The change in lateral lift distribution doesn't make much of a difference in stress on the wing. As to why crews level off and then keep going down with the boards out, it's usually a combination of speed restrictions on the STAR or a speed limit (e.g. 250 under 10k in the US) and needing to bleed excess energy. Unless winds play a big role, you want to go down to denser air first, then use that denser air to slow you down. Let's say your plane can do 350 KIAS and you need to get down NOW. You put out the boards, stuff the nose down to maybe 340 to give you some buffer, then keep going. If you need to meet the 250/10 constraint, you level at 10.1, bleed off to 245, and then stuff the nose back down.

As Galaxyflyer says, it's for deceleration. Either way, we are VERY far from the limits. The plane is built to take a lot more than that and the speedbrakes are a rounding error in context.

The rule of thumb in the A330/A350 is you lose 10 knots per nautical mile clean, and twice that with full speedbrake. Those boards come in handy.

Spoilers do not create an appreciable downwards force on the wing. On the contrary, they create drag and reduce the lift vector, which essentially unloads the wing.

If you don't increase the AoA to compensate, then you increase your rate of descent. If you do, then you increase your longitudinal deceleration.
And if you transition from one state to the other, as in your example, then you 'reload' the wing and feel the upwards acceleration (slightly increased g) that you mention in your scenario.

G loads are G loads regardless of attitude. A turn in level flight will produce a G load in excess of 1.0. Transition from a constant descent to level flight can also produce a G load but the airframe will not differentiate between the two scenarios so there is no concern. I would doubt that you have ever experienced more than 1.4 G on an airliner.

With regard to leveling off with the speed brakes extended; it's understandable if there is a need to reduce speed before continuing descent. However, I would take exception to leaving the speed brakes extended while leveling off and increasing thrust to maintain speed and altitude. This is like driving with one foot pressing on the brake pedal. Bad technique and likely the pilot not aware that the speed brakes remain extended while thrust is being increased either manually or by autothrottle. This I have experienced as a passenger (and pilot not flying) too many times.

I don’t think anybody’s saying advance thrust with the boards, which is a bad idea

113312 wrote:

G loads are G loads regardless of attitude. A turn in level flight will produce a G load in excess of 1.0.

So yaw dampered / fully coordinated turn will -- at level flight -- always increases G Load beyond 1.0? How come the passengers typically don't even feel a banking turn?

Yes, any level turn will increase G load, the normal maximum of 30 degrees of bank is about 1.1-ish G. Cosine of the bank angle, IIRC. Not really noticeable until about 45 degrees (1.4G) and much at 60 degrees (2G)

https://aerospaceweb.org/question/perfo ... 0146.shtml

GalaxyFlyer wrote:

I don’t think anybody’s saying advance thrust with the boards, which is a bad idea

At least a handful of times, I've had to crack the boards at cruise, and leave them extended from top of descent, just to make max landing weight. One Mexican airport kept giving us short cuts on the arrival, after we had told them we wanted to fly the full procedure to make MLW. Spent the last 60 miles with the gear down, flaps 15, and the speed brakes 3/4 or so, and made MLW by a couple hundred pounds.

AABusDrvr wrote:

GalaxyFlyer wrote:

I don’t think anybody’s saying advance thrust with the boards, which is a bad idea

At least a handful of times, I've had to crack the boards at cruise, and leave them extended from top of descent, just to make max landing weight. One Mexican airport kept giving us short cuts on the arrival, after we had told them we wanted to fly the full procedure to make MLW. Spent the last 60 miles with the gear down, flaps 15, and the speed brakes 3/4 or so, and made MLW by a couple hundred pounds.

I’ve seen that act, always a fix

First off, thanks everyone for the info. I was wondering about the structural loading associated with a level-off from, say, negative 3,000 fpm to level flight with the speedbrakes extended to, say, meet a published or ATC-imposed speed constraint on the arrival, but my error was clearly in thinking of the speedbrakes as exerting a substantial downward force instead of pure drag. I learn something every day here. Thanks for all the replies.

And to the ancillary comments about speedbrakes with more-than-idle thrust: A buddy who used to fly for Comair (the defunct US regional, not the SA airline) way back in the day told me they would have to reach MLW more often than they should have done by deploying speed brakes against thrust in level flight or in slow descent to burn off fuel because dispatch was relatively fast and loose with the release numbers. I don't think I've ever experienced that type of maneuver as a pax - never noticed speed brakes extended with thrust more than idle - but from my buddy I know it happens out there.