Why would the Speedbrake be required for such a long time on approach? [duplicate]

Question

As far as I was aware, the Speedbrake is designed to slow the aircraft down. On landing, I do understand why they are required. Once the aircraft has hit the tarmac, they are deployed to 1) slow the aircraft and 2) prevent the aircraft from bouncing up and down. The airflow over the wing increases doesn’t it? To ‘push’ the aircraft down, as I say, preventing it from jumping up and down.

I have (7 hours ago) just landed on a B757, but the Speedbrake was up pretty much for 70% of the descent and approach.

Thinking back to my previous introductory paragraph, the reasoning must have been excessive speed, no? Or a need for the aircraft to slow down dramatically. Is this the case, or am I missing something obvious?

I know Air France flight 447 considered deploying the Speedbrake when their Airbus gave them faulty warnings of speed and altitude; it was advised then that this would have been completely the wrong thing to do.

I have never seen it before, as much as I did today on descent, and I wonder why the Speedbrake was required for near on 70% of our approach?

Answer 1

There are two reasons I can think of off the bat:

• Its possible the controller asked them to hold a slow speed due to increased traffic ahead. The brakes may have been deployed to match the speed requested. I have heard this called on occasion over the radio in the terminal area I fly.
• The controller cleared them for a steeper decent than usual this would cause them to potentially deploy the speedbrakes to make the descent rate.

This article discusses it in reference to small planes but idea is the same. And another article by a CFI here.

Answer 2

Those are spoilers...not speed brakes. They do increase drag, but their primary function is that they kill lift. They allow the aircraft to lose altitude rapidly without pointing the nose downhill and picking up speed.

You will also see them deploy upon touch down. At high speeds they increase drag and thereby aid in deceleration, but again they kill lift making the weight on wheels higher allowing for much more effective wheel braking.

In the case you mention, the aircraft could have been held at a higher altitude longer than desired, and the pilot simply used them to get back on schedule.

Here is an example of a true speed brake. The sole purpose being to increase drag.

Answer 3

Speed brakes are often used on aircraft that are very clean (i.e., have very low drag), and, as a result cannot descend at a steep enough glide angle even with power at idle to satisfy minimum descent gradients for some specific phase of flight. This is especially true on final approaches, where the approach glide path may be steeper than the aircraft can descend with idle power and maintain a constant airspeed.

Another more legacy reason was in the early days of jet aircraft, centrifugal flow and early axial flow turbojet aircraft had poor fuel control systems and could not increase engine rpm very fast. In some aircraft, it could take 20 to 30 seconds to accelerate the engine from idle to full power. As a result, flying approaches in idle was very dangerous. In these cases speed brakes were used to allow the aircraft to be flown on normal final approach glide path angles with the power set at higher, medium throttle positions so that unexpected power requirements (e.g., for go-arounds) could be satisfied in less time.

Answer 4

Those are "flight spoilers" which may be standalone or may be the landing lift dumpers doing double duty, or a combination thereof, and are used to increase sink rate, not as speed brakes as such. They were being used there because the descent profile in the arrival clearance was steeper than an engines-idle glide. There are times when you have to use them but in general you try to avoid using them because when deployed you "wasting" energy and are below the most fuel efficient descent profile. Some airlines discourage pilots from using them unless really necessary because they can alarm passengers. You won't generally see them on final approach because the idle descent with gear and flaps down is more than steep enough so that some power is required, and on some jets there is a limitation that requires you to be somewhat above Vref (flaps down approach speed), say Vre +15 knots or something like that, to deploy them. If you see flight spoilers being extended on final with gear and flaps down, somebody probably messed up.