Why are the outboard Krueger Flaps retracted before the others on the B747?

Question

In this video, I noticed that the outboard Krueger Flaps on this Boeing 747 were retracted while the middle and inner sets remained extended (refer to 3:22 - 3:30 in the video). My questions are:

• Why the staggered retraction?
• Does each leading-edge flap configuration have its own unique setting on the pilot's flap-control lever?
• On approach, are the leading-edge flaps extended in a reverse staggered order?

Please note that I have already reviewed the below Aviation SE questions (and their answers):

• What are the benefits of using Krueger Flaps?
• Is it possible to takeoff without flaps extended?
• Is the A320 family allowed to take off with the flaps retracted?
• If flaps are used on takeoff, when should they be retracted?

But I haven't found anything related to how flaps (leading- or trailing-edge) might be extended or retracted in a staggered fashion like this.

Answer 1

This is from memory and applies to 747-100/200 aircraft, which I last flew in 1999. The video shows, I think, a -400. However, to the best of my knowledge, it has the same flap settings.

The flap selection handle detents and resulting device positions are:

• 0° — leading and trailing edge retracted
• 1° — leading edge inboard of the engines extended, trailing edge up
• 5° — all leading edge extended, trailing edge 5°
• 10° — all leading edge extended, trailing edge 10°
• 20° — all leading edge extended, trailing edge 20°
• 30° — all leading edge extended, trailing edge 30°

At least one carrier, TWA, but maybe others, ordered aircraft with additional setting. I seem to remember 15° and 40°, but the above was the 'standard'.

Why the staggered retraction?

On takeoff, it's all about changing the wing from a low-speed wind to a high-speed wing. Normal takeoff flap setting for the -100/200 was 10°. I think it's 20° for the -400 but I never flew them. As you retract the flaps (both leading and trailing edge), the minimum safe airspeed increases approximately 20 knots for each decrease in flap extension. That gave you a safe bank angle of 15°. Add 10 more knots and you could, as I remember, bank 30°.

So, in the video the airplane is accelerating, and flaps are being retracted in stages as it reaches the minimum safe airspeed for that stage.

I'm not knowledgeable enough to authoritatively explain why, if you want to change part of a swept wing to a lower-lift wing while leaving part of it as a higher-lift wing, you need to make the change to lower-lift from outboard to inboard, but I do know, at least I think, that you must avoid the possibility of the outboard wing stalling while the inboard is still flying. You need to retain outboard aileron control as long as possible. Thus, retract the outboard leading edge devices before the inboard devices.

Does each leading-edge flap configuration have its own unique setting on the pilot's flap-control lever?

No, the leading edge flaps do not have a separate selection. They're tied to the trailing edge flap setting. Note that flaps 1° doesn't actually put any trailing edge flaps down (although I seem to remember it took them out of the retract position and moved them backwards, but I may be wrong), but extends only two of the four leading edge flap sections.

On approach, are the leading-edge flaps extended in a reverse staggered order?

Yes. On approach you're slowing, and you must not slow below the minimum safe speed for your flap setting. Those speeds will be lower than the respective takeoff speeds because you're lighter. I just looked up the fuel for a -400 that I have on my weight and balance web page, and it shows 359415 max fuel. Thus it's not unreasonable that you might be as much as 340000 lbs lighter than at takeoff and the minimum safe airspeed for a given flap setting could conceivably be in the range of 40 to 50 knots slower than at takeoff.

For those interested in more detail about the -400 leading edge devices, go to this incident report detailing an un-commanded retraction of the leading edge devices inboard of the engines. The single paragraph that most caught my attention is:

The serious incident involved the un-commanded retraction of the automatic Group ‘A’ leading edge flaps on rotation for a period of about 23 seconds. Subsequent to the initiation of the retraction of the Group ‘A’ leading edge flaps, the aircrew was faced with unexpected stall warnings. The pilot flying was able to prevent the aircraft from stalling, with support from the other crew members and to keep the aircraft flying until the leading edge flaps re-extended and normal performance capability returned.

Answer 2

Flaps, Krueger flaps, and slats are all ways of modifying the wing's airfoil to effectively increase its camber.

Stalling happens when the air-flow separates. That is, rather than continuing to flow along the surface of the wing, it starts to simply flow away from the wing. It typically happens at high angles of attack.

When the wing's camber is increased, it's effectively operating at a higher angle of attack, even without moving the rest of the plane to a nose-up attitude.

As @Terry pointed out, you want to ensure that if any stalling happens, it happens toward the root of the wings, not the tip. To assure that, we typically want to ensure that the tips of the wings operate at a lower angle of attack than the roots of the wings. There are a number of ways to accomplish that.

One way to do this is to introduce washout--that is, the wing is twisted¹, so the tips are always at a slightly lower angle of attack than the inboard parts. The ailerons also have an effect here though: when the aileron is moved downward, that also increases the angle of attack over that section of the wing. To be fully effective, you need enough washout that the angle of attack with the aileron at its extreme downward position is still lower than the angle of attack at the root of the wing. Otherwise, using the ailerons can stall the tip of the wing--and it's the wing you're trying to raise that ends up stalling, which reduces its lift substantially (more than normal aileron deflection would) and increases its drag. As a result, the plane turns the opposite of the direction you were attempting to turn it.

Washout helps prevent this, but twisting the wing decreases the wing's efficiency all the time, even though most of the time there's no real chance of stalling, so you're losing a lot of efficiency in return for safety you only need a little bit of the time.

Deploying flaps/slats primarily toward the root of the wing increases the camber and effective angle of attack at the root of the wing without affecting the camber/angle of attack at the tip. When the aircraft is cruising, the flaps/slats are retracted, so the wing can fly (reasonably) optimally. Tip stalls are dangerous enough that in most cases, you'll still have a little washout, but substantially less than you'd need if your also had flaps and/or slats operating at the tips of the wings.

---

^{1. Note that twisting the wing is one obvious way to introduce washout, but there are also other ways. For example, theoretically optimal wing loading happens with an elliptical planform. If the tips of the wings have a chord larger than an elliptical planform would, that will normally decrease wing loading, and with it the effective angle of attack.}

Answer 3

Off the the top of my head.. not all parts of the wing generate the same amount of lift and the staggering could be a way of balancing .. equalizing the forces on the wing.

I believe the area nearer the wing tips have a higher lift to drag ration (ie high lift.. low drag) and will generate lift earlier than the rest of the wing. As a result, the requirement for flaps is reduced at a much lower airspeed compared to the thicker (higher drag) part of the wing.

Anilv