MD-11 Bounce

[nikitos_567 1]

I head that Md-11 was plagued with horizontal stab, problems, because it was short and positioned too up. That's why there have been accidents. I am wondering why else MD 11 could bounce on landing and why is it generally considered a really hard airplane to land apart that it has above normal high approach speeds.

 

Thanks

[Rations 102]

Welcome to AVSIM!

 

Perhaps it is also due to the small tail and third engine? I am not very familiar with the MD-11, so I am wondering the same thing. Is it difficult to land from the perspective of an Airbus or Boeing pilot, or would an MD-11 pilot transferring to the B747 find the two equally difficult to land?

[PHXMD-11 86]

I head that Md-11 was plagued with horizontal stab, problems, because it was short and positioned too up.

 

A long fuselage reduces the size of the stabilizer. The longer the fuselage the less force is required to change the attitude.The problem with the MD-11 is the aft center of gravity. This further reduced the requirement for a large stabilizer. However, in an effort to reduce drag, it also reduced the stability of the aircraft. Stability is acquired when the center of gravity is forward of the center of lift and the horizontal stabilizer pushes the nose down to provide stability. The wing needs to be able to provide enough lift to offset the aircraft weight and the downward force of the stabilizer (which is creating negative lift). So these create induced drag so the more forward the CG is, the greater the drag is. The further forward the CG is the more quickly the aircraft returns to its trimmed attitude (which is stability). The further aft the CG is (closer to the center of lift) the more unstable the aircraft is. If the CG and center of lift converge, there would be no stability and an upset would continue at whatever attitude the upset started.If the CG were to be aft of the center of lift the plane is uncontrollable because an upset would cause the plane to continue in the direction of the upset.

 

The MD-11 is really just a stretched version of the DC-10 with a DC-10 wing. And the MDD designers thought that they could tinker with the above paragraph and they moved the CG range closer to the center of lift in order to reduce drag. So the MD-11 is touchy at slow speeds.The longitudinal stability augmentation system will autotrim the aircraft to provide best performance and will help the pilot maintain stability during low airspeed conditions, such a takeoff and landing.

 

When you are designing an airliner, you MUST make it as safe as you possibly can. Boeing (even though the 787 has some problems at the moment) and Douglas know/knew this. The thing is McDonnell only had experience in military aircraft. So when McDonnell took over Douglas, they took shortcuts on how to design the MD-11. She was designed like a fighter. The goal was to meet the specifications, not safety. If the MD-11 had been designed by another company, I don't think we would see the problems that the MD-11 has.

 

Perhaps it is also due to the small tail and third engine?

 

I am not a pilot so I cannot comment on the actually flying the 747/MD-11/Airbus. This is just my commentary on something.

 

But it does not have to do with the number two engine. It has to do with poor design on McDonnell Douglas' part because of the wing and the center of gravity.

[JOR 0]

I think the major problem was with all the accidents (except the SW and LH ones) is because of a very critical (delta)wing. They all flipped over the wing which means that one of them had too much lift again. IMO an extra set of spoilers more to the wing tip would have made less trouble.

 

The AoA at the TD fase is pretty high. With a bounce you have 2 problems. No awareness of the height and you have to spool up the engines again which takes time.

 

But then again, when you land it like you suppost to do, there is no problem. Keep up the good speed and AoA in the flare and do it on time.

 

@PGMXD

As far as I know the MD11 is a complete new aircraft, only based on the DC10. So it also came with a new wing.

[BlueStar 323]

I head that Md-11 was plagued with horizontal stab, problems, because it was short and positioned too up. That's why there have been accidents. I am wondering why else MD 11 could bounce on landing and why is it generally considered a really hard airplane to land apart that it has above normal high approach speeds.

 

Thanks

 

There is nothing wrong with the MD-11. Those who criticize it have never flown it. I liked the "10" better because the FE made my workload easier. The "11" is like an ex- "fun to fly but not forgiving."

 

Billy Bluestar

[PHXMD-11 86]

As far as I know the MD11 is a complete new aircraft, only based on the DC10. So it also came with a new wing.

 

She wasn't re-designed from the wheels up. McDonnell Douglas was facing competition from Boeing and Airbus.They thought that going for cheap would mean more profits. They took shortcuts in the design to make the aircraft as cheaply as possible. Look what happened in the late 90s

 

It was the same wing as the DC-10 but with some upgrades. Other than the winglets and the dropped ailerons, it was esentially the same wing.

[nikitos_567 1]

Why is MD-11 has a tendency to bounce when landing, is it because of the stabilizer problems or high approach speed?

[PHXMD-11 86]

Why is MD-11 has a tendency to bounce when landing, is it because of the stabilizer problems or high approach speed?

 

The MD-11 can bounce because of stability issues (refer to the first part of my first reply), high approach speeds for the give conditions (runway wet/dry/contaminated, gross weight, winds), tendencies to overpitch/flare more than normal, windy conditions, pilots not flying the aircraft correctly.

[E6BAV8R 88]

Why is MD-11 has a tendency to bounce when landing, is it because of the stabilizer problems or high approach speed?

 

For starters, the horizontal stabilizer on the MD-11 is essentially smaller than it ideally should be; mainly as a result of its' shared components from the DC-10. The reason the stability and oscillations on landing are so bad on the MD-11 is because of the wings' location and spoiler affect upon touchdown. There are multiple reasons for this including: better efficiency in cruise flight, better longitudinal stability in flight, position of the CG in flight (to maximize fuel efficiency), etc... In short, they positioned and designed the wing in such a way to maximize efficient and stable cruise flight. However, as you see now and the by-product of those efforts, the airplane is rather unstable when it gets slow (e.g. on final or after initial touchdown in the event of a bounce) or in those other situations where the aircraft is at its' slowest.

 

Getting back to your point of instability on landing, essentially what happens on the MD-11 is that when the the airplane touches down and the spoilers extend because of weight-on-wheels, the spoilers shift the center of lift backwards enough to pitch the aircraft upward - this is also why the aircraft has a specific de-rotate system upon weight-on-wheels, to assist in this). However, if quick action is not taken, the aircraft can lift back off the ground (just as a bounce) and now the aircraft has lifted off, is incredibly slow and way below Vref, the throttles would be at idle with landing flaps and gear extended, and the aircraft is now extremely behind the power curve and is in flight. With older engine technology, the spool-up time from idle to go-around power is very large compared to today's engines (some can argue that going full power with 'below wing engines' can make the pitching moments worse - e.g. B737s). Compound the state of the aircraft at this point, with a smaller horiz. stab. than was designed for the aircraft, and the plane doesn't have enough elevator authority to overcome the pitching motions (oscillations) of the airplane at such an extremely low airspeed and high-drag configuration. You're essentially just along for the ride at that point. This is why in MD-11 landing crashes you see the airplane just bouncing and oscillating like crazy. That is because, beyond a certain point and if it gets bad enough, you just can't control the airplane any longer.

[WR269 367]

Another contributing factor is having the main wing gear legs attached to the wing spar. When the aircraft touches down, energy gets transferred to the spar, touch down hard enough, it bounces in an up and down motion, keep bouncing and the wing spar snaps off as it exceeds design limits. I recall having a discussion about this with an ex tristar and ex md11 skipper. He loved the MD11, and he said you learn a lot more about the engineering side of things than you ever expected.