Airplane rollover!

[koma]

Hi... I was watching a Mayday episode and there's one thing I don't really get... How can a rudder failure get an aircraft to rollover as it's not controlling the roll like with USAir Flight 427 and United Airlines Flight 585?? Or is it not the rudder that was the cause of rollover but the wind? I understand that it can push the aircraft to the ground if the aircraft is banked but I don't see how it could actually cause it to bank that much? Also, why is a rollover primarily countered with the rudder and not by banking(rolling) the plane to the opposite side and pitching the nose up smoothly? Could somebody explain? Thanks!

[DA900]

Here is a quick explanation on what can happen

To prove its theory, Boeing persuaded the NTSB to conduct a series of flight tests: A 737 would position itself behind a 727, as the USAir 737 had been when approaching Pittsburgh, to show how the turbulence could shove the 737 into what Boeing called a "sustained yaw" that might cause it to crash. (A yaw is a swing to the left or right.)

The tests were conducted one year after the Pittsburgh crash. Ironically, they not only failed to support Boeing's theory, they proved something else entirely. They showed that Boeing's long-held assumptions about how to safely handle an uncommanded rudder hardover at lower speeds were wrong.

The tests revealed that if a rogue signal swung the rudder to one side, with a 737 flying at 190 knots (218 mph) or less, it was NOT possible to stabilize the aircraft by simply turning the control wheel in the opposite direction.

Turning the wheel deploys wing panels, called ailerons, to roll the plane back to a level position. But the tests showed the ailerons' ability to counter a rudder hardover actually diminished quite rapidly as the plane slowed down — and vanished at a "crossover" speed of about 190 knots, the USAir jet's speed when it flipped over.

Here is what ALPA's conclusion was...

Based on the evidence developed during the course of this accident investigation, ALPA believes that the airplane experienced an uncommanded full rudder deflection. This deflection was a result of a main rudder power control unit (PCU) secondary valve jam which resulted in a primary valve overstroke. This secondary valve jam and primary valve overstroke caused USAir 427 to roll uncontrollably and dive into the ground. Once the full rudder hardover occurred, the flight crew was unable to counter the resulting roll with aileron because the B737 does not have sufficient lateral control authority to balance a full rudder input in certain areas of the flight envelope.

The B737 rudder control system design is unique among jet transport designs in that it utilizes a single panel rudder and a single rudder PCU. Since the B737 received its original FAA Type Certificate in 1967, the aircraft has had a history of uncommanded yaw incidents. The B737 rudder control system does not meet the current FAR requirements, FAR 25.671, with regard to malfunction probability and effects. During the course of the investigations of UAL 585, USAir 427, and Eastwinds 517 a number of failure modes have been identified with the B737 main rudder PCU which can lead to uncommanded full rudder hardovers and rudder reversals. The B737 main rudder PCU's design redundancy is ineffective if any of these failure modes occur and, as a result, the aircraft is not in compliance with the FARs. Some secondary valve jams leave no witness marks. USAir 427 experienced a secondary valve jam and reversal in the main rudder PCU that resulted in an uncommanded full rudder deflection. The B737 has limited lateral control authority which, at certain airspeeds and aircraft configurations, is unable to counter the roll due to sideslip caused by a full rudder hardover. In the case of USAir 427, the lateral control authority available was not sufficient to maintain a wings level attitude once the flight experienced the full rudder hardover.

Just to give you an idea of what is going on...

If you were flying a 737 and a uncommanded rudder deflection was to occur the aircraft would have extreme yaw in that particular direction. You would have no opposite rudder control to counteract the yaw. The aircraft would have one wing still providing some lift. The other wing's lift would be severely reduced. Thus the one wing would stall causing it to drop as well as the nose. You would have a "wing over" as the press likes to refer to it. Now if this happen close to the ground at a low airspeed as the other aircraft were. You see how the ailerons would not be very effective.

What in theory is happening is the rudder is deflecting full direction left or right. Since you do not have any control over it, trying to recover from it at a low speed and altitude is almost impossible. The older 737 are now being modified with a hydraulic system valve that will allow you add opposite rudder pressure that exceeded the normal pressure valve when closer to the ground.

Now that is what I was taught. If someone was told different or has a more clearer explanation please let me know.
Thanks

[Mitch Cronin]

Simple explanation for rudder causing roll: The yaw from a rudder deflection results in one wing having more lift, and one less. The wing into the direction of flight will have increased lift due to it's relatively increased profile - especially in a swept wing aircraft.

[Rockie]

Not to mention increasing angle of attack due to dihedral.

[small penguin]

Curious, does the increased drag from the in-airflow wing do anything?

[V1RotateV2]

You can spend a lot of time explaining the aerodynamics of rudder and yaw and roll, but the easiest way is to go ahead and try it. Next time you are flying apply some pressure on one rudder and watch the airplane bank.

Boeing tested the suspected rudder PCU (power control unit) and Yaw damper. The results were inconclusive, but there was a possibility of an uncomanded full deflection of the rudder. If I recall correctly, it had something to do with the fluid's temperatures and pressure through the PCU.

At low speeds the ailerons were not enough to maintain level flight. Boeing introduced the "cross over speeds". These speeds were higher during approach to maintain enough aileron authority. Eventually the PCUs were replaced with upgraded models and the Yaw damper went digital, eliminating the higher cross over speeds.

[koma]

This secondary valve jam and primary valve overstroke caused USAir 427 to roll uncontrollably and dive into the ground.

So, if I get it right, they are completely ruling out the turbulence as a cause of crash!? I still doubt that such a brutal "wingover" can occur because of a rudder malfunction. Notice that another aircraft suffered the same kind of problem, and recovered as quick as it rolled over, while pitching, yawing and rolling an airplane after a wingover would not result in a 2-3 second TOTAL stabilization of the plane. That's why I think that other significant forces acted on these planes and caused them to roll.. or recover.

[C-GPFG]

Koma...have you ever applied full rudder approaching slow flight and tried to counteract it with full aileron?

[koma]

No.. I haven't.. And I understand that part.. But my biggest question was how can a rudder malfunction get an aircraft to roll 90degres in 2 seconds.. and again 2-3 seconds later stabilize it like nothing happened. Is that possible?

[buster79]

didn't the ntsb determine that it was some kind of hydraulic check valve that buggered up, the pilots of the plane that crashed were holding what they thought were correct rudder imputs but they were actually opposite( right rudder peddle was infact left and vise versa) so they were just making it worst because they didn't understand what the hell was going on. in the other case the valve only stuck momentarly and the pilots were able to gain control again. i'm sure this is the same maday episode you were refering to.

[viccoastdog]

Read the whole NTSB report: It fills in a lot that's missing or dumbed-down in the Mayday episode.

[Liquid Charlie]

Thought Hedley wud dive in here -- did i actually say that -- anyway -- mechanics of a snap roll or flick roll -- rudder is what sets up the quick roll rate.

[SQ]

Koma...have you ever applied full rudder approaching slow flight and tried to counteract it with full aileron?

No.. I haven't.. And I understand that part.. But my biggest question was how can a rudder malfunction get an aircraft to roll 90degres in 2 seconds.. and again 2-3 seconds later stabilize it like nothing happened. Is that possible?

Koma, I can't believe you never did a spin. GPFG describe the perfect way a student tests your abilities to recover a f*d up stall and transforms it in a improvised spinnnn.



very interesting post however.

[seniorpumpkin]

Something I've always been curious about is differential power in flight. If the rudder failed and created a situation where the plane rolled to the right for example. Could you apply power to the right engine to bring the right side of the plane straight? Are there any other emergencies/applications that call for differential power in flight? I think I heard somewhere that for harsh crosswinds some planes are easier to land with differential power, is this true?

[seniorpumpkin]

Anyone?... Anyone?.....

[goates]

Anyone?... Anyone?.....

NASA has been playing with using differential engine thrust in emergencies for a while now. I think the idea originally came from the Sioux City crash where the DC-10 lost all hydraulics. Don't know if it would have helped with the rudder problems the 737s were having though.

[FamilyGuy]

Full rudder deflections on a Boeing product will definately roll the airplane over - it's a big control surface!

Full rudder deflections on an Airbus product will usually rip the vertical stabalizer off - it's made of plastic..

Pick your poison.

[Four1oh]

if we're talking boeing 737's and their rudder, then try this in the sim. When you simulate the hard over, deploy 1/2 speed brake along with the approved boeing procedure of full power and reducing angle of attack. 1/2 speed brake gives you full spoiler deployment on one side and none on the other and is the best way to get your highest roll rate. The roll rate on the 737 is very impressive.

[Tim]

Anyone?... Anyone?.....

NASA has been playing with using differential engine thrust in emergencies for a while now. I think the idea originally came from the Sioux City crash where the DC-10 lost all hydraulics. Don't know if it would have helped with the rudder problems the 737s were having though.

There's actually been a software program developed (I don't know to what degree) that will allow the stick to control differential throttle in the event of a hydraulic failure. Maybe it's too expensive to put into commercial planes, maybe it doesn't know the difference between a flare and a climb, I'm not really sure. Anyone else know anything about this one?

As the the rollover, I seem to recall that the hydraulic fluid became supercooled and as it went through the valve that controls the rudder the cold fluid actually cause the rudder jam, then to move in the opposite direction as the rudder input. So it wasn't just that the rudder fully deflected, but that opposite rudder inputs kept the rudder fully deflected.

The relationship between roll and yaw can be broken down pretty simply and it's taught to students before their first solo. You cannot have one without the other, it's impossible. We use rudder when we bank to control the yaw being produced by the aileron. If you apply either control without the other to go with it, the other movement will occue i.e. adverse yaw in a bank, or using rudder to roll back level after a wing drop in a stall.

[cgzro]

Can't speak to the heavy metal, but full rudder combined with low speed and any significant angle of attack produces a roll rate that greatly exceeds the aileron only roll rate. Somebody above correctly described this as a flick or snap roll.

Here is an example of the effect without ailerons. Look at second :48 of this little video shot with an aft facing camera showing the rudder/elevator and resulting effect.

http://www.youtube.com/watch?v=8nF2s6ZEKis

Peter