737 rudder question (again?)

[tallsandwich]

I'm sure the issue of uncommanded rudder deflection in 737s has been covered somewhere here at great length - where can I find info on the current state of this problem from an authorative and fairly un-biased source? How much of a risk does it (still) represent when compared to other risks associated with flying?

[x5uvt]

During my recurrent a couple weeks ago, i had a fairly interesting checkpilot. He contributed some very valid points to the NTSB during their examinations but never testified officialy. The first sentence when he entered the room was "have you ever heard about the rudder problem of the 737?" - yes sir i did. He went on and explained why he doesn't believe it. First accident with United, he said that the 732 was flying at 140 kts. with Flaps 5. However, the crew thought they had set Flaps 15. Then they entered a 30deg bank and here it happened. From his point of view, they stalled the aircraft and initiated a wrong recovery.
The second accident with USAir, he believes that wake turbulence presented a strong factor. Again, a wrong recovery technique was used.

He presented his points in a very believable way and focused his attention on wrong recovery techniques for aircraft upsets. However, i truly respect this gentlemen and his explanations due to the fact that he was an active 737 testpilot and he had done things with this plane i don't even dream about; he had been asked by the NTSB to testify his findings and explanations, and he provided us with some terrific insights and unusual attitude recovery training never seen before.

His last sentence was quite memorable: "... the only problem the 737 has, are the pilots flying it".

Cheers, x5

[tallsandwich]

Thanks for your info, very interesting.

In the light of your comments, would it then follow that the retrofit that is being applied to (some ?) 737s is an engineering enhancement which was (in his opinion) introduced to demonstrate that the (alleged) problem was actually being addressed by Boeing and being treated seriously (a techincal PR action to reassure people like me)?

Did anyone ever find undisputable technical evidence of an uncommanded rudder action, or was this just a 'sensible' engineering conclusion based on the available facts?

[prop jocket]

Tallsandwich - here's a posting I made on the subject elsewhere on the forum. Hope it is of use to you............

I don't know if this will help the debate, but here goes. In my day job, I was involved in an investigation into a 747 uncommanded elevator input a few years ago, and also in the 737 rudder system redesign conferences in Seattle.

The cause of the 747 elevator deflection was a combination of two things - a bit of grit in one of the selector spools on the tandem actuator preventing full closure, and return lead rather than return lag on the other spool. As the design of the 747 elevator and 737 rudder PCU's are almost identical, this caused a light bulb to illuminate over the head of one of the development engineers, and much finger pointing ensued towards the 737 actuator.

Parker Hannifin have redesigned the spools to prevent return lead, and have increased the overlap margins to make some allowance for contaminants in the system.

Mr. Boeing has redesigned the entire system; the only salvageable bit is the standby actuator. Even the yaw damper coupler is different, as is the wiring and the magic boxes. There is a redefined hydraulic limiter, and a mechanical limiter.

The Speed increase was an attempt to ensure that the aircraft operated above crossover speed and so give the pilot a fighting chance at controlling it if the rudder declared UDI. I don't know much about flying 737's, but from a systems perspective I'd suggest that as the 737 has manual reversion, the best course of action in the event of a rudder hard over would be to dump the hydraulics and fly manually.

Other Boeings have non return valves in their systems to prevent the wobbly bits flapping around in the wind in the event of a total hydraulic fluid loss; as the 737 has manual reversion, that would be a bit of an own goal, so it is quite normal for the control surfaces to move about in the wind, whereas it would be a bit worrying if a 747, 767 or any other true Boeing flying control did so.

I'm now sitting back and awaiting the inevitable character assassination..............

[tallsandwich]

Thanks

Thanks for the info.

Since posting the article I listened to (and read) a presentation which described the three conditions that could have caused the two 737 incidents which some have blamed on the rudder.

Unfortunately I can't find the URL of this info, however the direction the investigation took was that non of the three possible causes (one was dirty in the mechanism, one was heat expansion and interference of the moving components, and I can't recall the other) could be proved to have happened - but any (or all three) could have happened.

They sumarised by sayig that a fix of some sort was applied to each of the 3 areas. The 'fix' was either by redeisgn, change of procedures etc, so that the source of the problem (whichever of the 3 was the cause) would not exist and hence the incidenct would not re-occur.

It was rather odd to hear that a problem has been solved when the exact cuase was not know, but nevertheless a very convincing presentation. I'll try and find it again and post it if this topic is still alive.

Thanks,
Tallsandwich.

[LHmain]

I heard that it was thermal shock due to a supercooled valve body being activated by hot hydraulic fluid. The retrofit gives overide command to the pilot.