ANC 744 rudder report

[PaperTiger]

(Sorry, I can't find the original thread to put this in context.)

I see the NTSB has now posted preliminary findings on the NW 744 rudder problem at Anchorage: http://www.ntsb.gov/NTSB/brief.asp?ev_id=20021018X05344&key=1
Sounds considerably hairier than first reported - asymmetric thrust used for control.

[Cornish Jack]

Hmmmm!
Fascinating statistics on this. ;)
Over 100 views of this thread on an incident which must have been marginally close to a real nasty. How many responses? ..... Zilch. :confused:
So where are the usually verbose contributors who could have been guaranteed to drip vitriol had this been an Airbus incident??
Where are those who repeatedly regale us with their signatures of 'if it ain't Boeing, I ain't going' ??
Given the previous Boeing history with it's baby jets' rudders one might have expected rather more interest in this one. Could it be that a 'simple' Boeing structural failure doesn't excite the sort of comments that an Airbus fin failure would do? Ho hum - a funny old world :(

[PAXboy]

Yeeee-owchhhhh! That is one of the nastiest bits of news I have ever read.

No warning, in the middle of cruise and it only got worse? Rudder STILL out of place on landing for photographic evidence? As an outsider, I would suggest that crew deserve considerable praise.

[Intruder]

Cornish Jack:

The big difference between this incident and the 737 mishaps is that only half the rudder system failed on the 747 -- the remaining controls allowed the crew to keep the airplane under control. The fact that the airplane landed safely gives clear evidence that the split rudder system provides the redundancy required to guard against foreseeable failures.

As with any system failure that doesn't have a specific non-normal procedure addressing it, this one required the flight crew's knowledge of the airplane systems and the airplane's capabilities to successfully deal with the failure. In the 747 the rudder ratio changer (restricting rudder travel at cruise speed, but allowing full travel as airspeed decreased) would affect low-speed controllability as well as the general principle that control inputs are less effective with decreasing airspeed.

Since only a preliminary report was released, we don't know if the most significant cause is a design defect, maintenance failure, or a simple, unpredictable component failure. No need to go ballistic yet...

[Shore Guy]

Couple of questions for 747-400 folks...

(1) Is there a display of flight control position available to the crew in flight? Both upper and lower rudder?

(2) Is it possible to (hydraulically) unpower each segment of the rudder? Inflight/air mode?

Thanks in advance....sounds like they had a handfull of airplane.

[Shore Guy]

From the Australian Safety site..

Problem with the upper yaw damper computer on a 747

Safety Brief 200105429

A Boeing 747-SP38 aircraft was maintaining Flight Level (FL) 430 with autopilot ‘A’ engaged, when the aircraft yawed abruptly to the right and rolled to a bank angle of approximately 20 degrees. The autopilot was disengaged and the aircraft stabilised in a straight and level attitude. The uncommanded yaw occurred again. The flight crew broadcast a PAN (radio code indicating uncertainty or alert, not yet the level of a Mayday) and received a descent authorisation to FL380.

The upper rudder position indicator showed a rudder displacement of 5-degrees right and the lower rudder indicator showed zero degrees deflection. The flight crew began activating and de-activating the upper and lower yaw damper switches attempting to isolate the problem. During those actions, the aircraft commenced to ‘Dutch roll’ (lateral oscillations with both rolling and yawing components). The crew then successfully isolated the problem to the upper damper and turned the upper damper switch off. With the aircraft at FL380, normal operations ensued. Autopilot ‘B’ was then engaged and the flight proceeded without further incident.

Investigation by company maintenance personnel confirmed an anomaly of the upper yaw damper computer. The unit was replaced and the system tested. Normal operations ensued.

Analysis of Flight Data Recorder information revealed that during the event the upper rudder displaced 4.7 degrees. The data also indicated that the maximum roll encountered was 13 degrees to the right.

System redundancy had operated as required to limit the effect of the upper yaw damper anomaly.

[Intruder]

(1) Is there a display of flight control position available to the crew in flight? Both upper and lower rudder?

Yes, there are 2.

The Surface Position Indication on the Status display available on the lower EICAS gives approximate positions of the control surfaces. This display is used to verify control positions when doing control checks.

There is also a maintenance display available on the lower EICAS via the center CDU, though it is not "normally" used. The Hydraulics display gives a higher-fidelity display of control positions than the normal Status display.


(2) Is it possible to (hydraulically) unpower each segment of the rudder? Inflight/air mode?

Normally, no.

There is a set of maintenance switches on the overhead maintenance panel, "for ground maintenance use only." Whether these could be successfully used in flight for this purpose, I don't know off hand, but they supposedly shut off hyd system #1, 2, 3, or 4 (respectively) power to the tail.

To depower the lower rudder, you'd have to shut down hyd systems #2 and 4, which would also reduce the stab trim and elevator authority by half. I don't know if this would be a reasonable alternative to using the upper rudder (plus aileron and engine, when necessary) to compensate.

[411A]

Boeing...and rudders, can they NOT get it right?

Ever since an AA 707 rolled over inverted into Flushing Bay in 1959 (as I recall) off IDL (now JFK), Boeing has had a BIG problem with the rudder.
Question: WHY has this not been FIXED a LONG time ago, in ALL Boeing aircraft?

Good grief, is a COMPLETE grounding of the fleet required to fix this?
Lockheed/Douglas, no problem.

Rudders, a pox on Boeing equipment.

[lomapaseo]

411A

What's for Boeing to fix??

**** happens

That's what maintenance is for

I'm still awaiting facts

[Intruder]

411A:

"Boeing...and rudders, can they NOT get it right?

. . .

"Question: WHY has this not been FIXED a LONG time ago, in ALL Boeing aircraft?

"Good grief, is a COMPLETE grounding of the fleet required to fix this?
Lockheed/Douglas, no problem."


So, what is "right" to you?

NOTHING is infallible or totally fail-safe! The 747 in question landed safely. That indicates Boeing DID "get it right"!

Douglas didn't "get it right" in all cases, either... There was recently a [not so] minor problem with a MD-80 horizontal stab jackscrew...

[Cornish Jack]

Intruder
An interesting response re. 737 difference. Unfortunately you missed the other difference - ALTITUDE. The 37 which rolled into the ground did so from pattern altitude. Now, just apply a little hypothesising and put this 744 at pattern altitude, on instruments and slowed down to below 235 kts??? Now give your best shot at forecasting what would have happened with the same problem occurring?? :(
Methinks the outcome would have been somewhat different in a situation where there was no prior indication of potential failure and with full bottom rudder going in - especially when even at cruise speed and minimum rudder deflection and with time to do the drills and think it through, they still end up needing asymmetric thrust to stop the roll. :(

[John Farley]

Cornish Jack

Well said