tallsandwich
13th Sep 2005, 22:18
Can someone in the industry please decode this (NTSB?) quote concerning the crash of an A300 AA587 on 12-Nov-2001:
"The in-flight separation of the vertical stabilizer as a result of the loads beyond ultimate design that were created by the first officer's unnecessary and excessive rudder pedal inputs. Contributing to these rudder pedal inputs were characteristics of the A300-600 rudder system design and elements of the American Airlines Advanced Aircraft Maneuvering Program."
Are they really saying that a full-scale rudder deflection at climb speeds (they were at 2500 feet) will cause the vertical stabiliser to simply tear off from the fuselage? If so why is there no system to prevent this control surface making such big deflections at high speeds? Is this not possible or would such a system be unsafe?
What does "elements of the American Airlines Advanced Aircraft Maneuvering Program" mean in real terms - that the AA ops manual was seriously flawed?
Authorative answers only please - I have already heard a life time of speculation from bar room experts.
Many thanks,
tallsandwich.
It has been common knowledge since the early days of swept-wing jet transport flying that large (full) rapid rudder inputs will put the structure at risk.
This knowledge has passed on with the early guys, and the young turks at AA thought that the advanced aircraft maneuvering program was the 'proper' way to right an aircraft that had suffered a jet upset incident.
As to why hasn't the aircraft rudder power system been designed to keep the rudder from providing excessive vertical fin loads, clearly Airboos thought they had...just as AA thought they had all the answers.
Sadly, both did not, and now folks are dead as a doornail.
The usual 'maneuvering' speed limitation protections in a straight wing aircraft do not necessarily follow through to a swept-wing jet transport, especially those with a highly powered rudder.
DP Davies knew this, and so did the older folks at Boeing/Douglas, and the senior folks at AA then knew it as well, yet this knowledge was never passed along to the new guys or the new guys thought they knew better.
Clearly they did not.
This has all been covered before in a 30 page thread.
Basically ...
Flight 587 flight data recorder (FDR) shows three lateral accelerations of 0.3g and 0.4g right, and 0.4g left, in the approximately 7 seconds before it appears that the fin came off. Analysis show that the aircraft may have been in a full slip to produce the high accelerations. During the same period the FDR shows the rudder making about five deflections of 5 to 10-11 degrees, culminating in a rudder reversal immediately before the fin apparently came off. The 10-11-degree deflection is the maximum allowed by the A300's rudder limiter at that airspeed, suggesting it was working correctly.
The FARs paragraph 25.351 covers yaw manoeuvre conditions, and 25.341 covers gust and turbulence loads.
Paragraph 25.351 spells out a simple manoeuvre and requires that the manufacturer analyse the loads at four conditions. The manoeuvre is to:
Fly straight and level, and step on the rudder pedal with a large force (condition A),
Maintain rudder and let the aircraft swing to a peak sideslip angle that is beyond equilibrium slip due to fuselage momentum (condition B),
Maintain rudder and let the aircraft swing back to equilibrium sideslip (condition C), and
Neutralize the rudder while at equilibrium sideslip (condition D).
The rational for these design requirements is that each condition tends to load different parts of the fin, such as the front spar, rear spar, hinges, rudder, etc., but condition D can create the highest fin bending loads, as far as the regulations are concerned.
Ultimate loads as per FAR 25, only needs to be tolerated for 3 seconds (not 7 seconds) and can result in permanent deformation. There is no requirement on what the strength must be after surviving ultimate load.
The A300 rudder is relatively powerful because it is large, about 34% of the total fin chord. Rudder effectiveness also washes out with increasing sideslip, and this affects the critical anti-slip rudder more than pro-slip rudder. The A300 rudder has ±30 degrees of authority at speeds below 165 KIAS, and the limiter progressively cuts this back to 3.5 degrees at maximum speed. It may be tempting to further limit the rudder at higher speeds, but it needs enough authority to handle engine failure with some margin, and serve as a yaw damper. There also are unusual conditions such as multiple leading edge flap failure that may require a large amount of rudder to counteract.
Given the limited amount of FDR data released by the NTSB it is not clear if forces in the rudder exceeded ultimate loads, but the high sideslip and rapid full rudder motions are ripe for this possibility. The exact motions may never be known because the FDR only measured the rudder twice per second, while it can move at 39 deg/s the rudder could go from neutral to the stop and back between samples. And as the NTSB has stated fast rudder motions were distorted by being filtered.
In simple english, you need the amount of rudder travel provided for failure cases (engine, flap, etc). No rudder limiter prevents successive pilot initiated stop to stop rudder inputs.
What does "elements of the American Airlines Advanced Aircraft Maneuvering Program" mean in real terms - that the AA ops manual was seriously flawed?
Been suggested that senior check and training in AA used previous military upset recovery techniques for large airliners.
IFALPA has published correct recovery techniques for airliners, they do not involve the aggressive use of rudder.
tallsandwich
14th Sep 2005, 07:43
Many thanks for taking the time to answer - much appreciated.
http://www.pprune.org/forums/showthread.php?s=&threadid=71106&perpage=15&highlight=AA587&pagenumber=30 and http://www.pprune.org/forums/showthread.php?s=&threadid=154775&highlight=AA587