View Full Version : CF6-45/50 URGENT recommendation

16th Feb 2011, 11:30
Are we all aware of this please?

Four Recent Uncontained Engine Failure Events Prompt NTSB to Issue Urgent Safety Recommendations to FAA @ AMTOnline.com Top News (http://www.amtonline.com/article/article.jsp?siteSection=1&id=11011)

16th Feb 2011, 11:51
Ouch! That will hurt.

16th Feb 2011, 12:11
Yes...very much.

Very much like the T972 on the stricken Qantas.

We are on a steep learning curve!

16th Feb 2011, 12:17
Hardly breaking news. This NTSB recommendation was nearly a year ago.


16th Feb 2011, 12:18
Warning, there are two threads running which intermix the GE problem with the RR problem without further substantiation! other than they both quack like a duck. :)

The substantive facts concerning the GE problem are contained in the details of the AD action as:


The FAA defines safety-critical parts as those parts of an engine whose failure is likely to present a direct hazard to the aircraft. Safety-critical parts are subject to special engineering, manufacturing, and maintenance restrictions intended to ensure that the parts are removed from service before a hazardous condition develops.
The design process for a safety-critical turbine disk includes consideration of the disk’s critical frequencies and mode shape information so that the part is designed with an adequate margin of safety. In the case of the LPT S3 disk, an adequate margin of safety should have existed between the disk’s natural response frequencies and the engine’s operating frequencies, including the frequencies that result from common fault conditions, such as HP rotor unbalance. 7
Further, an engine vibration monitoring (EVM) system
13 can be used to alert flight crews of impending part failure to increase the margin of safety. However, when the CF6-45/-50 engine was being certified, the EVM systems installed on the airplanes that the CF6-45/-50 engines were designed to power were considered marginal in their ability to detect HP rotor unbalance.
14 As a result, the FAA placed a special condition (No. 33-36-EA-9, dated November 8, 1971) on the CF6-45/-50 type certificate that required GE to show that "the engine would operate without inducing detrimental stresses in any engine part while operating with an increased vibration level, such as that which might result from one or more broken or missing rotor blades, if the increased vibration level cannot be detected in flight."
The FAA considered this special condition satisfied with its acceptance of a GE certification report that concluded that CF6-45/-50 engine unbalance levels high enough to have detrimental effects would be easily detectable by perceived noise and vibrations in the cabin area and in the controls, enabling the flight crew to take corrective action. However, the flight crews of the airplanes that experienced the uncontained CF6-50 LPT S3 disk failures cited above did not report any unusual engine vibration preceding the events. In addition, there were no reports of vibration associated with the removal-from-service of the eight CF6-50 engines in which the cracked LPT S3 disks were discovered during normal teardowns. Thus, the NTSB concludes that GE’s FAA-approved special condition has failed to provide an adequate margin of safety to prevent catastrophic CF6-45/-50 LPT S3 disk failure. Therefore, the NTSB recommends that the FAA immediately require GE to redesign the CF6-45/-50 LPT S3 disk so that it will not fail when exposed to HP rotor unbalance forces. The NTSB also recommends that, once GE has redesigned the CF6-45/-50 LPT S3 disk in accordance with Safety Recommendation A-10-100, the FAA require all operators of CF6-45/-50-powered airplanes to install the newly designed LPT S3 disk at the next maintenance opportunity.

Therefore, the National Transportation Safety Board recommends that the Federal

In my view the "special condition" was poorly substantiated and accepted by the FAA on the basis that an unregulated function (under Part 37) would suffice to justify certification.

16th Feb 2011, 12:26
OK so you don't like change. I happen to enjoy looking at NEW TECHNOLOGY especially when it is applied to stuff WE THINK we know well. viz. HARMONIC EXCITATION LEADING TO UNCONTAINED ENGINE FAILURE.

The question now is: how can we analyse this effect with the Bayesian method? and wait... could we get a nice fat research grant? That is what some see this as. A business opportunity, rock and roll!

Turbine D
16th Feb 2011, 14:32

You may have missed a previous post I made concerning this AD. So, here it is again, please read it carefully:

AD Aimed At Delinquent CF6 Operators | AVIATION WEEK (http://www.aviationweek.com/aw/generic/story_channel.jsp?channel=mro&id=news/avd/2011/02/07/08.xml&headline=AD%20Aimed%20At%20Delinquent%20CF6%20Operators)

I think it is explained pretty well, at least as far as the CF6-45/50 engines are concerned. I am not sure you can make the same connection between this and the problem encountered on the 972.

16th Feb 2011, 14:53
The spline wear on the T972. The only thing that can cause this is vibration outside of that encountered before. It means there is a fundamental characteristic in the dynamic mass geometry in the 972.
Basically it is shaking itself apart. It was unforseen at the design stage OR they did not have the knowledge or hardware to map it.

New frontier in understanding aerospace turbines?


The math to support this theory is over on the A380 thread in Tech Log.
Not rocket science.