To: Lungs
Quote: "What is the certification standpoint, bearing in mind that there is real potential for accidents with this type of incident"?
"Is it just a case of testing to ensure that it is so unlikely that its not worth considering or are procedures written for these events in the operating handbooks"?
From a certification standpoint the FAA requires a detailed FMECA and a Safety report. From an operational standpoint the manufacturer must demonstrate the operational capability of the flight control system using what is termed an Iron Bird. The Iron Bird cycles the flight control system under controlled flight loads to simulate the air loads during various phases of flight. The Iron Bird will simulate a very high number of cycles to demonstrate both operability and reliability and the iron Bird will in most cases be still operating after the aircraft is certified and in revenue service.
The FMECA defines the relationship of a piece parts failure to its’ effect on the next higher level all the way to the aircraft level. The FMECA also includes the failure rate of the part, which is determined in the Reliability analysis. This is where the system gets skewed due to the methodology of determining the failure rates. Sometimes the analyst will select failure rates that are not representative of the part under analysis in order to meet the failure rate for the unit under analysis. In most cases, the failure rates are determined through the use of K factors that allow the analyst to determine the ultimate failure rates by adjusting the failure rates through the use of these k factors. As such, the final number in no way relates to the actual part in the unit.
These numbers are then inserted in the safety hazard analysis and are then mathematically manipulated to determine the safety level of the unit under analysis. To boil it down to the lowest denominator it is GIGO (Garbage In Garbage Out).
All of this is then turned over to the certification authority and if it meets their requirement certification is approved.
Even though the Safety report indicates that the flaps will never move uncommanded at a frequency no more frequent that 1 10 9 it has happened at a frequency much higher than the requirement and the FAA does nothing about it. The NTSB may require a modification but the FAA will overrule that requirement after conducting a cost benefit analysis.
It’s just the manipulation of numbers and that is what safety is all about.
