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Final Rule, affecting Mitsubishi MU-2B series airplanes

Wed, Nov 28, 2007 — David Evans

Briefs

21 August 2007
FR Doc E7-16288 – Docket No. FAA-2007-27191 – AD 2007-17-09
Federal Aviation Administration (FAA)

Previous airworthiness directives (ADs) involved actions to reduce maximum elevator nose-down trim. This AD requires modification of the trim indicator scale dial to be consistent with the reduced elevator trim capability. As the final rule indicates:

“AD 93-07-11 and AD 94-04-16 currently requires you to reduce the maximum deflection of the elevator nose-down trim [from -10 degrees to a -1 degree to -3 degree range]. When the above AD actions were issued, there was no associated elevator trim indicator change. Without such change, the trim reaches the maximum nosedown limit and the indicator still shows additional nose-down trim available. In attempting to force additional nose-down trim, pilots have manually jammed the trim system, preventing subsequent electric trim changes until the pilot manually freed the trim wheel. Consequently, this AD retains the actions from AD 93-07-11 and AD 94-04-16 and adds the actions of modifying the elevator trim indicator scale to be consistent with the reduced elevator trim capability. We are issuing this AD to prevent the above scenarios from occurring with consequent loss of control.

In plain language, attempting to force additional nose-down trim beyond the mechanical stop may jam the trim system. Operators have 100 flight hours after the effective date of 25 September to modify the trim indicator scale.

The AD affects 400 airplanes in U.S. registry.

Note that the first two ADs were issued more than a decade ago. This third AD results from “several incidents caused by excessive control wheel force,” according to the FAA.

This would seem to be a classic case of an incompletely thought-out fix inadvertently generating a further associated and potentially serious problem. Because it becomes – quite directly – a flight control issue – it is particularly egregious. The lesson is obvious: one shouldn’t fool around with primary or secondary flight controls without a full failure mode effects analysis (FMEA). Evidence of the missing FMEA comes from the notice: “When the above AD actions were issued, there was no associated elevator trim indicator change.” [Emphasis added]

The manufacturer, and the FAA, apparently forgot to follow through, until incidents over the past decade revealed that the original fix spawned another problem.


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