NTSB took aim at some contradictory FAA guidance on maintenance programs in a recommendation letter issued Thursday (.PDF)
In the probe of the November 2005 crash of a Business Air Embraer EMB-120
in Manchester, N.H.--in which maintenance issues played a role, but weren't fingered as the probable cause--NTSB found that the plane's engines were maintained in part under an FAA-approved 'on-condition' maintenance program. But the board also found inconsistencies in how, exactly, industry was being told to conduct such a program. From the letter:
"According to FAA Advisory Circular (AC) 120-17A (.doc), “Maintenance Control by Reliability Methods,” which was issued in 1978, on-condition maintenance is:
… a preventive primary maintenance process. It requires that an appliance or part be periodically inspected or checked against some appropriate physical standard to determine whether it can continue in service. The purpose of the standard is to remove the unit from service before failure during normal operation occurs.
This AC also states that hard time, on-condition, or condition monitoring are the primary aircraft maintenance processes. As written, the AC was intended to provide conceptual guidance for operations conducted under 14 CFR Parts 121 and 127. However, no ACs specifically address on-condition concepts for Part 135 operators such as Business Air.
However, NTSB continues:
AC 120-16E (.pdf), “Air Carrier Maintenance Programs” (another active AC), conflicts with AC 120-17A in regard to FAA on-condition maintenance philosophy. According to AC 120-16E, paragraph 602a(2), Parts 119, 121, and 1357 air carriers “should not use terms such as hard time, on-condition, or condition monitored in [their] maintenance schedule.” The AC further states that “these terms represent obsolete 1960s methodology [and] are vague” and that use of these terms runs the risk that needed maintenance may not be performed according to a set schedule.
And while NTSB notes that the ACs are intended for Part 121 operators and Business Air is a Part 135 operator, the inconsistency is a concern -- as is the way Business Air was left to run its maintenance program. Lots more in the full NTSB report (.PDF)