When does adherence to an engine manufacture’s TBO (Time Between Overhaul) become mandatory? Ah, a question for the ages—yes, both calendar time and running time may matter. But seriously, there’s a lot of confusion in the industry about engine overhaul compliance. Granted, it mostly comes from pilots and aircraft owners, but aviation maintenance professionals are not immune to occasional confusion.
Let’s start by reviewing a fairly recently issued (March ’19) FAA Legal Interpretation, otherwise known as a Chief Counsel Opinion. An aircraft owner asked if his Citation Bravo, powered by two Pratt & Whitney Model PW530 engines and operated under 14 CFR Part 91, must adhere to the 4000 hour TBO noted in the engine maintenance manual. The short answer was “…no, unless that TBO is specified in the applicable FAA-approved Airworthiness Limitations section or is required by an Airworthiness Directive (AD) or other FAA regulation.” The FAA Opinion goes on to point out that TBO was listed in Scheduled Maintenance Checks section of the manual and not in the FAA-approved Airworthiness Limitations Section. If it was listed in the latter, then it would be mandatory as provided by 14 CFR sections 43.16 and 91.403(c).
Ok, so what if this same aircraft was operated under the commercial operating standards of 14 CFR Part 135 instead? Part 135 operators are required to have operations specifications and these op specs may require compliance with a manufacturer’s TBO depending on which maintenance program is selected or approved. The Bravo is certificated for a seating configuration of nine seats or less (excluding any pilot seat) and therefore section 135.421 applies. This section simply says each certificate holder must comply with the manufacturer’s recommended maintenance programs, or a program approved by the administrator, which leads us back to the particular op specs; whatever it says rules. Variations of this basic construct also apply to aircraft operated under Part 125 and Part 121.
One final note, even if engine overhauls are deemed mandatory either by op specs, airworthiness limitations, or AD’s, TBO extensions are routinely granted by the FAA. For more information and guidance on obtaining engine time in service interval extensions see Advisory Circular 120-113.
Kathy Yodice is the Managing Partner of Yodice Associates, a private practice law firm with a special emphasis on aviation law. She has more than 30 years experience representing airman in FAA enforcement actions, regulatory compliance matters, accident investigations, and aviation-related business issues.
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