time between overhaul

TBO (Time Between Overhaul) – To Be or Not to Be?

Sorry about the play on words, I couldn’t resist it! The subject; TBO or Time Between Overhaul, is a really fascinating subject when considered for flight operations under FAA Regulations 14 CFR, Subchapter F, Air Traffic and General Operating Rules, Part 91(not for hire ). No, operations running under Part 91 do not have to follow manufacturer’s recommendations for TBO. TBO stands for Time Between Overhaul and means having to overhaul an engine at the manufacturer’s recommended times or an approved TBO extension time.

time between overhaulThere are lots of arguments pro and con regarding engine overhaul. Most engine operators accept some form of TBO limit and submit their engines for overhaul more or less in keeping with the recommended TBOs. In the United States, PT6A Agricultural Aviation operations (FAR Part 91/137) don’t follow TBO recommendations for the most part. Some operators, private and Ag, have the mistaken idea that PT6A engines run forever and, don’t have to be overhauled! They are right in that they don’t have to overhaul the engine but, very wrong in the idea that they run forever!

I’m obviously not going to try to force anyone to overhaul their engine in the paragraphs of this article. But, I am going to point toward some information that any operator of a PT6A engine should be familiar with! The information is contained in the Pratt Service Bulletins governing TBO. That bulletin is the third service bulletin of the bulletins affecting any series of PT6A engine. Example; SB 1403 – PT6A-34, SB 1803 – PT6A-20, -21, -27/-28, & -135/ 135A, SB 3003 – PT6A-41, -42/42A and, SB 13303 – PT6A-52/60A.


SB 13303 places a recommended replacement time for first and second stage power turbine blades of 12,000 hours! Another bulletin identifies a certain part number Compressor Turbine Blade that must be replaced at 6000 hours. Main-line rotor-bearings #1 and 4 as well as the first stage planet gear set (including sun gear) are recommended for replacement at 12,000 hours’ time since new. Interesting information for an engine that is supposed to run forever!

PT6A-34 SB 1403 recommends an inspection of the compressor turbine disc / blades at 5000 hours’ time since new and subsequently at each 3000 hour interval. This inspection is known in the maintenance industry as The 5000 Hour Check. The inspection involves removing the disc from the hot section, removing the blades from the disc, cleaning up everything and conducting non-destructive testing (NDT) on the blades and disc. A stretch-check of the turbine blades is also called for. That inspection compares the actual length of the blade with the manufacturer’s dimension marked on each blade. You’re allowed a few thousands inch of blade growth but no cracking. Wouldn’t you want to know if your turbine blades were cracked or stretched beyond allowable limits?

I know certain operators don’t have to overhaul their engines and, I know the engines run and run and run. However, the concern for certain parts reflected in the service bulletins governing Time Between Overhaul and TBO extensions should alert you to the possibility that some things may not last forever! I have been heard to say; “the cheapest time to overhaul a PT6A is 30 minutes before it fails”. The hard thing is…if you miss-judge that 30 minute lead-time, you lose everything!

In considering warranty requests and other customer support venues, Pratt puts a lot of weight on how closely an operator follows their recommendations for maintenance. Those recommendations include TBO for engine overhauls and hot sections. Please educate yourself on these service bulletins! It could help you avoid engine failure, the loss of an aircraft and possibly save your life! At the least, it will help you develop a maintenance plan to enhance your flying safety!

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