Tag Archives: TBO

TBO: Time Between Overhaul. Even the PT6A Needs Maintenance

Covington Aircraft has now been in business since 1979.  Over the last 30 plus years, we have seen some Agricultural Aircraft owners who were riskier than even the most daring stunt pilots. How so? Well, they try, much like those who push the oil change on their vehicle, to put off as long as possible the overhaul of their engine. We focus on what to look for and Time Between Overhaul in this post.


The Ag industry often overlooks FAA Part 91/137 (Agricultural Operations) regarding recommendations for TBO, or Time Between Overhaul.  TBO is overhauling an engine at the manufacturers recommended times.  Often, Ag industry pilots are under the mistaken impression that PT6A engines don’t need these recommended overhauls, when this couldn’t be further from the truth.

SB 14503 (-67AG)

Pratt Service Bulletins governing TBO for this engine indicate that the first stage power turbine blades should be replaced at 12,000 hours, while second stage power turbine blades should be replaced at 5,000 hours.  Compressor Turbine Blades with certain part numbers must be replaced at 6,000 hours.  Main-line rotor bearings #1 and 4 as well as the first stage planet gear set’s sun gear should be replaced at 12,000 hours time.  Considering these low hours, it stands to reason that the PT6A will not fly forever if the recommendations are not followed.

PT6A-34AG SB 1303

pw-pt6aThe 5000 Hour Check, which involves removing the disc from the hot section, removing blades from the disc, cleaning and conducting non-destructive testing, is recommended for the first time at 5,000 hours and at 3,000 hour intervals after that.  This inspection also requires comparison of the actual length of the blade but they cannot be cracked.  Cracked turbine blades or those stretched beyond limits could be devastating to an aircraft.

These two simple recommendations indicate that PT6A engines do not last forever.  Failure to follow these recommendations could result in engine failure.  In addition, Pratt & Whitney places significant importance on maintenance recommendations when honoring warranty repairs.  Therefore, it is important that owners educate themselves on service bulletins and overhaul recommendations to avoid catastrophe.

If it has been too long of a TBO for you or you have general maintenance, sales, or overhaul questions, make sure you visit the Covington Aircraft website!  Happy Flying!