They obviously wear out eventually, but I don’t think there is such a mechanism.
So what led to the (say) 2000h TBO number?
Could it be like Maximum Demonstrated Crosswind – a CYA figure?
It must be based on average data. The O-360-A4M in a PA28-181 does not have a turbo and with regular use and 25 hour oil changes I’ve heard of 3,500 hours + and still all fine. We all know that most engines sit around and don’t get used every 10 days. They go into “long Annuals” by mistake and the engine does not get dehydrated or laid up properly for 2 months.
It’s a shame there is no data on the few well looked after engines. Flight Schools would be a great source of information but they have to rebuild (probably) perfectly good engines and replace them with more dangerous rebuilt ones which come with the 0-300 hour danger zone!
This FAA Advisory Circular on engine certification is available. On Page 30 it says only that engine operating limitations, including component life are “established by the Administrator”. Starting on Page 55 there is a description of endurance tests required for engine certification, totaling only 150 hrs, with a tear down inspection afterward. I imagine the recommended TBO is set after discussion between manufacturer and Administrator based on post-test inspection data, but it is not stated.
When a totally new type of engine is first offered to the public, its TBO may be intentionally set low, but as the type matures and the TC holder collects the statistics, the TBO is usually increased – sometimes a bit, sometimes dramatically. Once you have the statistics, TBO can be reasonably estimated from the bathtub-shaped failure rate curve – somewhere near the end of the “bottom”. Accordingly, it’s unsurprising to see at least some engines making it well past TBO defined this way, especially if the given engine is properly pampered.
Ultranomad wrote:
Once you have the statistics, TBO can be reasonably estimated from the bathtub-shaped failure rate curve – somewhere near the end of the “bottom”.
The question is if the curve really is bathtub-shaped for an engine which is properly inspected. And if it is, where on the bathtub TBO is located. The only interesting failures are those that happen in flight. An engine failing an inspection doesn’t count, nor does an engine that gives clear warning signs in flight without actually failing.
I think for most engines the issue that drives overhaul is probably oil consumption and/or blow-by (i.e. mess on the belly), not imminent engine failure. If it is a six cylinder high power engine that may need a top overhaul at half TBO to maintain compression or limit oil consumption, many people will do it but not many people will commit to a second top overhaul without overhauling the bottom end. If its an engine like the O-320 that rarely needs anything between overhauls, when oil consumption or blow-by rises the hours are enough that most owners figure its time to do the whole engine.
The other driver is corrosion detected by borescope but given that engines rarely fail as a result of high time since overhaul, I don’t think hours SMOH is the driver for most owners. Also weighing into this is that many components that could fail as a result of time is service (e.g. due to metal fatigue) are not replaced at overhaul.
Airborne_Again wrote:
The question is if the curve really is bathtub-shaped for an engine which is properly inspected. And if it is, where on the bathtub TBO is located. The only interesting failures are those that happen in flight. An engine failing an inspection doesn’t count, nor does an engine that gives clear warning signs in flight without actually failing.
Yes, it certainly is bathtub-shaped, and the TBO, as I said, should be somewhere around the far end of the “bathtub bottom” (how exactly you define it is a more complex question, though). As to sudden failures without warning signs, I have no reliable data on Lycoming and Continental, but just one week ago I participated in a disassembly of another manufacturer’s piston engine with a connecting rod ruptured in flight. This failure mode in old engines is well-known, occurs suddenly with no warning signs, and two recent cases prompted us to analyse the phenomena behind it. Unlike those in Lycoming, rods are made of tempered 2024 aluminium alloy. At high temperatures, it gradually loses its strength; specifically, around 190°C it would take only about 20-something hours to go from the optimal temper point to a point where a risk of rupture starts to increase seriously, but at lower temperatures the process is much slower. Our preliminary assessment shows a contribution of about 30 seconds per flight towards this 20-something-hour limit. In typical operations, this limit may be reached somewhere at 2500-3000 hours TTSN. The official TBO for this engine is 1600 to 2000 hours depending on the series, and during an overhaul, the rods should be tested for hardness. The rear cylinders are known to run hotter, and the results of hardness tests (which I am doing personally) are fully consistent with that. An engine analyser with data recording and CHT sensors for each individual cylinder might provide useful information, but nobody really uses them on these engines…
LOM (or Walter) engines have aluminum con rods. From the link (which is BTW quite interesting): “The connecting rods, with the stem in the form of “H” section, are aluminum alloy forgings” This is in common with other inline aircraft engines originally developed in the 30s or before, for example the DH Gipsy also has aluminum con rods.
From the Australian study: During the period, January 2000 to December 2005, 20 in-flight engine failures occurred in Australia. Sixteen of the 20 failures involved Lycoming TIO-540-J2B engines fitted to Piper Chieftain aircraft. With that in mind I think the data may be of some interest if you have a Piper Chieftain or other type with that specific type of 350 HP angle valve turbocharged engine, otherwise less so.