Vic, the ~1980 valve issue with the BMW engines was pretty clear: they changed valve seat material to an harder material to prevent seat recession due to fuel with no lead deposits to protect the valve seats. The valve itself then overheated, deformed and recessed into the intact seat. Then later, circa 1984, new replacement valve/seat materials worked in combination, but when people tried to use only the new valve material (since that was the part that was now failing visually) in ~1980-83 engines without replacing both valve and seat, the valve still overheated in the same way because the root cause of the issue was that the harder seat material had lower thermal conductivity and didn’t cool the valve as required.
In a different engine with better cylinder head cooling you can get away with using hard valve seats to fix valve seat recession. However, in a more thermally stressed engine, racing engines being one example, this may just transfer the problem to a now undercooled valve unless the thermal conductivity of the seat material is maintained.
In order to assess what is going on with the UND engines, one would first have to look at what is causing the dry valve clearance to close up as reported. If it’s recession of the seat then the selection of harder material to solve that problem will have to take into account the cooling of the valve too: aircraft engine cylinder heads and valves are not overcooled, their heads run hot and their valves are large and therefore have small valve seat cooling circumference in relation to their area. Aircraft engines are highly thermally stressed, more like the circa1980 air-cooled BMW example and less like liquid cooled engines.
12.000 hours of flight training PER MONTH. Am I the only one who gets amazed by that number? That is 400 hours of flight time PER DAY on a 30 day basis. If each plane flies 5 hours a day that’s still 80 aircraft, at least, because that’s including downtime and weather.
UdoR wrote:
Am I the only one who gets amazed by that number?
No – it is amazing. It will be very interesting to see how this story develops. These guys have a lot of aircraft, and must have a huge maintenance facility to service them. I would hope they have a lot of data from all these aircraft that they will share at some point. One of the open questions for me is how do you measure valve seat recession? And why do you do it? How can you be sure of what you are measuring? Did they see actual failures or performance issues? Did the engines not make it to their expected TBO? Many more questions than answers.
how do you measure valve seat recession?
On Lycoming engines (other than the O-235 which has solid lifters and is not used by UND) you collapse the hydraulic tappet/lifters that automatically adjust to allow for valve train wear, and measure the resulting clearance. If the seat or valve (either or both) has worn the collapsed or ‘dry’ clearance is reduced, but unlike with a solid tappet/lifter engine operation would not be affected unless wear was really extreme.
Given that valve recession is known to be the most likely issue with converting engines to unleaded fuel, this measurement was done periodically, I’d guess at 100 hr inspections.
Well what is actually “a problem”?
They did 46.000 hours in 4 months without failure.
If you translate that to normal European club environment (let aside personal use aircraft) that is so incredibly many hours that 99.9% of the planes will have many other problems evolving much earlier. It’s just noise in the statistics.
But still the final results will be interesting.
Maybe as seems to be suggested here by others an initial use of Avgas for some hundred hours and continue the rest unleaded could be a solution ..
Well what is actually “a problem”?
Eventually the worn valve will stop sealing and cylinder compression will lost. It is logical to assume that UND saw increased wear rates that would extrapolate to this occurring on a given engine well before it reached their normal TBO.
ND winters are like Siberia’s.
I guess UND has a very seasonal activity and take advantage of the summer like homesick angels. They surely fly in the winter but must be very restricted by weather.
I guess they overhaul the engines in the winter.
These large scale, controlled experiments are great.
Silvaire wrote:
Eventually the worn valve will stop sealing and cylinder compression will lost. It is logical to assume that UND saw increased wear rates that would extrapolate to this occurring on a given engine well before it reached their normal TBO.
The interesting question is why this does not happen in those places in Europe where unleaded fuel has been used for decades – it would certainly have been noticed. Is there something special with UL94 as compared to UL91 or 91/96UL which are used in Europe? Is there something unusual in how UND operate their engines?
How do the annual hours compare with UND’s very high numbers?
In Europe, an awful lot of aeroclub activity is quite low annual hours on the aircraft – as well as low annual hours on the pilots (5-10 average in some places).
It could also be strange engine operating practices but IME of one US school I am sure they do all kinds of weird stuff. The one I did my IR in was using the CHT gauge to lean the engine (for peak CHT, no kidding). That PA28 was doing 700hrs/year and that was IR training which was generally two 1hr flights per day which works out about right for 700. But PPL training can be flying almost all day.
Peter wrote:
In Europe, an awful lot of aeroclub activity is quite low annual hours on the aircraft – as well as low annual hours on the pilots (5-10 average in some places).
That is true, but we still run our engines past TBO. Going past TBO regular compression checks are required so any compression issues would have been noted. Swedish aeroclubs alone fly about 40000 hours every year, most of these are flown on unleaded AVGAS.
