M20K231 max. MP during maintenance ground run

[Emmet]

Hi,

I just received a report from my mechanic where he pushed the MP over the 40" redline during the incoming inspection ground run. Is this a common value for that procedure? 

Thanks,

Emmet

 

 

[Shadrach]

I believe the pressure controller is spec'd higher than redline so as to ensure that redline is attainable under various conditions.  Did he indicate why he exceeded redline? 

[Emmet]

Haven't talked to him yet. The OM states that the max MP can be exceeded by 1-2" for max. 2-3min.

I thought that he might have checked the max reachable MP. I still don't see any value in it as it is standing on the ground. The docs from the previous annuals show only 40 ...

[kortopates]

There is no controller on the 231's, just the fan, a bolt in the exhaust acting as a fixed wastegate and the overboost valve that pop's when ~3-5" (from memory) above redline. Very minimal without any added mods. I don't know why he would go over redline, but what does  "pushed the MP over the 40" redline" mean? 40.5"? 41? You asked if it was a common value without giving one.

But if it was hot out, just maybe he really wanted to test it at 100% power and was adjusting with another 1" MAP for each 10C above ISO. I know your POH only talks about this for setting cruise power. But the more advanced controllers that measure density of the air (such as the one used in the Bravo) actually do this and therefore have a redline range rather than a fixed number since it varies by temperature (which makes them very hard for the tech to setup).

I think the caveat you mention from the TCM maintenance manual (and/or O/H manual too) is very true but stems more from those engines with hydraulic controllers (like the 252) that will naturally overboost when the oil is isn't fully warmed up (below 180F). So some overboost on the first flight of the day is common and normal in such aircraft and doesn't hurt when its not excessive.

[Guest]

It shows that your mechanic has little understanding of how the turbocharging system on your engine works.  With a fixed waste gate as is standard on the 231, over boost control is the gray matter between his ears.  

At lower field elevations the maximum rated manifold pressure will be reached before the throttle is fully opened.

Clarence

[Emmet]

The precise numbers were 43.3 on the incoming run-up. Then the turbocharger was changed (installed o/h unit). Therafter the run-up went to 43.8 !

Temperature was 19C.

The aircraft has a Merlyn wastegate.

Just looked up the overhaul manual which in the chapter for the wastegate adjustement clearly says "Do not exceed maximum rated power manifold pressure ..."

The OM I was referring to in my initial post was the Operators Manual. This only allows 1-2" overboost.

[kortopates]

Now that you clarified it doesn't sound like a tech had a plan or purpose except to maybe verify the overboost valve popping off. I am sure from your engine analyzer data you could look at the data from both run-ups and see the MAP become erratic (along with fuel)  when peak MAP was achieved showing the overboost valve was opening. Personally, I don't see the value in testing the overboost valve on the engine like that but that is the only think I can imagine was his intention - unless he really didn't know what equipment he was working on as Clarence suggest. I'll add the turbo overhaulers do not call for that nor does the TCM manual. Nor does Merlyn since it too does not limit maximum boost. The turbo overhaulers do suggest you make a test flight to verify the plane's critical altitude.  Anyway, TCM says you are allowed 2" overboost for a full 2 minutes. 3.8" overboost would irritate me as well  but personally I  would not be worried about 3.8" for a couple seconds (I am assuming that, but your engine analyzer can confirm how long it was.)