Engine shutdown

[Redglyph]

Hello!

 

At several occasions, I've seen pilots revving their engines for 30 seconds or so before the shutdown, and I'm not entirely sure for what purpose(s). Here are a few examples:

[ame=https://www.youtube.com/watch?v=SG2pqITEl8w]P-51[/ame]

 

(at about 10'40")

 

[ame=https://www.youtube.com/watch?v=-7HUDAVUcUg]P-38 and F-4U[/ame]

 

(at about 3'25" and 5'58", though the latter is shorter and might be for other reasons, the corsair has a radial engine).

 

Possible reasons:

• Cleaning-up spark plugs? Would that need to be so long, and with such high RPM as the P-51 video? And does it really clean up potentially fouled plugs? I've seen advises from mechanics to run the engine above 1000 RPM for a little while before shutdown when an engine (or a pilot...) tends to get fouled plugs, but the best is probably to avoid the situation, by leaning or not letting the RPM too low.
  
• Making sure the scavenge pump removes any exceeding oil (dry sump engine), possibly with extra oil coming from the propeller governor when the unit is set back to high pitch at shutdown (to ensure the oil goes to the engine cylinders first on startup). But isn't the oil poured back into the cylinders constantly, and even more so at high revs?
  
• Loading the batteries? But it seems to me that they had plenty of time to load during the flight.
  
• Doing a magneto run-up check? That would make sense, to detect any potential problem that could occur during the flight and may cause harm after shutdown or on the next startup (like auto-ignition if the ground wire has a contact problem). But it seems a bit long and, again, high RPM for that test in the case of the Mustang.
  

 

I'm really curious about this, could anyone shed some light?

[Chris1012]

One and four are on my A/C's checklist - RPM 1200, dead cut mag check

[aaron886]

It's for the oil scavenging, so good guess! The pump is more effective with that increase in RPM. The effectiveness of the scavenge pump at low RPM is of greater importance than the flow rate back to the engine. At low idle RPM the oil is collecting excessively at the bottom of the crankcase.

 

Full disclosure, I don't fly Mustangs. :D

[Redglyph]

Thanks, interesting info! (I suppose you meant its effectiveness at high RPM is of greater importance ;) )

 

Easier than removing the bottom plugs and getting messy on a radial :D