WhiteWillow

So I was showing a student today how a multi-engine reacts to different control inputs, power, etc from an engine out from around 12,000ft.

I started the simulation by slowly retarding the right throttle to "sim" the engine failure. I then left the engine near idle and showed how feathering the right propeller took away drag and made flying the plane much easier. We putted around for maybe 2-3 minutes max.

This is where it got a little weird. I went to add power back in and I couldn't get any manifold pressure back. The prop RPM's were around 1100-1200 RPM's. No matter what I did with the throttles, I couldn't generate over 12-15" of MP. I can't quite remember what finally got the prop RPM's back up and MP back up, but I think it was going to feather and back to full RPM which subsequently made the engine run rough briefly and then the MP came back up. I also noticed the cabin pressure fell pretty rapidly and the light went on.

I'm curious as to why this happened? Is it because of the geared engines? I wish I knew more about the geared engines, but I was hoping another MEI or mechanic could shed some light onto what was going on so I can better explain this to a student.

DirectTo

Ok first off, why are you teaching in a 421? I cannot imagine a much worse multi-engine trainer. Expensive, large, tons of gas, probably not forgiving in a single-engine scenario.

Secondly, if you're teaching in it, you probably need to understand the principles behind the aircraft, especially on something as (excuse me mods) ****y as a 421.


On that note, it sounds like the engine did not come out of feather. I'm presuming you went to full RPM first, then attempted to add power. 2-3 minutes at a lower power setting could also provide time for the turbocharger to spool down, which would explain the lack of manifold pressure. I would say the root cause of the engine refusing to come out of feather however would be an oil issue, possibly a valve sticking in the prop (not allowing oil to return to the prop to drive the blades to a coarse pitch).
I cannot think of ever seeing anyone feather and not shut down an engine in a piston airplane. I'm not saying that's not permissible (I haven't taught in any 400-series Cessnas), but that seems odd to me and hard on the engine.

WhiteWillow

1 - I had a multi-engine student with me on a paying cargo run
2 - We were not out beating up the patch or doing anything other than a quick Vmc demo
3 - I did not fully feather the prop

Thanks for your help and answer!

N9373M

Could it be a cable going bad?

WhiteWillow

Never thought of the cable issue - although if memory serves me correctly, it didn't feel like the cable was loose or anything.

STR8NLVL

It's not the prop being stuck as that would've caused your MP to rise as the engine labored under high drag. The turbo spooling up maybe? How long did it take?

WhiteWillow

Well the MP did nothing with any throttle adjustments...it was stuck right around 14". Only after bring prop all the way back and the full forward (high RPM) did the engine begin to perform normally. MP slowly rose and ran rough for 5-10 secs and then she roared back to life.

DirectTo

Willow, sorry if I came off rude in my post last night, few cervesas last night with the lady.

Turbo spoolup sounds reasonable. Couple minutes at idle, plenty of time for the turbo to spool down and cool down. That would explain the delay in manifold pressure once the RPM was back up.

Just to confirm, you went idle, feather, demo'ed it to him, then back to full RPM and got nada? Back to feather, back to full RPM, then couldn't get MP? Then finally the MP recovered?

WhiteWillow

DirectTo - no worries at all...been there myself!

Your synopsis is correct other than I never actually went to full feather, just to a low RPM setting.

wizepilot

Watch out for bootstrapping. You bring that MP back too much, besides cooling the cylinders to quickly, that geared prop is turning the engine instead of the other way around. Not good! Never feather an engine on a 421 unless you REALLY have to do it.