Backdriving the GTSIO-520

D

denver

New Member
Everyone I have talked to says to NEVER let the prop drive the engine with this engine found on the Cessna 421's. However if it was so bad than why is there no limitation published in the AFM, placards or TCM engine manual? Supposedly it has to do with crankshaft counterweight dampner bushing wear and gearbox backlash. They do make a nasty noise at the neutral point but the noise goes away at full idle in flight. I have yet to get to the bottom of this one and cannot for the life of me figure out why Continental would design an engine that could not be run in this area. Could it be a carry over myth from radials? Makes it tough to practice engine outs and come in at a reasonable speed and altitude on final.

I could see if one went on noise alone that the bad noise comes from the neutral thrust point as the free play in the gears bangs back and forth which cannot be good. But with a positive drive back on the engine (prop fully driving engine) someone has yet to convince me why this is bad. Why would they design the gears to only drive in one direction?
 
Really, that's the first I have ever heard of anyone say that. Every 421 driver I have ever talked to says NEVER do THAT. If this is the case than it is the most limitating OWT in aviation history other than shock cooling or a following sea.

Do you guys have experience running these engines and how confident are you with this determination?

I can't ever get anyone from TCM to return my phone call, maybe they got taken over by the government.
 
Roger said:
Old wives tale.
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Not necessarily. Crankshaft detuning and nastiness DOES happen on the GO-480, but it isn't the world ending occurrence that everyone bills it as. Prolonged high IAS, low MP and high RPM descents are not good for the -480 (decades of Twinbo operators can tell you all about it), but I can't vouch for the -520.

Proper descent planning prevents bad things in powerplants (rapid cooling still no bueno) and in other operational areas. (Make the airplane do what you need to do, of course, but start down early.)
 
We maintain about 8 421's at our shop. One customer manages 3-4 of them and he has over 18k hours operating 421's. He will tell you NEVER to let the prop drive the gearbox. Ever. We have a damaged gear assembly on a shelf somewhere in the shop from another customer letting the prop drive the engine. I will try to get a picture of it if I can find it.

Im not surprised that TCM won't talk to you. Try Bob Mosely; I think his shop is in Missouri. He was a lead A&P/IA for TCM back in the day and is reknowned in the Twin Cessna community as being one of the formost experts in GTSIO-520s.

As an old 421 driver once told me, "You want to treat the GTSIO like a fat girl, nice and easy."
 
Hey thanks for that, If you can find a picture that would be great. It has been starting to make more sense to me as of late as I visualize how the gear reduction works. I think it is that neutral crossover point that is really bad. This is where you get the nasty noise and vibration. All the power strokes on the 6 cylinders happening alternatively create a push at 1/4 turn and the last 3/4 turn in pull. As you get to the crossover point where the engine starts to unload you will have a range where the backlash in the gears bangs back and forth very rapidly due to the balance of cylinders coasting to cylinders pushing through the rod to the crank. This all happening at a 3:2 ratio with a higher RPM on the engine by a considerable amount. Then as the engine fully loads in reverse this noise goes away due to the fact that the gears are now being firmly negatively driven. This would not seem as bad but they tell me it is. The reasoning is that the back side of the gears were not designed to be driven on. To me it seems like they could have designed the gears for that since it is an airplane but I am not an engineer so there is probably allot I do not know as to why this would be and possibly a good reason for it.
 
TwinCessnaDriver said:
How often do engineers and pilots see eye to eye on aircraft development?
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How an engineer thinks:
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How a pilot thinks:
anthair.jpg
 
I was told the same thing about the letting the props drive the engine in the 421 back when I was flying one. I did have it happen once and let me tell you. Those noises and vibrations that come from that engine when that happens are not something you want to feel. That GTSIO is letting you know that It ain't happy at all. Don't know much about there being a technical reason for why not to do it, but once you feel it you will do your best never to let it happen again.
 
Wow... I've never had a problem... or even a reason to let the props drive the engines. I've always left the props just where they are during the entire landing:bandit:

Passengers appreciate it... I don't understand why anyone would increase the props in a decent unless you are seriously gasping for drag/borderline emergency.
 
ryan1234 said:
Wow... I've never had a problem... or even a reason to let the props drive the engines. I've always left the props just where they are during the entire landing:bandit:

Passengers appreciate it... I don't understand why anyone would increase the props in a decent unless you are seriously gasping for drag/borderline emergency.
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Propellers are not speed brakes, and I'm with you on leaving them where they are. If shooting an approach to the bottom in the Bo, the props get stood up to climb power at gear extension, but I rarely push them up in any other circumstances. Too noisy, and I'm capable of pushing them up in a hurry if need be.
 
ryan1234 said:
Wow... I've never had a problem... or even a reason to let the props drive the engines. I've always left the props just where they are during the entire landing:bandit:

Passengers appreciate it... I don't understand why anyone would increase the props in a decent unless you are seriously gasping for drag/borderline emergency.
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As far as I can tell, this discussion isn't about prop RPM settings. It's about managing power in an airplane with geared engines. You can't just reduce the throttles to a point where the prop is windmilling and driving the engine because then bad things happen.
 
If you fly props there is always a time you will have to let the prop drive the engine and that is during landing. In a non-geared engine it is not noticeable at all when the change happens. If you fly power off approaches in a single engine airplane the prop is driving the engine heck if you get below about 12-13" this begins to happen. So you are letting the prop drive the engine more than you think and if you land properly this will happen once per flight, it is inevitable. If the prop slows down on the landing roll all by itself than the prop was driving the engine. In a non geared FLAT engine it is no problem at all. In a geared FLAT or non-geared radial it is apparently a problem and the engine will definitely let you know with a noise and vibration that will make you never want to do it again. I land with the props set a cruise and even then your minimum power setting is 14". You have to bring the throttles back at sometime unless you want to go off the end of your 4000 foot strip and it is at this instance that the prop will drive the engine until you are under about 30 knots. You can have the props at min rpm and this will still happen when it is time to begin the landing flare and the power comes back to idle. The issue is really in prolonged descents in this condition and should be avoided.

In a 421 it is a major inconvenience to say the least to be left high and dry on an ILS by ATC in moderate turbulence with this limitation when in a Cessna 182 or a king air it would be absolutely no problem. If I had it my way every time all descents would be at 500 fpm and under the glideslope in this aircraft. But ATC has other plans.
 
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