Backdriving the GTSIO-520

I only flew a 421 once or twice but I have a fair amount of time (4-500) hours in the un-pressurized brother, the 404. Our cardinal rules were simple: power reductions done at 2 inches of MP per adjustment, per minute. Don't let the props drive the motors. We got TBO out of engines probably 90% of the time, but they were also Gold Seal Engines. Now for approach and landing, I could literally cruise up to the G/S, drop the first flaps, slow below gear speed, drop them, slow to final flaps, drop them, and barely make any power reductions except to pull it back to approach power for my final approach speed. Backdriving issues originally came from round engines, as the seals were not secure when the prop pushe the shaft back into the engine. This caused leakage around the main thrust bearings and caused lots of concern amongst mechanics and pilots alike.
 
denver said:
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?
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My family has been in the engine business for 60 years. This is not a wise tale by any means. You can destroy a geared engine by having the propeller driving the engine. All one has to do is listen to the harmonics when it occurs. It does not take a scientist to figure its not doing the engine any good. Counter weight bushings are the first to take the hit. When those fail the counter weight will make an appearance out the side of the case. We have torn down thousands of engines in our 60 year history in the engine business and have verified this more than once. Radial engines have the same issue and have a specific operational requirement to keep the manifold pressure above the RPM. You simply need to maintain a positive load on the propeller during flight. When the aircraft reaches approach speed the issues diminishes significantly. Its not difficult and the engine will love you for it. Just because the manufacturer does not include a warning in writing doesnt mean its a wise tale.
 
Thomas Jackson said:
My family has been in the engine business for 60 years. This is not a wise tale by any means. You can destroy a geared engine by having the propeller driving the engine. All one has to do is listen to the harmonics when it occurs. It does not take a scientist to figure its not doing the engine any good. Counter weight bushings are the first to take the hit. When those fail the counter weight will make an appearance out the side of the case. We have torn down thousands of engines in our 60 year history in the engine business and have verified this more than once. Radial engines have the same issue and have a specific operational requirement to keep the manifold pressure above the RPM. You simply need to maintain a positive load on the propeller during flight. When the aircraft reaches approach speed the issues diminishes significantly. Its not difficult and the engine will love you for it. Just because the manufacturer does not include a warning in writing doesnt mean its a wise tale.
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Many a jump operation failed to reach anything resembling TBO on the GO-480s on their Twin Bonanzas.

(Of course, jump people are savages, but still.)
 
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