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It has been suggested that this is the case, but I can’t see how one could design a gearbox which is guaranteed to shed teeth in any possible propeller contact scenario before any possible damage occurs to the engine, but at the same time is robust enough to last say 2000hrs.
FWIW: when I broke my prop, close on two years ago, the gearbox was briefly, informally, visually inspected, and no damage found. Later discussion confirmed that, IF the slipper clutch is installed as it is on my 912, damage to the gearbox is very exceptional.
http://i.imgur.com/Ew8mD.jpg
Who says there are only gears in a gearbox? There are also clutches, torsional vibration absorbers and other mechanisms that provide a shear limit. None of the geared diesel engines in aviation have a direct gear connection between engine and prop.
If the engine manufacturer says that a prop strike requires a gear box service but not an engine teardown, then that is all you have to do. Also I would not demand the gearbox to last 2000h or more without service but rather that service is at reasonable cost. The job of the gearbox is not only to reduce the engine revs but also to decouple the engine from the torque impulses from the prop. This is hard on the gear box and makes it a maintenance hotspot but at the same time it is great for the engine and makes it last longer.
With Rotax engines, you regularly insert a pin to block the crankshaft and then you measure the torque required to turn the prop by 15° This is to determine the condition of the friction disk which has to be in a certain Nm range and needs service if outside. This is a standard maintenance item.
Well, AFAIK it is even considered a propstrike if the composite prop of a Cirrus hits the towbar … so one that breaks a gearbox must be a propstrike too …
Turboprops and GiTSOs need a lot of shockloading work after propstikes. Hasn’t happened to me (yet), but I know plenty of cases. The latest one here at my homebase when a Kingair hit a fire extinguisher on the ramp. The engine was away for almost two months and has just been re-installed.
I had a chat with the guy who maintains most of the remaining Porsche fleet. In case of a prop strike, it depends whether the prop came to a halt and where the prop broke off. Depending on that certain inspections are required. These start with the gearbox which needs inspection for cracks and a replacement of the torsional damper. Then the crankshaft (with the gearbox removed) is measured and only in case it is bent, an engine teardown is mandated. This means that engines never have to opened unless it is evident that the crankshaft has seen damage. Unless the engine needs to be opened, the inspection and repair can be carried out in the field.
In Rotax engines (which typically have a clutch opening at max 50Nm), the procedure is very similar. The gearbox has to be overhauled in all cases (as opposed to inspect and repair as necessary with Porsche) and the crankshaft head is measured for damage. This means a repair in the field is not possible.
I personally think that when looking at the pros and cons of gearboxes (pro: smaller engine, higher engine efficiency, decoupling of crankshaft and prop, con: complexity, wear), an aero engine with a gearbox is the better aero engine.
