I am with chilli on this. the engine fund has grown out of the hourly rate and is ring fenced. think of it as deferred expenditure. it will have to be spent eventually.
if there are 4 in a group, the aicraft is valued at £20k then a share is worth £5k. If the engine needs replacing then then engine fund is spent on it as the fund exists to cover the flying that has already occured. The remaining bill is covered by the group.
Perhaps it would be easier to think of it as paying the engine fund money directly to the maintanance company every hour and when the engine fails you get a cheap engine with the cost split between the group.