Thanks again, Tom, once gain I can't say I'll be able to fully answer your queries and Joe stepping in helps...
How do we measure this for this thread? We can use maintenance man hours to get something of an idea, particularly for the airframe types that were detail maintained. But does this apply to the armed forces and/or airframe types that used a fair amount of what were effectively LRUs? Also what types/levels of maintenance and repair do we use.
Regarding maintenance hours, yes, that's a good measure of reliability, or more relatably time spent on the ground. Using my own experience, which doesn't equate to the same as WW2 aircraft maintenance, but might help contextually, airlines make money when their aircraft are in the air, so having them on the ground undergoing maintenance is anathema to their very existence, hence why line maintenance is done at night between day operations. Heavy maintenance is a fact of life and affects aircraft availability, the schedules that Mike Meech provided give us a good clue as to aircraft servicing times for the RAF, even though they are post-war they are probably closer to wartime than the way maintenance is performed today. Aircraft that routinely spend time on the ground owing to defects suddenly arising are naturally regarded as being less reliable, and again it depends on various factors. The equipment fitted, like for example the Darne guns had their own reliability issues, which affected the aircraft's serviceability.
The concept of LRUs is not a new one but has certainly been refined and made easier compared to wartime. Each rotable unit today has its own time between overhaul and this record is kept separate to aircraft maintenance schedules, so even while in service and in the middle of a run, an aircraft can be taken off the line for replacement of an LRU at any time as these have regulatory expectations to be met. This is usually recorded on their CAA Form 1 documentation, which gives serviceability info.
As for what levels of maintenance do we use is a moot question. Knowing and understanding each system of maintenance is the key and that might not be translatable between armed services. I certainly can't add any more to that as I simply don't know the answer. The table that Mike provided is a good insight as to how the RAF did it.
Organisations can certainly chop and change when maintenance is done to an aircraft, as long as it meets the criteria as stipulated in the maintenance manuals. If the manuals state the aircraft are to have 100 hour servicings, then this needs to happen. The organisation might wish to separate each task out into individual jobs spread out over time or at different bases, but as long as the work is done in accordance with the manual and its TBO stipulations.
The A6M Zero. The overall maintenance record of the Zero shows that it was relatively reliable and easy to maintain. But, as has been brought up in other threads, the center fuselage section and wing were built as a unit - so any major serious to either required the replacement of both, with the center section being sent back to the factory if salvageable. I assume that the initial purchase orders/costs included some spares for rebuilding an airframe, but out on the islands they would have quickly become unavailable due to logistics issues. How does cannibalization count toward reliability and maintainability? How often did the center section need to be replaced? Did the concept payoff due to operational considerations?
Those are good questions and directly pertaining to the A6M offers a different scenario to, say, even F4Fs and F6Fs at jungle strips simply because of the logistical situations arising between each of the forces. I couldn't answer directly about rates of serviceability for the Zero, certainly unit availability rates can assist with that, if anyone has pertinent figures. Again, this is where environment and quality of engineers come into the fold. In situations where logistics dries up, more inventive means of coping with that situation have to be taken into account. Iran keeping F-4s, F-5s and F-14s serviceable despite a dearth of parts is an example of this. Using the Hawk SAM instead of AIM-54s is an imaginative solution to a thorny issue, and for the IRIAF, it appears to suit its needs.
Using canabalised parts is an obvious and often necessary measure and I've seen it done by airlines even today. We used to often park aircraft that were awaiting parts from overhaul facilities outside our base and use them as parts trees if we didn't have what we needed in store to get another aircraft back on the ramp for the next morning. These days there are regulatory issues surrounding that, as everything has to be traced and every step we make has to be made in accordance with some piece of legislation, but during the war on a coral atoll, I reckon the regulations might not have played such a decisive role. You can guarantee that parts were canabalised from spares trees based on the logistical availability of parts.
By the end of the war, to a certain extant the USN was treating entire airframes operated off of CVs as one-use articles. If an airframe would require more than a certain number of man-hours to repair the aircraft would be set aside or tossed over the side. This last example may not apply to the OP, as it is replacing an airframe as opposed to maintaining it, but the range of the concept goes from the Darne to what % of the airframe?
I'm not so knowledgeable of US Navy practise, but common knowledge/perception (?) is that US aircraft were a little more expendable, or should I say in riposte, other air forces were a little more attentive regarding the recovery of downed aircraft and parts - again, a perception rather than a fact? I remember reading a personal account of action by an RAF unit in Europe post Overlord, the individual commented on a wrecked P-47 he had seen sat forlorn for ages at the FOB he was at, and he wondered whether or not it was going to be removed for spares. The US and its industrial might certainly had options, but I wouldn't want to say the US forces were less careful regarding airframe serviceability because they had the option to leave aircraft because there were ten more arriving by boat next week, so to speak. Some of our other members might be able to elaborate more on US military policy regarding this during the war. The US had maintenance ships that were parked at strategic locations offshore from battlefield theatres that were outfitted with workshops specifically for the maintenance and overhaul of components, which was a clever idea.
The same goes for propellers to a certain degree (I think). I do not know much about the maintenance/repair of the cans/hubs, but I am under the impression that all types of blades could be repaired if the damage was not too serious - the limits depending on the type of blade and extent of damage. Aluminum and steel blades were often(?) straightened and had minor nicks and such repaired and/or tips reduced. Wood blades tendered to suffer more permanent damage due to breakage, but I have read that due to the wood blades breaking the hubs and/or reduction gear boxes were less likely to suffer damage. If the air force had replacement blades galore, and they simply replaced the blades or hubs rather than repair them, do we count the man hours needed to manufacture the blades and hubs - or just the time to replace the blades?
Yup, Blade repair obviously is measured and even during the war was done in accordance with the maintenance manuals, which outlined tolerances. Those aluminium props could take quite a beating and I've seen the extent that they can be straightened if bent. Servicing the hubs is like any component, really, high tolerances and measurements taken with Mitutoyo gauges and DTIs in a sterile workshop environment. When I worked in a prop shop, we had to dismantle and inspect props from aircraft that had been involved in accidents, mainly to determine if prop failure was a contributory factor in the accident's outcome. The policy today is that no component from a crashed propeller can be re-used, even if it passes tolerance testing. That is certainly a luxury that wartime engineers didn't have, so again serviceability is dependent on the manuals, which even back then were very detailed and specific.
As for man-hours as a quantifiable measure, it depends on the facility, what the manuals state, which'll differ between prop manufacturers, experience levels, and spares availability and so on. As mentioned before, efficiency or lack of is measured using various yardsticks. If you can retrieve such figures for individual props
in a wartime setting, I think you might be lucky.
So, in summary, a key to this is understanding the maintenance structure within an organisation or service. Understanding the premise that aircraft require maintenance and the types of maintenance required is important. The foundation of aircraft maintenance is the humble inspection and carrying these out on a frequent basis is decided by the aircraft manufacturer, which ultimately decides an aircraft's maintenance schedule within an organisation.
There's a difference between scheduled and unscheduled maintenance. The former is planned inspections based on time in service, during which defects are found and work arising from those inspections determines how much time the aircraft can spend on the ground. These are your alphabet checks in airline parlance and your '100 hour' servicings and so forth. Unscheduled maintenance is stuff that happens, a burst tyre, stone damage to a fuselage or wing, bird strike etc, which all has to be fixed at some stage or another, requiring the aircraft to be taken off the line.
I hope that helps a bit.