The theory of aid climbing ratings is very different from the reality (see below).
The theory is fairly simple but it doesn't really tell you much about the climb.
Here's the theory, but don't pay too much attention to it:
Aid Climbing Ratings - The Theory
The grades range from A1 to A5, and from C1 to C5. 'A' grades refer to anything that will need a hammer (placing pitons or copperheads), whilst 'C' grades are used if the pitch can be climbed without using a hammer, i.e. 'clean'.
A1/C1 – If placed correctly, every piece of gear would hold a fall. These routes are 'safe'. A fall is unlikely to do any more than give you a scraped knee or a bruised ego.
A2/C2 – You might place one or two 'bodyweight only' pieces in between good gear. These pieces wouldn't hold a fall, but your good gear beneath it would. A fall should be fine, though quite exciting!
A3/C3 – Falls may be long, but unlikely to cause serious injury – there's good gear somewhere down there, and no ledges to hit!
A4/C4 – You'll wiggle terrible gear into bad rock for around 10-20 metres until reaching a good piece. A fall at certain points will cause broken bones or worse.
A5/C5 – If you place your skyhook or copperhead one grain of granite in the wrong direction at any point, you will fall and rip out every piece of gear, hitting ledges and pulling off huge blocks as you plummet endlessly into the abyss. You will probably die.
In general, aid pitches get easier with more ascents. Piton scars widen and become more reliable cam or nut placements, copperheads become fixed, ‘chicken’ bolts and rivets get added and route-finding generally becomes more obvious.
Aid Climbing Ratings - The Reality
In reality, the grading system doesn’t really work.
Aid ratings are based entirely on the danger involved. The rating does not tell you how physically challenging a pitch is, or how difficult it is to figure out the moves.
Unfortunately, there is no way of accurately measuring how dangerous a pitch is – we can only guess. This works fairly well up to A3. But in the harder grades, it becomes a measure of fear. And fear is different for each climber and each situation.
Aid Climbing Ratings - What is A5?
Aid grades of A5 or harder impress the masses, but no climb can really be given the A5 rating without proof of certain death if you fall. Those tiny copperheads could hold, but you don't know until you fall off. And no aid climber is crazy enough to test this theory. Not even Ammon McNeely.
A grade of A5 cannot even be confirmed if someone falls off. This is because every climber protects pitches slightly differently. Some climbers place more gear, equalize pieces and add shock-absorbing slings. Other climbers back-clean, don't bring enough gear or miss out key placements in the pursuit of moving faster.
A3 gets upgraded to A4 due to fear. A5 gets downgraded to A4 due to lack of proof. Therefore, the grade of A4 becomes a vast spectrum of difficulty, which is only possible to describe when you've climbed enough of it. Here's my view:
A4 – What It Really Means
A grade of A4 could mean there is one well-travelled and straightforward section of fixed gear in solid rock which is really A2 if you spend time climbing it well (e.g: Lost in America, Zenyatta Mondatta, many El Cap trade routes).
Or it could be a 30+ pitch nightmare of rotten rock and death blocks. On an average pitch, the unfortunate leader will suffer in a perpetual state of mind-boggling terror as they sketch from one horrendous placement to another. It will often take over 30 minutes to construct a science-project placement which enables the leader to tremble one foot higher up the 3000-foot wall. Fear builds exponentially as they become further and further removed from anything secure and completely uncertain that they will ever reach a belay.
After 8 hours in a new and unfamiliar state of panic, dehydration and delirium, the exhausted leader will be forced to mantle out of their aiders into a long, unprotected free climbing section of unknown difficulty, protected below by a string of worthless ironmongery. This must be climbed while wearing a massive clustered aid rack.
Balanced at the top of these desperate free moves, the leader must stretch high to place a tiny copperhead and transition back to the final section of improbable aid moves to a belay which they must construct from pitons and duck tape. At least, that’s how it felt on Continental Drift.