[Luno13]

I believe the game HUD message for damaged fuel tanks is simply: "fuel tank: leak". It doesn't give any info as to whether or not it has or will re-seal. You can only watch the gauge and find out.

The game also models 3 levels of fire: thin grey smoke, thick black smoke, and full-on fireball. The thin grey smoke typically burns all the fuel in a matter of minutes. There is no way to stop this, which is why I'm lobbying for fuel-tank selectors. The thick black smoke also burns fuel quickly, but after a random amount of time, it will almost certainly develop into a fireball, killing the player, exploding the tank, or melting the wing spar.

The fire can be put out with great speed, but will always return. I think some more variability should be implemented here, and I think an empty tank shouldn't keep burning forever (unless I'm wrong about that).

The engine burns in a similar way to fuel tanks, which is why a fuel-cutout valve should be added. Some planes had extinguishers for the engine, but as far as I'm aware, this is only implemented in the Avia B.534 and bombers. The PZL.11 had a jettisonable fuel tank.

Thin grey smoke from an engine however, typically is caused by burning oil. It's not the only indication of damage, though. I've had shot-up engines whine and seize without any smoke at all, or even damage textures visible on the cowling. Maybe this is an indication of coolant damage?

As far as I know, coolant was mostly some glycol mix, and should look like leaking fuel (thin white stream). If it's in the DM, it certainly isn't represented visually.

There are no streaming fluids from hydraulics, but I imagine the reservoirs for those aren't as large as for fuel and oil tanks. Hydraulic damage only affects landing gear, but some aircraft had a system which used a valve to control flaps, cowl flaps, etc. from the same reservoir. Other aircraft had pneumatic systems, which can't be damaged, or simply act like hydraulic systems. Hand-pumping landing gear systems are impossible to damage.

O2 lines and bottles aren't modeled, but it would add an interesting element to the DM.

It's worth mentioning, though a little OT, that engine controls can be damaged, causing the throttle to be stuck at the last setting it was left at. There is no HUD text to indicate this. Damage to control lines affects the control and trim, but this wasn't always the case (there are stories of guys flying back using only trim after controls were shot out).