My original comment about jets was sort of directed at Rainboe who said that a shutdown engine looks like a running one....which surely only applies to jets (presuming that one is talking about a feathered prop).
Actually, your comment quoted me. That's okay, though. A shutdown engine does look like a running one...and that doesn't just apply to jets.
This is particularly the case with turboprop engines....especially free turbine engines...in which shutting down the engine isn't the same as feathering the propeller...two separate subjects. In many cases, feathering should happen...but an engine can run in feather and not be shut down, or the enigne may shut down and not feather. I've seen both.
Even a piston engine when shut down and not feathered...looks very much like a running engine.
For an engine that can be feathered...did it feather properly, will it come out of feather? If you haven't had an opportunity to feather propellers in flight, some stop, some don't. Some continue to windmill to some degree and I've seen that happen plenty of times, too. In the case of the hydromatic ham standard props previously mentioned, if one didn't get the feather button popped out at the right time, it would go right through feather and drive back into a running state again, windmilling and creating an enormous amount of drag. With one prop out of four windmilling, it didn't take a camera to know that full rudder was required for an outboard engine...with over 70 lbs of force to hold the rudder...it was fairly obvious.
Some years ago when I was going for my PIC card in a piston prop tanker, I had to take a checkride with a government official riding along. As I approached my drop point, my "copilot," a very experienced captain, said "low oil pressure on number 3." I glanced at the gauge, and it showed oil pressure in the green (just fine). I said "I don't see a problem." Again, he said "Low oil pressure on number 3." In my mind I was quickly reviewing the reasons he might say this. Perhaps he was seeing a massive oil leak from a cracked propeller governor stephead base...not uncommon...but then he'd have told me. Perhaps he was seeing something else...why was he saying this. Again, I said "It looks fine," and concentrated on my run to the drop point.
Suddenly it dawned on me (I'm a little slow)...this is a checkride, and this is a test. Okay, proceed past the point, jettison the load, shutdown the engine and feather it, and exit the area. Go home. We jettisoned the load, and went home.
Did I ever get a tongue lashing. Both from the experienced captain and the government observer. Why, they wanted to know, did I not jettison the load when I was told I had a serious engine problem? Why did I not abandon the approach to the drop and exit down canyon and jettison? Was that the time to be trouble shooting? Didn't I know the procedure? What was I thinking?
The upshot was strong counsel to not wear two hats in the airplane. We had procedures given us. I didn't need to be a mechanic while I was flying that drop. Just a pilot. I didn't need to explore all the reasons I might have low oil pressure, or even try to fix the problem. Low oil pressure was serious because that oil came from the same resorvoir that supplied the feather pump..and without oil I wouldn't have been able to feather the engine...and that airplane with a load and one prop windmilling wouldn't hold altitude...even with power on the other three engines. It's serious. I had good information from a reliable source...the experienced captain, and I had a procedure to follow...I didn't follow it.
Compare that to the use of the camera. We have procedures in the airplane...we have a job to do and a way to do it. A very good way, developed by millions of dollars of research and development, overseen by hundreds of thousands of hours of flying experience, with input from aeronautical engineers, captains, first officers, and flight engineers, through decades of flying...these procedures cover what needs to be done, have had the benifit of hundreds of thousands of hours of real world experience and the real emergency...and they work. None of them require a camera...I could have used a dozen well placed cameras to look for the oil leak...but that would have been just as bad as what I did...it would have been the same thing. I had a procedure to follow and should have followed it. Trying to look at a picture to somehow add my own twist to it or modify the procedure would be trying to second guess and improve on far more experience, training, and education than I'll ever have in this lifetime or the next...it would be me, trying to reinvent the wheel.
The wheel works well, as do the procedures...without need for a camera. From a camera, one doesn't always see what one wants to see...one sees an image...but from the cockpit we see a detailed picture spread out over multiple systems from multiple angles. If I were to look at a picture and see a feathered propeller, I could say ah-ha! The engine has failed...but that may not be true. Perhaps I have an oil leak, and I have an engine fire. The picture may tell me to reach for the engine failure in flight checklist...which is not an emergency procedure. Instead, I may have an engine fire problem...which certainly can be an emergency and is part of the emergency checklist. It also has memory items that the engine failure checklist doesn't (depending on the airplane, of course).
You can see, then, that what one sees on a camera might be entirely different than what's really going on. An excellent example would be an engine fire during the engine start...torching, with flames blowing out the tailpipe. Seen by people on the ground, it's a scary, quite possibly very loud, very hot, dire emergency. Long flames trailin out the tailpipe, with burning liquid fuel dripping on to the ground beneath, pooling, and burning. Black smoke. The ground crew goes running away. People point and yell. The camera would say we're on fire, shut it down.
In truth, we have a different procedure. Yes, we're on fire, but if we shut down that engine, then we may create an uncontrollable fire. We have tools to deal with it and a procedure that's very simple. We simply shut off the fuel and keep cranking. If we pull the fire handle and shut everything down to fight a fire, we can't do that. The pylon bleed air valve has shut off...effectively preventing us from putting bleed air to the starter motor to keep turning that engine. Now we can't do anything about it from the cockpit. Instead, we follow procedure. We don't respond to it the way people seeing it from the outside (the camera view) would do. We shut off the fuel manually with the start lever. We apply ground start ignition, which supplies air to the starter motor, and we turn to our EGT gauges to wait until we see 180 degrees, before we stop motoring. Now, if we have a fire indication and the engine is indeed on fire...now we can T-handle it, shutting off the hydraulics, bleed air, fuel, and electrical...and arming the fire bottles. Now we can go to the next phase...but in the case of both events, even if they're combined to be the same event, acting based on the visual image from outside the engine could very easily lead us to do the wrong thing.
Instead, we have instrumentation inside that engine; very detailed, very sophisticated instrumentation to give us a lot more information than what might be offered by a camera on the outside. We've got some fifteen instruments feeding us information during that start, as has been previously discussed, to give us critical need-to-know parameters...and this is what we use to make our decisions. Very often, what's going on outside isn't at all like what's going on inside...and we're very concerned with what's going on inside the various parts of the airplane.
Cameras have their place...but for most of our practices and procedures, very little application in the cockpit.