Quote:
Originally Posted by Crumpp
Exactly. It is no different than the Take Off rating in a Lycoming O-360. As soon as the trees are cleared, it is time to back off. Otherwise you risk having the engine fail as it is not designed to maintain 28mmHg @ 2700 rpm.
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It's the prop that might fail, not the engine. This is arguably worse because losing a blade will cause huge amounts of vibration, which will make it very difficult to fly the aeroplane (because you won't be able to see very well) and may cause secondary failures if the engine isn't rapidly shut down. If the engine is shaken out of its mounting then the CoG will probably shift so far that the aeroplane will be uncontrollable, and unless you're wearing a parachute and are able to use it, that's almost certainly going to result in a fatal accident. Whereas just having the engine fail means a forced landing, which one would
hope to have a good chance of walking away from.
28 mm Hg is a pretty severe vacuum... I suspect that you mean 28" Hg, which would be almost atmospheric pressure, roughly what you'd expect for a naturally aspirated engine running WOT at sea level on a standard day.
Actually, you'll find that because something like an O-360 is naturally aspirated, it's more likely to tolerate WOT operation than a supercharged or turbocharged engine because the manifold pressure (and therefore the power) simply lapses away as you climb. (Of course, a lot of GA aeroplanes don't climb very well, so YMMV, especially if you subscribe to the old-fashioned view that takeoff = 1 minute unless something has gone wrong.)
Equally, you risk engine failure at all times. Even
before you start the engine (which is why you should treat all props as live, because magnetos can fail ON).
You should never fly an aeroplane into a situation in which losing an engine will leave you with no options, because as Chuck Yeager wisely advises:
Always leave yourself a way out.
So I would modify your first statement to say that "Otherwise you
increase risk having the engine fail
beyond that assumed during certification".
This increase might well be pretty small, especially if the engine has a long overhaul life and you only misbehave very infrequently. Indeed, if you're prepared get the engine overhauled more frequently than the nominal TBO then you may actually be able to charge about the sky WOT all the time and still have a very safe engine.
Does this mean that you can ignore the limits? Of course not.
But it's important to understand what the limits are about. They're intended to deliver acceptably high reliability at the end of the certified overhaul life of the average engine,
based upon a set of usage assumptions.
This means that the same engine might have very different ratings for different applications. For example, given the same total number of flying hours, if you were going to fly lots of short sorties, you'd spend a higher proportion of each flying hour at takeoff power than if you were going to fly a smaller number of longer sorties. You'd therefore feel comfortable in allowing a more aggressive takeoff rating in the latter case than in the former.
If you've got a short duration manifold pressure limit, a naturally aspirated engine, and a reasonably good power:weight ratio, you may well find that you can actually operate the aeroplane WOT from takeoff to top of descent, without exceeding the limits, because the aeroplane climbs fast enough that the manifold pressure has lapsed to less than or equal to the maximum continuous rating before the time limit is exceeded.
Something like a GTSIO-520 might be a bit less forgiving, but if you look at the
TCDS, you'll find that the manufacturer helpfully just flatrated it such that the takeoff power = max continuous.
Engineers do this because it is generally assumed that pilots can't be trusted to obey the limits in the Pilot's Notes. Sad but true.
Now, actually AFAIK these engines didn't have the best reliability record in the world, but that was more due to how people operated them in the cruise than due to people charging around with all the levers rammed into the firewall.
If you go to the average flying school in the UK, at least in my experience, most people will have no idea what the actual limits for the engines in their knackered old Cessna 150s and 152s are.
They'll almost all be flying the aircraft outside of their certified weight & balance envelope (indeed, when I did my skill test, the examiner from the CAA, who was 6 ft tall and not exactly thin, looked over my shoulder as I performed the weight & balance calculations and told me that he weighed 4 stone - which made us "legal" - to which the response was of course "yes sir", given that I wanted my PPL). Very few of them will perform as well as the book says they should, and a lot of the time they'll be "cruising" with the throttle crammed into the firewall (for the first few weeks of my PPL I actually had the Cessna throttle shape embossed into my hand...).
None of this is good, but it is reality.
People get away with it because the engines are pretty much agricultural (and the airframes are forgiving). They drink fuel and oil at a prodigious rate, they have a pretty low piston speed, are naturally aspirated and actually quite heavily built.
That O-360 makes about 180 bhp, which is only 0.5 bhp/cubic inch.
The O-360 has a stroke of 4.375 inches and turns at a maximum of 2700 rpm. This gives a mean piston speed of 1968 feet per minute. The TBO is about 2000 hours.
An early Merlin on 100 octane fuel had a combat rating of about 1300 bhp from its nominal 1650 cubic inches, which is 0.78 bhp/cubic inch. Late war engines on 150 grade were putting out about 2050 bhp for war emergency, which is almost 1.25 bhp/cubic inch.
The Merlin has a stroke of 6 inches, and turns at a maximum of 3000 rpm, giving a mean piston speed of 3000 feet per minute.
The nominal overhall life of a a fighter Merlin was 240 hours at the start of WWII, increasing to 300 hours (360 for twins) at the end. A substantial proportion of engines didn't make it to their nominal life.
[I've left power:weight out of this equation because otherwise we risk a chart war over different engine dash numbers and mod states for different power ratings, resulting in rather different weights, not to mention the secondary argument about the weight of the Merlin's cooling system. But I think it's probably fair to say that the Merlin's installed power:weight was also a bit better than that of the O-360.]
The military would accept rather higher accident rates in wartime than anybody (regulator, manufacturers, pilots) would accept for GA flying in peacetime. So if you wanted to match the engine failure rates without de-rating or changing the usage schedule you'd probably end up with considerably shorter overhaul lives.
The fact is that the margins in GA are sometimes pretty generous.
A couple of renewals ago, I had to fly off some hours, so I bashed the circuit in a PA-38. It was a beautiful sunny day, about 15ºC with some reasonably strong thermals and quite large cumulus development; I rather regretted not being in a glider. The engine was sometimes a little sluggish when I opened up to touch & go, but nothing dramatic.
The next day I went in to the flying school to do some more hours and got asked all manner of pointed questions about whether I had checked the carb heat before I flew, which of course I had. It turned out that some time after I checked it, the cable broke, and I'd been flying touch & goes for probably the best part of an hour without it in almost perfect carb-ice conditions.
Does this mean that you can get away without using carb heat? Certainly not. But it illustrates the fact that failure to do so doesn't cause instant/rapid, guaranteed engine failure (as it does in some flightsims, such as X-Plane).
It just increases your exposure to risk.
The point here is that IRL there is no re-fly button. This
dramatically changes the way in which the average person will behave.
If you've got a 1% chance of having to hit re-fly because of questionable decision, most people will shrug their shoulders and press on. Swap that out for a 1% chance of actually being killed, and a lot of people would change their behaviour.
For example, it's a legal requirement in most places that you wear a seat belt when travelling by car. But even in places with really bad road traffic accident rates, most people would expect to manage more than 100 journeys between even minor crashes.
To put it another way, imagine for a moment that 1c make the Wellington flyable in a future expansion. IRL, RAF bomber command sometimes suffered a 10% loss rate. This was unsustainable, both for reasons of morale (given that the probability of surviving a tour was therefore pretty low) and also for the simple practical reason that it was difficult for the training organisation to produce sufficient crew to replace those losses. Very few people would want to step into a bomber pilot's shoes IRL.
But if you look at the way most people fly online, they'd consider a 10% loss rate per sortie to be somewhere between good and excellent (though they'd find the actual flying to Germany and back very very boring, given that almost 90% of the time nothing much would happen, and they'd probably bomb an empty field due to the limitations of their navigational abilities).
Quote:
Originally Posted by Crumpp
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This isn't really especially dramatic. 8700 hours is quite a lot of life.
It seems to me that this document is largely aimed at lawsuit avoidance.
I'm not saying that the information it contains isn't valid & important, but we're really talking about different orders of magnitude of safety margin from those associated with WWII aircraft and engines; compare and contrast with the nominal overhaul life of a Merlin.
Quote:
Originally Posted by Crumpp
It is much better to be down on the ground wishing you were flying than flying and wishing you were down on the ground.
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Amen to that. But if you get away with it then it's an extremely effective learning experience.
Quote:
Originally Posted by Crumpp
It amazing that most gamers just think changing a few parts is no big deal.
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That's because they don't have to pay the bill!
Quote:
Originally Posted by Crumpp
They don't seem to understand the low safety margins required for flight. Facts are it is the simple things that can kill you in aviation because of those low margins.
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Yes, but most of the time they don't. One of the biggest problems in aviation safety today is that aviation is safe enough that people get complacent, and this makes it very difficult to improve the accident rate, because people feel safe enough to cut corners, and
Smeed's law comes into play. This seems to be especially the case with helicopters, which have suffered a stagnant accident rate for quite a while. People just seem to have collectively accepted that helicopters crash far more regularly than fixed wing aeroplanes, shrugged their shoulders and elected to either take the risk or (in a minority of cases) avoid using helicopters.
I get the strong sense from reading accident reports that quite often people get themselves killed after putting their head into the tiger's mouth dozens or even hundreds of times, probably without even knowing that was what they were doing.
Some people smoke for a lifetime and don't get lung cancer. Some people never smoke and do. Likewise, some people do stupid things with aeroplanes for a lifetime and get away with it, and some people are just downright unlucky, and get killed through no fault of their own.
The latter group are thankfully in a tiny minority, and the size of that minority is subject to reduction via the application of what may broadly be termed "engineering methods".
But it's very hard to prevent people from killing themselves through stupidity, because idiots are both persistent and ingenious.
I'm at an entirely post graduate university. At a minimum, everybody here has at least a BSc; a substantial proportion have enough degrees to make a rudimentary thermometer, and most speak multiple languages. So these are clever, well educated people. But an
amazing proportion of them smoke. Wandering around the place, you'll see them clustered outside doorways sucking on their cancer sticks. Even outside the healthcare building, where there are so many smokers that they've had to put up signs to remind them not to litter the place with cigarette butts. Not that they seem to take much notice.
That basically sums up the general problem with risk. Once things get safe enough that people don't see their peers dying around them as a consequence of questionable risk management, they tend to just go with the flow and get into bad habits, even if they objectively understand that they're taking risks.
By the time the population of post-graduate smokers start to suffer an increased cancer rate, not only has the damage been done, but a lot of them will have become so habituated to smoking that they'll have real difficulty stopping, because of course by then their friends will contain a disproportionate number of smokers, and if they quit then they'll be left alone whilst the smokers all go outside to suck their cancer sticks. I've watched this happen to several of my friends. They start smoking, then all their friends smoke, their girlfriend smokes, and so when they quit they suddenly find their whole peergroup basically ostracises them for a large amount of the time whilst they all go outside to smoke. So unless a majority of the group decides to quit, they can't make it stick.
Also, people are generally happy to take a large number of small risks (for the same total risk exposure) than a small number of big risks.
So the smokers will happily gradually erode their life expectancy one cigarette at a time, but if you could arrange to lump even about a year's worth of smoking risk into one event, they'd probably feel much less comfortable. I suppose this is due to perceived variance.
In a similar way, some pilots will happily take lots of small risks (e.g. pushing decision heights a little), despite the fact that if you re-arrange the overall risk into one big hit (e.g. by saying, "if you guarantee never to take this small risk again, I'll give you a one-time-only get-out-of-jail-free-card to fly under/through Tower Bridge, the Eiffel Tower or some other suitably dramatic landmark of your choice - you can even do it upside down if you want!") they'd feel very uncomfortably about taking it.
Overall, aviation safety has now reached a point where a lot of people do questionable things without seeing consequences, and so they build up bad habits, and don't feel uncomfortable about them. I've seen people at some gliding clubs move powered aeroplanes by pushing or pulling on the prop. It makes me feel extremely uncomfortable. Especially when I challenge them and they say things like "It's OK because I checked that the switches in the cockpit were off".
In fact, the people who do behave sensibly are made to feel uncomfortable because they're abnormal. Walk out to a club aeroplane wearing a flying suit, flying boots, gloves and a parachute, and people will make snide comments.
I think that at some level it reminds them of their own mortality. Personally, I've never used my parachute, and I hope I never will. The flying gloves are a pain most of the time. But if the aeroplane were to catch fire, the gloves would protect my hands, giving me much more time to get out, and if I used my parachute then the boots greatly reduce the chances that I'll break an ankle on landing.
Therefore I'll gladly suffer the inconvenience 99.999999% of the time, just in case one day I find myself on the 0.000001% flight.
I'll also actually wait 2-3 minutes after a jet airliner has landed before using the same runway to takeoff, even if the controller moans at me, because clearance to takeoff does not magically guarantee that it's safe or sensible.
But I'm not your average GA pilot.
TL;DR- I don't think that you can make a direct comparison between modern civil aviation and 1940s military aviation.
- I also don't think that you can expect people to react to game risks in the same way as they would react to real-life risk.