Originally Posted by
birb
Hello! This is my first post after many years of lurking...
Why don't the wings fall off? Or rather, what causes structural failure in small aircraft? I know that the wings don't fall off planes very often, but sometimes the wings do fall off, and I don't know how to deal with this fact!
In really simplistic terms, a structural failure occurs when the flight envelope is exceeded - normally by at least 50%, as that is the minimum margin most airworthiness standards require between the maximum permitted load in normal flight, and the structural failure point. So that's more than 150% of the g-limits (in a light aeroplane those are usually at +3.8/-1.4g, in a microlight +4/-1.5, in an aerobatic aeroplane at-least +6/-3, in an airliner almost always +2.5 / -1.25, fighters typically sit around the +8g point, display aerobatic aeroplanes in the bracket 8-12g).
There are more complex cases - control deflections above 1/3rd deflection at Vne, tapering up to full deflection at the manoeuvre speed, Va (which is the stall speed multiplied by the square root of the g-limits - in most cases
I've had lessons in a C42 and a CT.
C42 is 4g x 1.5 = 6g to break it
CT is 4g x 1.5 x 1.5 = 9g to break it. That's when new - the extra safety factor is for the composite wings, which are assumed to degrade in service.
I've also been up in a Grob, a Cub, a couple of Cessnas, some bizjets and even a Harvard, so I wouldn't say I was scared of flying. But I don't know what I have to do to prevent catastrophic failure in the aircraft's structure,
Don't exceed the operating limits.
and I don't know (for example) how much turbulence could shake a fixed-wing microlight apart.
Probably an infinite amount - the low inertia means that the whole aeroplane moves, not breaks. Airliners can be threatened by turbulence - microlights are only threatened by the risk of loss of control, not structural failure.
A couple of years ago I was passengering in an AX3 and the windscreen folded in around 400ft into our initial climb. The pilot got us back to base, albeit with the door flaps hanging off, but it makes me wonder what else can go wrong on these little flying machines.
Canopies and doors are secondary (aerodynamically necessary) as opposed to primary (structurally necessary) structure, so may be tested to lower standards. Bloody annoying, but almost certainly you were not in any danger at any point.
I'm instinctively nervous about structural failure, and instinctively reassured by the presence of a ballistic parachute, but seems to be the opposite of what experienced pilots think.
In flight structural failure on small aeroplanes, is possible, but fantastically rare. The BRS is in many cases, particularly on lower speed aeroplanes like microlights, nothing but a comfort blanket as the vast majority of life-threatening risks occur too close to the ground to use them anyhow.
On an aeroplane like the Cirrus, it makes more sense as it has a lot more energy - even at the stall, so will be much harder to safely land in a field.
On gliders personal parachutes are common - because they operate in a way (lots of aeroplanes chasing the same thermal...) that make mid-air collision much more likely than any other civil aeroplane class. It is far to say that basically no aeroplanes are built to take a mid-air collision - avoiding those is a piloting issue.
I like microlights but what stops them falling apart? Are they less sound than a Cessna? How am I supposed to be confident in the wings not falling off when sometimes the wings do fall off!?
Microlight structural requirements are simplified compared to light aeroplanes, but that simplification is compensated by requiring slightly higher g-limits. Basically, there's no real difference in structural strength.
Am I being a massive wet blanket?
No, but in the politest possible way, you're being ignorant. That's probably not your fault - as this is poorly taught, particularly to PPLs - and if you're not even at that level, you've just not been exposed to this.
If you don't mind spending money, I can recommend some books - but as I may have written some of them (and they are quite expensive) PM or email me for references rather than my causing a daft public p*****ng contest on the subject as happened last time I mentioned them on PPrune.
G