Comment by throwaway0a5e | Hacker News Reader

Comment by throwaway0a5e | Hacker News Reader

Parent Full thread

> The weight is so critical that the 7-seat plane can realistically only seat 4-5 adults. I remember being shocked that they had to weigh the paint they applied when he had the plane re-painted.

I used to work in general aviation. If my eyes could fly a loop in my skull they would have.

Were the occupants required to use the bathroom before flying? That's how much weight you're potentially saving by weighing the paint on a small aircraft.

They make you weigh the paint because they want you to spray on a certain thickness so they say "X oz paint, Y oz thinner/hardener" (or something like that) in order to get your mixture into the right ballpark so it will work with whatever procedure they want you to spray it on with and get the thickness/finish/hardness the OEM wants you to get.

In aviation there's a ton of treating simple systems as black boxes and "do X and exactly X" type maintenance that happens in order to smoothly transfer liability. You paint a cowl the way the OEM says not because you couldn't get an equivalently performing cowl a different way but because you don't want the NTSB coming after you trying to determine if you did it different but right or different but wrong.

The specifications to which general aviation stuff is done isn't really any more exacting than stuff in automotive or heavy industry. The service literature is just more verbose and the service procedures are more tightly defined.

>in order to get your mixture into the right ballpark

This makes perfect sense. I'm using a kitchen scale to measure the 2-part silicone mixture that I'm using for making toys, not because weight is critical but because it needs to be right for curing. I should do similar when mixing epoxy, but I always eyeball that for some reason. Maybe has to do with cost, it's $10-20 worth of silicone I'm mixing, and usually a quarter worth of epoxy, just due to quantities involved.

I'm building an experimental, and we tend to re-do weight and balance after painting. An elaborate paint job can add 30+ pounds and can shift your CG rearward. The regs don't require it, but many builders do it anyway.

Very interesting. I remember thinking it was positively "nuts", but then when you're in the aircraft being slapped around by the breeze, it starts to click.

>The specifications to which general aviation stuff is done isn't really any more exacting than stuff in automotive or heavy industry. The service literature is just more verbose and the service procedures are more tightly defined.

This reminds me of how often I quote weight limits on cars to people and their eyes go wide at how easy it is to exceed the OEM's recommended limits. I'm fairly sure I'm one of the few among my friend groups that has read through every owners manual for the cars/vans I've owned.

I know that glider maintenance procedures in my country call for redoing the center of gravity measurents after painting. Might have been related?

In the US, the FAA requires recalculation of the weight and balance unless the change is "negligible", which AC 43.13-1B defines as "any change of one pound or less for aircraft whose weight empty is less than 5,000 pounds".

I think the commenter was making the point that the weight of the paint is not a significant consideration, not that recalculating the weight and balance after a paint job should not be done.

Well... ok, but you usually still need to weigh and check the balance of the control surfaces after painting to ensure that they aren't going to flutter at less than Vmc (post-paint control surface balancing is often explicitly called out in the maintenance manual).

Painting a plane is one of those times that you often strip everything out anyway, so it's a convenient time to check the weight and balance against the logbook.

Reply on news.ycombinator.com