# If I get on a plane with my 500 lb parakeet Buster on my shoulder...

… and once we’re at cruising altitude, Buster decides he needs to stretch his wings and starts flying around inside the cabin, obviously the whole plane gets a little lighter (or does it?). Would that mean that the plane would increase its cruising altitude a little?

The weight would remain the same. Because although Buster’s movement is producing an upwards force on himself, he would also be producing a downwards force on the plane.

That’s not a parakeet it’s a pterodactyl!

We have done this one several time and I am pretty sure Cecil himself has done it. I am sure that people will come along shortly to introduce variables that complicate the crap out of the issue but this one is easy to explain.

The airplane is a tightly sealed container. It is filled with air and people and a parakeet. Now just switch the conditions so that the plane is carrying a big container full of water and you are in it in your scuba gear with your pet stingray on your shoulder. It flies off. Does the tank and plane weigh differently now?

The conditions are the same because although we tend to think of air as nothing, we are constantly swimming in it the same way a sea creacher moves about the ocean.

Consider this:

Air is just a fluid.

When you put a fish into tank full of water (which is another fluid), the weight of the whole thing increases by the weight of the fish; regardless of whether the fish is resting on the bottom, or swimming about.
(this is on thew assumption that the tank is not so full that some of the water overflows and is lost when you add the fish)

Damn you and your stingray, Shagnasty

If you have a 500 lb parakeet and are able to carry it on your shoulder…I’m willing to let the damn plane do whatever you want.

What if the giant parakeet is on a giant treadmill?
Man, that never gets old.

Now I know what’s going on when the altitude jumps to 30001ft.

In Soviet Union, 500lb parakeet flies you.

That does get old, though.

I thought a “Parakeet Buster” was a surface to air weapon; that we would be discussing airline security.

OK, how ´bout this*:

If all passengers on the plane (say 350 on a DC-10) were to jump up simultaneously…
Maybe the plane wouldn’t get lighter, but the landing… it’s still ~52.500 crashing down at the same moment. Would that effect the plane?

*I know Cecil did this - Chinese jumping off chairs. But the Earth is a lot bigger as compared to a billion Chinese, as compared to a plane and 350 people.

Buster is my dog. I just needed a name for the parakeet from hell that weighs in @ 500 lb.

Somebody would put a foot throught the fuselage (sp) and confuse the hell out of the FAA investigators.

And the mod says,

"Why me?’

Again and Again!

What if Buster got outside and pushed? Would the airplane go faster because it was lighter?

Cecil partially answered your question in this column, which I assume was the basis of your question. In summary: if the plane is sealed, the plane does not get lighter. If the window is open, the plane gets lighter.

Assuming the plane does get lighter, you are asking whether the altitude increases. Let’s assume that everything else is kept constant (fuel consumption, angle of attack, etc). This means that the plane is producing enough lift to balance the original gravitational force, but gravitational force has now decreased. The unbalanced forces result in a net acceleration, causing the plane to move upwards faster and faster as time goes on. However, I assume that as the air gets thinner the plane will not be able to generate as much lift, so eventually the forces will be balanced again and the altitude will stabilize. The altitude will have certainly increased.

Realistically, the pilot (or the autopilot) will make a slight adjustment to the controls to keep the altitude constant. The result will either be that the engines burn less fuel and to travel at the same speed, or that they burn the same amount of fuel and the plane goes faster.

Hmmm… air pressure won’t make any difference here, so if we consider the plane and the people to be a system, there are no outside forces acting on the system (other than gravity and lift, which balance each other out). Therefore the centre of gravity of the system will not move. So the plane will move down a bit when they jump up, and up a bit when they fall down, but in the end it won’t make any difference to the altitude.

Breaking it down step by step, when the people jump their feet accelerate the plane downwards. As soon as their feet leave the ground the plane is lighter, in a way. The gravitational force on the people is pulling them toward the floor of the plane, but it isn’t balancing the lift generated by the plane at all. Therefore there is an unbalanced force pushing the plane up towards the people. This causes the plane to stop moving down and start moving up, just as the people stop travelling upwards and start falling. When the people land they will apply just enough force to stop the plane from moving upwards. A little math would show that the final altitude of the plane is the same as it was before the jump.

It took me a while to figure out what was going on (the second paragraph stuff), so I hope you like it. Otherwise, feel free to tear it apart and enlighten me as to the Truth. I’m pretty confident in my answer though.

Is there a volume the plane could reach when it starts acting as an “open” environment? Or can it reach any volume and still behave the same way (does Earth actually behave this way, as in if it were possible to weigh the Earth, would it weigh the same with planes grounded as in the air?