[yor_on (OP)]

We have a aircraft,  in it a elephant jumps, as it do so does it count to the aircraft's mass?
Just argue, don't need to be perfect :)

[Bored chemist]

No, because elephant's can't count (they also can't jump).
If they could it would be a rehash of this  old chestnut.

http://www.telegraph.co.uk/news/uknews/11345183/Birds-in-a-lorry-riddle-finally-solved-by-Stanford-University.html

[Janus]

Even if we assume that an elephant could jump, there are a number of factors to consider during the three stages of the jump to come to any conclusion as to what would happen.
The three stages of the jump are:
1.Push off
2.Mid air
3. Landing.

The factors to consider is the Elephant to plane mass ratio, the nominal center of gravity of the plane and the location of the elephant with respect to the lift point (the wings).

If we assume that the elephant is located at some point aft of the lift point, then the GG of the plane will be shifted aft of the nominal CG.   And the elevators will be trimmed accordingly to maintain level flight at the desired airspeed of the plane.   We will further assume the the pilot makes no corrections either by control surfaces or throttle during the jump.

Stage 1, the push off:
The elephant pushes downward on the floor in order to accelerate itself upward.  This has two effects on the plane. the first is to increase the load force on the wings, and the second is to effectively shift the CG of the plane aft. The tail will lower and the nose will rise increasing the angle of attack of the wings,  the increase in angle of attack will increase lift which will help counter the increase in load, but at the expense of increased drag and loss of air speed.  The complication here is that if the plane/elephant mass ratio is small enough, this could cause the angle of attack and decrease in airspeed to produce a momentary stall where the wings lose all lift.

Stage 2, mid air:
The elephant is no longer pushing against the floor nor is its weight pressing against it.  The effective CG will shift forward past the where it was when before the elephant started its jump.  The load on the wings will drop, the tail will rise, and the nose will lower, This decreases the angle of attack and lift and the plane will gain airspeed. Since the elephant is no longer in physical contact with the plane, the plane will move slightly faster than the elephant and in effect the elephant will drift further towards the aft of the plane.

Stage 3 landing.  The elephant lands imparting a force to the floor of the plane. The magnitude and duration of this force will depend on how the elephant absorbs of the impact.  This force will be downward and slightly towards the aft of the plane (the towards the aft part being due to the present difference between Elephant's and plane's present horizontal speeds.   Once again, this will increase the load on the wings and change the angle of attack.  However this time, with the additional sideways component to the landing force, there will be a slightly greater decrease in speed and since the elephant is landing a bit further back in the plane, a greater shift in the CG. Even if everything else is the same as the take-off,  this difference could be enough to cause a momentary stall.

Finally, after the landing is complete, will have just the weight of the elephant producing a load on the wings. It will be slightly aft of  where it started however, and the plane's trim would need a slight adjustment in order to maintain level flight.

This is just a rough analysis of the scenario and the smaller the Elephant's mass compared to the aircraft, the smaller these effects will be.

[timey]

Lol!  Excellent, I really enjoyed that... I'm now visualising the flight path of the plane...

[CPT ArkAngel]

In terms of mass, the mass will be lost by heat over time. The force is weight not mass. The mass loss is the work done by the elephant. The elephant will land on the same spot, not behind if it jumps up. The stability of the airplane will not be affected seriously if the total weight never exceed the maximum load of the airplane and if the center of mass is maintained within the allowed surface. The exact effect on the stability depends on the type of airplane and the position of the elephant.

[Colin2B]

If we assume that the elephant is located at some point aft of the lift point, then the GG of the plane will be shifted aft of the nominal CG.   

It's also worth looking at the situation where the loadmasters has placed the elephant over the CG.
Stage 1 is very similar except that there is no change in angle of attack and lift, so depending on the lift to elephant ratio the plane would drop slightly as the elephant accelerates upward.
In stage 2 the weight of the elephant is removed and, because the plane is still maintaining the same lift, the aircraft will begin to rise until it meets the proverbial elephant descending.

All in all let's hope the pilot doesn't decide to try out the vomit comet manoeuvre.

[yor_on (OP)]

Ahh BC, so you did know?
I've heard it too, but what we have here is a very special elephant, it's spherical and of a new and much improved jumping variety.

A new breed I admit, but cunning.
hmm, when it comes to the mass you made a choice I see.
Everyone agrees?

[Demolitiondaley]

We have a aircraft,  in it a elephant jumps, as it do so does it count to the aircraft's mass?
Just argue, don't need to be perfect

I'd say that if the plane was grounded and static on a very large set of scales then as the elephant was of the ground mid jump so to speak there would be a reduction in the weight  reading on the scales of whatever the elephant weighed. If however the plane was in flight then it would be weightless but of course still have mass. I think that the elephant and the plane are two entities and therefore have two separate masses while they are apart. So in answer to the question I don't think so as it jumps.