My f450 with apm2.6 nd ublox gps works fine in stabalize mode but when put in altitude hold or loiter or land mode the throttle automatically increases nd decreases...the quad starts jumping in the air....

Plz help urgent....

I think there Maybe some issue with the barometer in the board. Barometer is used to sense the pressure from the atmosphere so if it's blocked or something it will get wrong signals. Also the altitude hold mode only works between a throttle range of 40 to 60%

Problem with the APM. I had that issue before and it was because I had a bad board.

This is because the Barometer is air pressure sensitive and needs to be covered with small holed foam to ensure its not getting propwashed.

Or a piece of cotton..

Gyz  recently i have built my first quadcopter on s500
Someone told me flying quadcopter is ilegal in india
Is it or we have to register our  quad somewhere
Plz tell me

Yes you are right. It seems it is illegal. If you plan to fly it then go in open field where there is less crowd and fly there or if you are a student then fly in college premises. As of now there is no registration for the quadcopter. Where ever you fly fly responsibly and try not using fpv systems as much.

Thanq so mch

When you cover the barometric sensor, make sure you cover it with something dark. The sensor is also sensitive to light. I have used a piece of black muslin cloth folded over 4 times with great success.

If you can then post the specification of your quad and pics of setup.
the common problem in this case is that your quad is overpowered .
i.e. apm think 50% throttle can hold the altitude but due to less weight quad gain height at 50% then barometer shows it's gaining height apm reduce the throttle and you see oscillation in height.
Go to parameter section and decrease the half throttle value.
Or just add extra weight like a bigger battery or two battery in parallel.
Or run a autotune in flight mode section hopefully it will set the Half throttle value.

If anything of the above doesn't work then go and buy a new APM board. These cheap clone boards some times have faulty barometer on them. If possible buy a ImmersionRc Apm they are genuine.