Like in the real life, taking certain gear with you in a vehicle is limited. While you would normally store a fully packed, 30kg rucksack either in your cars trunk or at least at the backseat of your car, ACE enforces players that crew vehicles to do the same.

Excluded from this particular limitation are players that simply board vehicles as passengers or cargo. This is in favour for gameplay and also due to some game limitations.

Jet Pilots

You are not able to take a rucksack or anything larger than a handgun with you into the cockpit.

Your weapon and  rucksack are stored in a bag outside of your aircraft.

Helicopter Pilots & Crew Chiefs (Doorgunners)

You are not able to take a  rucksack and anything larger than a carbine with you into the cockpit.

Your weapon and  rucksack is stored in the helicopters cargo hold.

Upon disembarking normally from the helicopter you are automatically grabbing your rifle. Your  rucksack , if any, can be retrieved from the cargo hold.

Exiting in an emergency situation (Ejecting) causes you to loose your gear, since you would not have the time to grab your gear from the  weapon mount and cargo slot. 

According to this you may want to disembark normally if on ground instead of jumping out.

Tank and APC crews

Rucksacks are stored in your vehicle cargohold. You can retrieve it from there when you disembark.