From Grand Theft Wiki

AT-400
An AT-400 in flight GTA San Andreas.
Vehicle type:	Civilian fixed-wing aircraft
Body style:	Passenger jetliner
Appearance(s):	GTA San Andreas
Based on:	See #Description.

The AT-400 is a large passenger plane in Grand Theft Auto: San Andreas.

Description

Based on the Boeing 737-400 commercial jet, the AT-400's large size and sluggish response makes it harder to land than the other planes in the game. The plane also features other unique features. A primary disadvantage to this plane is once you land and get out you can't get back in because the door is very high from the ground. The only way you can get back in it to find a Baggage with a ladder attached to the back, land the plane at Las Venturas Airport and go to the hangar again, or find a Yankee and jump over it, using its back side as a platform. The AT-400 is also only one of two aircraft that can fly to the maximum height in the game, the other being the Hydra.

Unlike most other planes, this plane is also the only one that can be flown over Area 69 with less chance of being shot down by homing missiles, providing you don't hang around.

Like the other planes, it can also be seen flying in the distance, but it is hard to take down the flying AT-400, even with homing rockets or kamikaze style attack. The plane is also used for paid commercial flights between cities in San Andreas. As a controllable vehicle, the plane is considerably rare, appearing in only one location, inside the large airport hangar in Las Venturas Airport in Las Venturas.

Location

• Only from a large airport hanger in Las Venturas Airport. Only appears when you approach from the south.

Trivia

A parked AT-400 at the Easter Bay Airport, with a Utility Van parked nearby for comparison in size.

• The AT-400 is the largest vehicle in the game, the Andromada being the second largest. Interestingly enough, when comparing their real life counterparts the cargo plane (Andromada) is actually bigger than the passenger jet (AT-400).
• The AT-400's initial is similar to that of the Air Train's, a similarly sized airliner that repeatedly appears as a static prop earlier in GTA III and GTA Vice City.
• Certain modifications, like the "Fly Mod" and the "Anti-Material Rifle", can knock an AT-400 out of the sky but it still takes a lot of practice to bring one down even with modifications.
• You can't keep AT-400 in Verdant Meadows hangar, because the wingspan and the tail of the plane is larger than the hangar door.