Artificial Horizon - Android or iPhone Bubble/Level App?

An AHRS (Altitude Heading Reference System) must use at minimum three sensors: Accelerometers, Gyros, and Magnetometers (or GPS).

Algorithms that use just the accelerometers to correct for the gyro bias will diverge (get a VERY wrong answer) over time if you are in a coordinated turn. (The filters in question try to observe the long term trend of which way is "down." Circling in a thermal will fool these fairly quickly.)

The magnetometer or GPS input must be used to observe the turn and prevent incorrect estimation of the gravity vector.

A fairly well-known university (with which I am not affiliated) discovered this about a decade back when they were commanding their fixed-wing UAVs to loiter by circling. After a couple minutes they would overbank, enter a spiral dive, and crash rather dramatically...