When flying a multimotor aircraft without an autopilot, even if all connections seem "complete", the error: Simulation model is not globally balanced, having n variables and m equations (with n > m) is displayed.

Not all aircraft engines have a throttle command input from the source.

Add the missing connections of throttle command to the deltaThrotCmd[nEng] input of aircraft so all engines have an input. E.g. two connections are needed for an airplane with nEng=2: one for deltaThrotCmd[1] and another for deltaThrotCmd[2]. Define which input signal matches which engine on the Add Connection window.

When flying a multimotor aircraft with an autopilot, even if a throttle connection has been done, the error: Simulation model is not globally balanced, having n variables and m equations (with n > m) is displayed.

Not all aircraft engines have a throttle command input from the autopilot vector. 

Redo the throttle connection and add a colon deltaThrotCmd[:] (the default suggestion) on both input and output at the Add Connection window. I.e. From: autopilot.deltaThrotCmd[:] To: airplane.deltaThrotCmd[:]

When using autopilot, the error: Incompatible dimensions. The 1st dimensions of autopilot.deltaThrotCmd and airplane.deltaThrotCmd evaluate to n and m respectively, with n ≠ m is shown.

Assert shown stating: Aircraft.Atmospheres.atmosphericData: Height, h, out of range

This assert occurs when the aircraft has reached an altitude lower than zero. This could be due to:

1. Initial conditions on the aircraft model not declared.
2. The airplane crashed. 

1. Set the initialAltitude and initialVelocity parameters in the Aircraft's General Tab to reasonable values according to the type of aircraft. 
2. Analyze the simulation results (by, for example, plotting flightData.alt or by viewing the animation) and correct the necessary parameters for your airplane crashes.
   • Try adding some negative deflection to the elevator input to achieve trimmed flight.
   • Use a higher altitude for the simulation. When flying without autopilot, the phugoid oscillation might cause it to crash if flying at low altitudes.
   • Add more thrust!

One or both of the aircraft initial conditions (initialAltitude and initialVelocity) are significantly different from the Autopilot input commands. The autopilot cannot correct for such drastic differences.

Analyze the simulation results (by, for example, viewing the animation) and correct the parameters that make the aircraft try to perform unrealistic maneuvers. These could be excessive control surface deflection, too low thrust, excessive angles of attack, inverted stalls, etc.