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====== Lesson #6 ======
In this session we will add the Altitude Hold flight mode and repeat the flight patterns above.  Altitude Hold uses the barometric sensor to maintain altitude.  Utilizing this mode may require some tuning of the Altitude Hold settings.  The multirotor may slowly decrease in altitude during flight, this is common as the barometric sensor may sense an increase of air pressure from forward movement, thus retards the motor speed to return to  its original pressure value.  The faster the flight speed the more the altitude decrease may be.  You can compensate for this by decreasing the flight speed or increasing the throttle stick during flight and returning back to ~50% when hovering. Compensating for uncommanded altitude changes can be easily by adjusting throttle position while in Altitude Hold.  Holding altitude becomes more critical in Waypoint flights as the AutoPilot (AP) function disregards all control stick movements (except for yaw) during AP flight. Adding a optional LIDAR sensor should help maintain more accurate altitude hold during varied flight conditions.  More information on using and adjusting the Altitude Hold can be found here: [[ http://ardupilot.org/copter/docs/altholdmode.html | Alt Hold]]
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===== Part 1: Flying in Altitude Hold =====
These are the following steps to fly in Altitude Hold...
  * Configure the transmitter n the "Flight Modes" menu of Mission Planner so that the middle position of the switch is set to Alt. Hold and the highest and lowest points of the switch are set to stabilize. This is so that if at any point the vehicle move unexpectedly while in Alt. Hold, the pilot can easily switch back to a fully manual flight mode.
  * Arm your quad and take off in stabilize mode until it is approximately ten feet in the air.
  * Switch to Alt. Hold mofde.
  * Set the throttle stick to ~50% to hold altitude.  You will notice there is no/very little actual throttle reaction with the throttle stick is in the 40%-60% range.
  * Perform the flight patterns from previous lessons in Altitude Hold.

**Video of demonstration to be added**

**WARNING:** The FC is controlling the actual throttle signal to the motors while in any flight mode where Altitude Hold is an active function,.  The throttle stick is more of a input 'guide' to the FC telling it to maintain altitude when the stick is between 40%-60% position, slowly decrease altitude when below 40% and slowly increase altitude with above 60%.  The FC will be applying more throttle signal to the ESCs as the flight battery voltage decreases to maintain altitude.  When you exit out of an Altitude Hold function the actual stick position is __immediately__ used to determine motor speed. This could result in a drastic drop in altitude if more than 50% throttle is required to maintain altitude.  Be ready to quickly increase throttle when switching out of an Altitude Hold flight function.