Why it takes a long time to start the optical flow in gazebo?

Hi All,
I was trying to repeat gazebo simulation with optical flow, but what puzzled me is that the optical flow needed a long time to start, which means the quadrotor was unstable before it started.

Here are some logs:
(FCU began to work at 1487590473.95, but flow at 1487590486.50 after 13 seconds delay, why that’s happeded?)

[ INFO] [1487590473.954136396, 21.444000000]: FCU: no gps
[ERROR] [1487590474.003861923, 21.494000000]: FCU: failsafe mode on
INFO  [local_position_estimator] [lpe] flow init: quality 175 std 11
[ INFO] [1487590476.355594643, 23.842000000]: FCU: [lpe] flow init: quality 175 std 11
INFO  [local_position_estimator] [lpe] flow fault,  beta 12.36
[ INFO] [1487590476.405386423, 23.892000000]: FCU: [lpe] flow fault,  beta 12.36
INFO  [local_position_estimator] [lpe] flow OK
[ INFO] [1487590476.455397098, 23.942000000]: FCU: [lpe] flow OK
WARN  [commander] failsafe mode off
[ERROR] [1487590476.505152724, 23.992000000]: FCU: failsafe mode off
[ INFO] [1487590476.555089537, 24.042000000]: FCU: Using minimum takeoff altitude: 2.00 m
WARN  [local_position_estimator] [lpe] flow timeout 
[ERROR] [1487590480.106711890, 27.590000000]: FCU: [lpe] flow timeout 
WARN  [navigator] global position timeout
INFO  [tone_alarm] gps_warning
INFO  [commander] no gps
WARN  [commander] failsafe mode on
INFO  [tone_alarm] fast_bat
[ INFO] [1487590485.405694093, 32.880000000]: FCU: no gps
[ERROR] [1487590485.455676699, 32.930000000]: FCU: failsafe mode on
INFO  [local_position_estimator] [lpe] flow init: quality 184 std 10
[ INFO] [1487590486.505347391, 33.978000000]: FCU: [lpe] flow init: quality 184 std 10
WARN  [commander] failsafe mode off
[ERROR] [1487590486.605542792, 34.080000000]: FCU: failsafe mode off
[ INFO] [1487590486.655380095, 34.128000000]: FCU: Using minimum takeoff altitude: 2.00 m

Any help will be greatly appreciated.