Kin O. Yau has written:

'Quantitative measure of controllability'

Neil D. Evans has written:

'Nonlinear controllability via the initial state'

Mohamed Rabie Abdallah Mobarak has written:

'Controllability for infinite dimensional systems'

It depends on the weather condition and the pilot controllability...

When there is no gust wind and the pilot got good control over the helicopter then it can float or stand in the air for longer time

poles are the plot of the transfer function of a system on the left side of the origin, in s-plane. zeroes are the right side plot. poles and zeroes specifies the absolute stability of the system.. they also gives the observability and controllability of the system..