In the aspect of Trajectory Based Operations (TBO), accurate trajectory predictions are essential for meeting future concepts defined in SESAR and NextGen visions. The 4D trajectories are results of generated flight plans by Airline Operation Centers to be executed by pilots and monitored by Air Traffic Controllers. Trajectory generation is a process with many input sources such as flight route, speed schedule, initial conditions and environmental conditions. However, the uncertainty in these states generates discrepancies between planned and actual trajectories. This paper distinctly investigates the wind uncertainty by analyzing the forecast data in airline's operational flight plans and actual wind computed by aircraft's onboard systems (i.e., Quick Access Recorder). The study goes through the methodology of speed and altitude optimization for cruise phase. The simulation studies reveal the impact of wind uncertainty and potential fuel saving benefits when the precise wind data is available. The case studies are based on historical planned and flown flights. (C) 2018, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved.