## 3 Additional Clearance over Mountainous Terrain  
The Figure 7 shows a diagram summarising the calculation process to obtain the additional clearance in a mountainous terrain, with the estimates and expressions used.  
Figure 6: Vertical displacement of category D.  
The following graph shows the results obtained from the altimeter error and vertical displacement due to turbulence for the different categories. As both parameters depend on the wind speed, it was possible to develop functions depending only on wind speed. Since aircraft altitude losses vary according to the category and angle of attack, twelve lines were plotted, three per category (A, B, C, D), referring to each angle of attack (AOA = 2; AOA = 4; AOA = 6), in order to show the variation in the additional margin between categories and the angle of attack involved in the phase of flight. Thus, the altimeter error obtained from each wind speed was added to the vertical displacement, resulting in the following graph.  
As Figure 8 shows, the variation in the angle of attack does not greatly affect the final value to be considered as an additional margin, since its impact just affects the component of turbulence and not the component of altimeter error. The altimeter error is the most significant factor in determining the additional clearance and remains consistent regardless of the category or angle of attack, so the differences observed in Figure 8 correspond to the turbulence clearance.  
Figure 7: The process for determining the additional clearance in mountainous terrains.  
From 15 to 30 kt of wind speed, there is a greater difference between category A's values and those of the rest of the categories due to the fact that the load factor variation in category A's aircraft is more affected by these wind speeds, while the other categories are not as susceptible to those intensities of turbulence. This results in higher vertical displacements for category A's aircraft and, therefore, in greater turbulence clearance. In other words, lower-category aircrafts are more susceptible to the effects of turbulence, requiring a greater additional margin over mountainous terrain.  
Likewise, it can also be noticed from Figure 8 that the discrepancies between the additional margin values begin to remain almost constant above a 40 kt wind intensity, where the maximum difference is in the order of 60 ft and occurs between categories A and D, since they present the greatest disparity in size and weight and, therefore, in vertical displacement due to mountain wave turbulence.  
The results displayed in Figure 8 are presented in the following three tables [Tables 8-10] for \(2^{\circ}\), \(4^{\circ}\), and \(6^{\circ}\) angles of attack, where the additional clearance for mountainous areas is indicated as a function of the wind speed, constituted by the altimeter error and the loss of aircraft altitude. As observed in the figure, the additional clearance of categories B, C, and D are practically the same, so they have been combined in the same row, since the maximum difference in their values is between categories B and D and is less than 3 m (9 ft), which is negligible compared to the altimeter error. Therefore, the values shown in the row correspond to category B, as they are the most conservative.  
In summary, the clearance to be applied over a mountainous terrain should be the sum of the nominal MOC, the clearance due to the barometric altimeter error, and the clearance related to the loss of altitude caused by turbulence effects.