LT. COL. JOSHUA DAY
State Army Aviation Officer
Colorado National Guard
            
What is power management? This term means many things to many people. Most agree it has something to do with engine performance and torque. The instructors at the High-Altitude Army National Guard Aviation Training Site like to think there’s more to power management than just the torque gauge. At HAATS, power management encompasses three things: understanding the environment; understanding the aircraft; and understanding yourself. This article will focus on understanding the environment, which encompasses many things.

One of the most important components is the interaction of wind and terrain. At HAATS, we call this Wind and Terrain Analysis. This analysis maintains that wind flows over and around obstacles in a consistent and predictable manner. The ability to predict the flow of the wind is the result of understanding and practicing WTA principles, rules and methodologies, which have been developed through research and experiment, both in the laboratory and field.

The first requirement to achieve this ability is to believe it can be accomplished. Most pilots dismiss the notion as unnecessary or believe it’s far too complicated an issue. This is particularly true in mountainous environments. Airflow responds to the same laws of fluid dynamics as water or any other gas. While we often cannot see the movement, we can always detect it directly or indirectly. It is this ability which allows us to develop the skill to predict and, ultimately, see the wind. The components we need to know and integrate will follow below.

Air flows much like water and has characteristics aviators should note and test while flying in their area of operations. Air follows the path of least resistance. It will take the shortest and/or least obstructed route to fill any lows created by high winds over rough terrain. In canyons and drainages, the wind accelerates in the resultant venturi due to increasing pressure differentials. In winding turns, they accelerate to the outside of the turn, exactly like water, leaving eddies on the inside of turns. When colliding with an equal and opposing force, pilots can expect an opposite and turbulent flow. This opposite reaction can take the shape of a cliff face or another air current.

It is imperative aviators combine the principles in the preceding paragraph with the characteristics of stability and the mechanics of prevailing and valley winds to understand and apply the cornerstone of mountain wind predictions — the Wind Zone Model. The five zones are updraft, downdraft, turbulent, dispersal and stable zones (as depicted in Figure 2 below). In addition, two other terms require explanation: The demarcation line is the point separating the updraft and downdraft zones; the curl, or low pressure, is created by the wind’s passage over or around an obstacle.

The demarcation line’s angle and height is established by three factors: the velocity of the wind, steepness of the slope and angle at which the wind strikes the slope. It can be considered an extension of the slope as it rises above and beyond the obstacle. It is bent downward horizontally as it interacts with winds aloft. Its actual location becomes important in cross-country operations and when approaches are being considered to pinnacles and ridgelines.

The low pressure area is created on the leeward side of the obstacle by the very passage of that wind and is the “engine” that drives the ensuing turbulence. The wind will attempt, via the path of least resistance, to fill the low. In the diagrams below, the wind must come back from the downdraft zones to attack the low-pressure areas. This initiates a pattern of turbulence, rotating on a horizontal axis, which extends leeward until frictional interaction with other air molecules slows the swirling patterns, allowing the air currents to sort themselves out (dispersal zone) and return to a stable flow (stable zone).

The updraft and downdraft zones are a result of the intervening obstacle. The remaining three zones are a result of the creation of low pressure leeward of the obstacle. If the obstacle has sharp drop-offs on either side, then the movement to fill the low is lateral, or “wrap-around,” and the rotational plane of eddies and ensuing turbulence changes to reflect this direction. The rotational axis moves from horizontal to vertical and all points in between. This is particularly noticeable around isolated, sharp peaks, shoulders (abrupt change in terrain relief) or buildings.

The zones expand with an increase of velocity, slope angle or impact angle and contract when the above decrease. Knowing this is important for two reasons. First, a pilot with a little experience can judge the effects of the wind by simply studying a topographical map if the upper wind’s direction and velocity are known. He can plan safe routing to avoid the worst of the zones. Secondly, while en route, the pilot can judge the severity of the zones by how far leeward of the obstacle he encounters the dispersal zone (light turbulence). The farther the dispersal zone is from the obstacle, the greater the severity of the turbulence and downdraft zones.

When additional obstacles follow immediately after the initial obstacle, then some zones may be eliminated altogether. This is often the case as in the series of peaks or ridgelines as depicted in Figure 4 below.
    
In this situation, most of the turbulent zones are abbreviated or absent, as the turbulent, dispersal and stable zones can be eliminated on the initial and middle ridges. The key is if and where the downdraft zone impacts subsequent obstacles. The ensuing updraft zones can be compressed due to the strength of the downdrafts. Due to compression, the ensuing updrafts become very powerful. This has serious implications for aircraft transitioning narrow valleys.

In high winds, there is very little safe maneuver room in such valleys except within the narrow confines of the updraft zones or the “curl” or low pressure. Aviators needing to execute a landing or to maneuver in this confined airspace must use great caution and have intimate understanding of the environment and their aircraft. Powerful rotational patterns are trapped between the downdrafts and the upwind ridges depicted in Figure 4.

An additional note must be made about Figure 4. The point where a downdraft descends and impacts subsequent terrain is known as the strike point. Due to the lateral resistance of other air molecules, the airflow at this point can only go up or down. In freshly fallen snow, this area is visible. If there are no visual indications and the goal is to remain above the strike point, then the pilot needs to fly at altitudes equal to the ridge tops. When the updraft zone is compressed as in the previous paragraph, then the pilot needs to fly laterally as close to the terrain as safety permits to remain in the updraft. In these conditions, the route and altitude are dictated by observed or suspected conditions.

This is a brief synopsis of WTA and the Wind Zone Model, which should explain some of the nuances of mountain flying. Having an understanding of the wind and its interaction with terrain can mean the difference between success and failure.

(Editor’s note: This article was written by then-Maj. Joshua Day when he was the commander of the High-Altitude Army National Guard Aviation Training Site in Gypsum, Colorado)