10.2 Turbulence, Orographic Effects and Terrain

What Turbulence Is

Turbulence is irregular, non-steady motion of air, with eddies and gusts that change speed and direction over seconds. For a crewed aircraft it is uncomfortable; for a small remotely piloted aircraft (RPA) it can be operationally limiting because the aircraft's mass, control authority, and structural margin are all small. The same eddy that bumps a Cessna can roll a 2 kg multirotor past its recovery angle.

Two sources dominate low-level RPA operations:

1. Mechanical turbulence — generated when wind interacts with obstacles.
2. Orographic turbulence — generated when wind is forced over terrain.

Mechanical Turbulence

Mechanical turbulence is produced when the wind breaks around and over obstacles, shedding eddies downwind. The size of the eddies is roughly proportional to the size of the obstacle and the strength of the wind.

Common sources in RPA operations:

• Buildings — a warehouse, hangar, or row of houses in the launch area generates eddies that roll off the roof and around the sides, strongest in the first 100-200 m downwind and within a height of about 2-3 times the building height.
• Trees and tree lines — a treeline or windbreak produces a turbulent wake that extends several tree heights downwind and a height or two above the treeline.
• Ridges and escarpments — covered below under orographic effects, but the same mechanism (separation of the flow) applies.
• Line features — powerlines, fences, and embankments generate smaller, sharper eddies that affect a small RPA disproportionately.

Mechanical turbulence is strongest when the wind is strong and the lower atmosphere is unstable (cool air over a warming surface, typical of a sunny day). On a still, stable morning the same obstacle may produce no noticeable wake.

Estimating Mechanical Turbulence From a Report

The gust spread in a METAR or AWS observation is a useful proxy. A report of 27015G22KT has a gust spread of 7 kt; a report of 27015G25KT has a spread of 10 kt. A larger gust spread indicates a more turbulent flow.

Key habit: if the gust spread is large and you are operating near buildings or a treeline, treat the launch and recovery zone as turbulent even if the mean wind is within limits.

Orographic Effects and Terrain

Orographic means terrain-forced. When wind meets a ridge, hill, or escarpment it is compelled to do three things: rise on the windward side, accelerate over the crest, and descend and separate on the lee side. Each of these affects an RPA.

Orographic Lift (Windward Side)

On the windward side the air is forced upward as it flows toward the crest. For an RPA flying upwind toward a ridge this can appear as a boost in climb or an unintended altitude gain; for an aircraft flying downwind away from the ridge it can become a downdraft that has to be flown out of. The lift can be smooth and is not itself hazardous, but it signals that a lee-side hazard lies just over the crest.

Rotor and Lee Turbulence (Lee Side)

On the lee side the flow separates from the ridge and forms a rotor — a turbulent, rotating circulation with a sharp boundary between the descending flow above and the reversed, rolling flow below. The rotor is the most dangerous orographic feature for a small RPA because:

• Strong downdrafts on the lee slope can exceed the aircraft's climb rate.
• Reversed surface flow below the rotor can blow the aircraft back toward the ridge while the pilot expects it to drift downwind.
• Severe shear across the rotor boundary can exceed the airframe's gust limit.
• Turbulent eddies persist for several ridge heights downwind.

The safe operating guidance is simple: avoid flying downwind of a ridge in anything more than light wind. If an operation site sits on the lee side of a ridge or escarpment and the wind is blowing over that ridge toward the site, treat the site as unsuitable until the wind drops or the operation is moved to the windward side.

Worked Example

A pipeline inspection task runs along a ridgeline running north-south. The wind at a nearby AWS is 27018G26KT — a strong westerly.

• The western (windward) slope of the ridge sees orographic lift; a launch on the western footslope will encounter rising air and a moderately turbulent boundary layer but no rotor.
• The eastern (lee) slope sits under the rotor. An RPA launched there can encounter a downdraft that exceeds its climb rate, reversed surface flow that pushes it back toward the ridge, and shear that exceeds its gust limit.
• Decision: do not operate on the eastern slope in a 26 kt gust; either move the launch point to the windward side, wait for lighter wind, or re-schedule.

How Turbulence Affects a Small RPA

The effects of turbulence on a small RPA are more immediate than on a crewed aircraft:

• Attitude excursions — rapid roll and pitch changes that the autopilot may not fully damp out, particularly on aircraft without a fast attitude controller.
• Altitude excursions — vertical gusts push the aircraft above or below its planned altitude; in a survey this degrades data quality and may breach the 120 m AGL operating limit.
• Control workload — the remote pilot has to work harder to maintain track, especially in turns and during launch/recovery.
• Structural load — repeated gust cycles accumulate fatigue; for a small airframe a few flights in heavy turbulence can damage arms, mounts, or gimbal locks.
• Launch and recovery — the most critical phases. A gust during rotation or touchdown can drop the aircraft into an obstacle or flip it on landing.

Key habit: if the launch/recovery area is turbulent (large gust spread, obstacles upwind, rotor from a nearby ridge), abort or re-schedule. Launch and recovery are where most turbulence-related RPA incidents occur.