NTSB Safety Alert SA-052 - Visual Illusions The Ground May Be Closer Than It Appears

Visual Illusions : The Ground May Be Closer Than It Appears

Prevent controlled flight into terrain in flat light and whiteout conditions

The problem

Flight operations in geographic areas

that are susceptible to flat light and

whiteout conditions can lead to accidents, as visual references are greatly reduced for pilots.

• Flat light occurs when the sky

is overcast, especially over

snow -covered terrain and large

bodies of water. In flat light conditions, no shadows are cast and terrain features and other visual cues are masked, making it difficult for pilots operating under visual flight rules (VFR) to perceive depth, distance, or altitude. The photograph in this figure shows how these conditions combine to create an environment where it is difficult, if not impossible, to distinguish the sky from the ground.

• Similarly, whit eout conditions can occur in areas with snow cover. Pilots can

experience a loss of depth perception and become spatially disoriented, unable to maintain visual reference with the ground and unaware of their actual altitude.

Related accidents

The NTSB has investigated many general aviation (GA) accidents involving flat light or

whiteout conditions. These accidents serve as important reminders about the critical need to

ensure that pilots are aware of the challenges associated with these operating conditions

and are adequately prepared for safe operations. The following accident summaries help to highlight the issues involved.

• An Airbus AS350B2 helicopter sustained substantial damage when it impacted terrain

following a loss of control while landing. The comm ercial pilot, who was the sole

occupant, reported that the weather was deteriorating and he encountered an area

with flat light conditions over snow -covered ground. As the visibility decreased, he

Figure. Photograph of an accident site showing the visual

effects of flat light and snow -covered terrain

SA-052 March 2016 slowed the helicopter and stayed low to the ground; he reported that blowing snow

from the main rotor downwash reduced the visibility to whiteout conditions with no ground reference. ( GAA16LA031

)

• A Bell 206L- 1 helicopter sustained substantial damage following the commercial

pilot’s attempted precautionary landing due to poor visibility conditions. The pilot reported that while flying over an area of flat and featureless, snow -covered terrain,

deteriorating weather conditions with low ceilings, light snow, and flat light conditions reduced his visibility. He said that , while attempting to land, blowing snow from the

main rotor downwash reduced his ability to discern topographical features on the snow -covered terrain. During touchdown, the helicopter drifted to the left, the left skid

struck the snow -covered terrain, and the helicopter rolled onto its left side.

(ANC13CA041

)

• A Cessna 182B airplane, being operated on a cross- country flight that included flying

through a narrow mountain pass , sustained substantial damage when it collided with

mountainous, snow -covered terrain. The commercial pilot and two passengers were

fatally injured. The typical route through the p ass required making multiple turns, and

the pass intersected with a box canyon. The airplane’s wreckage was located at the bottom of the box canyon a day after the airplane was reported overdue. A friend of

the pilot, who attempted to cross the mountain pass the day of the accident , reported

flat light conditions and difficulty discerning terrain features. A nother pilot who flew

through the mountain pass on the morning of the accident reported 4,400- foot

ceilings, severe turbulence, and flat light conditions. (ANC13FA027

)

• An Aviat A1- A airplane, being operated on a cross -country flight under VFR,

sustained substantial damage after colliding with snow -covered ice (a frozen

drainage reservoir). The commercial pilot, the sole occupant of the airplane, was not injured. The pilot reported that he intended to over fly an airstrip without landing and

that flat light conditions hampered his ability to judge his relative distance to the

ground. He further stated that he did not realize he was descending over the snow -covered terrain until he impacted the snow. (SEA06CA043

)

What can pilots do?

• If possible, look for, use, and don’t lose sight of multiple visual reference points.

• Obtain an instrument rating and become proficient and comfortable with operating in

instrument meteorological conditions (IMC). Trust the cockpit instruments and

develop good cross -check practices .

• Understand that the ability to judge the height and determine the contour of terrain is

difficult in conditions where the sky and ground (or water) are similar in color. When

landing on snow -covered terrain, conduct an overflight and consider using weighted

flags or other markers that can be dropped from an aircraft and provide contr ast.

Shorelines may also provide needed contrast.

• If you regularly fly in snowy conditions, b ecome proficient and comfortable with

taxiing, taking off, landing, and conducting en route maneuvers and go- arounds in

areas with snow. If visibility drops, use y our instruments and land at the nearest

suitable airport.

SA-052 March 2016 • Install instruments that can enhance situational awareness (for example, radar

altimeters, ground proximity warning systems, onboard weather systems, and GPS

displays).

• Check all available weather sources before and during a flight. Study the flight routes

and avoid potentially hazardous areas (such as rapidly rising terrain, towers/wires,

and large open water areas ), particularly if the weather is conducive to flat light or

whiteout conditions .

• Set and use personal minimums , taking into account your skill level and the demands

of a situation. Sticking to your personal minimums helps manage risk .

• Use other sources of information during a flight to determine the weather conditions

(for example: company dispatch, other pilots operating in the area, or weather cameras that may be in place, particularly in Alaska).

Interested in more information?

• Federal Aviation Administration ( FAA) Pamphlet AM -400-00/1, “ Spatial Disorientation

Visual Illusions ,” explains both phenomena and how to avoid them during flight

operations.

• The FAA pamphlet “ Flying in Flat Light and White Out Conditions ” provides

information about how to safely operate in these conditions .

• FAA Pamphlet P -8740 -24, “ Winter Flying Tips ,” provides information to remind

pilots about winter weather operating procedures .

• The FAA Safety Team provides access to online training courses, seminars, a nd

webinars. The course “Weather Wise: VFR into IMC” and many others can be accessed from www.faasafety.gov

• FAA Advisory Circular 60- 22, “ Aeronautical Decision Making ” provides information

about how to implement the various aspects of aeronautical decision- making into

flight operations.

• FAA Advisory Circula r 61 -134, “ General Aviation Controlled Flight into Terrain

Awareness ,” provides information about controlled flight into terrain and mitigation

strategies .

• The NTSB issued Safety Recommendations A -02-33 through - 35 and A -79-75 to

improve, respectively, the safety of helicopter operations in flat light and whiteout

conditions and GA safety in Alaska and other regions with similar environmental

conditions.

• NTSB Safety Alert SA- 020, “ Reduced Visual References Require Vigilance, ” provides

information about operating in reduced visibility conditions and historical accident information.

The NTSB’s Aviation Information Resources web page, www.ntsb.gov/air

, provides

convenient access to NTSB aviation safety products. The reports for the accidents

referenced in this safety alert are accessible by NTSB accident number from the Aviation

Accident Database link, and each accident’s public docket is accessible from the Accident

Dockets link for the Docket Management System . The safety recommendation letter s

referenced in this alert can be accessed from the Safety Recommendations link. This safety

alert and others can be accessed from the Aviation Safety Alerts link.