Why Are D.C. Plane Crash Investigators Focused on Night-Vision Goggles?

As divers and salvage crews begin pulling parts and pieces of an American Airlines regional commuter jet and an Army helicopter from the Potomac River near Reagan National Airport in Virginia after the crash last week, investigators will start looking for clues as to how the disaster, which claimed 67 lives, happened.

One of the open questions is whether or not the military aircrew flying the UH-60 Black Hawk was wearing night-vision goggles at the time of the collision.

How common is it for military helicopter pilots to fly with night-vision goggles?

For military pilots, night-vision goggles are used just as commonly as they are by troops on the ground.

According to an Army report, the service began issuing night-vision devices to its aviators in 1985 and has continually acquired goggles that allow pilots and aircrews to see more clearly and with better depth perception at night ever since.

Pete Hegseth, secretary of defense, said the Army crew members involved in Wednesday’s crash were performing a required annual night training and that they “did have night-vision goggles.” But Mr. Hegseth did not say if the goggles were in use at the time and National Transportation Safety Board investigators said they did not yet know.

How do they work?

The devices work by amplifying light in the wearer’s field of view. With them, dim objects become brighter — even making stargazing possible in light-polluted cities.

Conversely, intense lights can overwhelm a user. Military helicopter pilots often wear their goggles a couple finger-widths away from their eyes to allow them to “see around” the devices with the naked eye when needed.

Why don’t investigators know yet whether night-vision goggles were being worn?

The night-vision goggles the Black Hawk crew members almost certainly carried would be mounted onto the front of their helmets, allowing them to be flipped up or down quickly.

But while those mounts might keep a pair of night-vision goggles in place after being lightly bumped in a cockpit, they are not meant to survive the incredible violence of a midair collision like the last week.

Then there was the impact of the helicopter’s wreckage with the water after falling roughly 300 feet.

Either event could have torn the aircrew’s goggles from their helmets, or knocked their helmets off altogether. Such crashes often result in dismemberment of their victims, which could make it even more difficult for investigators to definitively say whether the Black Hawk’s crew members were using their night-vision devices at the time of the collision.

What will they be looking for and how do they hope to figure it out?

The investigation is examining physical evidence from the crash site, including the condition of the bodies and the positioning of the crew’s helmets and equipment. Investigators are particularly interested in finding the night-vision goggle storage bags. If they are empty, that would probably indicate that the goggles were in use. If the bags are intact with the goggles inside, it would most likely mean that the goggles were not being worn at the time of the collision.

The severity of the crash means investigators will also be looking for signs of damage or detachment to the goggles and related equipment. Given the extent of the damage, it may take further analysis to determine whether the goggles were in use and how they may have been affected during the crash.

“If the goggles were down, they would push into the face and cause more facial structure injuries,” said David Soucie, a former Federal Aviation Administration safety inspector and accident investigator.

That might be apparent in autopsies or postmortem examinations of the aircrew’s remains, Mr. Soucie said.

But the violent impact of the midair collision and then the fall to the Potomac may have made that impossible to determine.

All of that evidence is still being examined.

How much do helicopter pilots rely on these devices when flying around Washington at night?

A military helicopter pilot who routinely flew patrols along those same routes over Washington and Northern Virginia said that many pilots typically flew with night-vision goggles attached to their helmets at night, doing so with them just far enough from their face to allow them to see around the goggles with the naked eye at times.

While the image seen through the goggles would present light as either green or white, it does not allow pilots to distinguish between the red or green lights on another aircraft that would signal its direction of flight, which pilots could see by looking just to the side of their goggles.

Flying in highly lit areas, like cities, could also prove challenging, said the pilot, who requested anonymity because he was not authorized to speak publicly about standard flight procedures in the nation’s capital.

Any limitations to a pilot’s peripheral vision or depth perception caused by wearing night-vision goggles are mitigated through frequent training and night-flying, the military pilot said, and it would be up to the pilots whether to wear their goggles up or down at any given moment — depending on the situation or environment.