Smoke rises from the helicopter crash site in Calabasas, Calif., amid thick fog and mist. (Los Angeles County Sheriff's Department/AFP/Getty Images)

On Sunday afternoon, many mourned the loss of basketball legend Kobe Bryant, his daughter Gianna and seven others aboard a helicopter that crashed into the Calabasas hills of California, northwest of downtown Los Angeles. Weather in the crash area at the time featured fog and mist, with reduced visibility probably obscuring hills — raising concerns that the conditions may have played a major role in the accident.

Here’s what we know about the weather, as well as the flight rules that were governing the helicopter’s operation at the time, and how they might have contributed to the disaster. Specifically, attention is focusing on the role played by fog, high terrain and the pilot’s use of “special visual flight rules” conditions. Pilots can request them when flying in controlled airspace, if flight by visual cues alone is not possible along their entire flight route.

Weather and flight history

The helicopter involved in the accident, manufactured in 1991, was stored overnight at Long Beach Airport in California. It was flown to John Wayne Airport in Orange County about 30 minutes before taking off with Bryant, his daughter and six other passengers aboard.

At 8:53 a.m., observations from John Wayne Airport showed light east-northeasterly winds at 5 mph, overcast skies and visibility around four miles. The cloud ceiling, or base of the clouds, was around 1,000 feet. The temperature was 57 degrees with a dew point of 50, indicating enough atmospheric moisture to generate low cloud cover. Conditions were sufficiently compromised there and at other L.A.-area airports to ground helicopters operated by the Los Angeles Police Department on Sunday morning.

The Sikorsky S-76B helicopter, registered N72EX, departed John Wayne Airport at 9:06 a.m. The Sikorsky S-76 has historically held a strong safety record.

About 15 minutes into the flight, N72EX entered a holding pattern just west of Glendale. It remained in this pattern, circling for about 10 to 15 minutes, unable to cross from Los Angeles airspace into a region controlled by Hollywood Burbank Airport. Part of the delay was caused by a transition to “special VFR rules.”

When a pilot is granted permission to fly, each flight is operated under “VFR” or “IFR” regulations.

Visual flight rules (VFR) describe conditions that allow the pilot to fly by sight. Assuming the weather is calm and visibility is high, operation and navigation of an aircraft can largely rely on what a pilot sees. According to the Federal Aviation Administration, “VFR flight is based on the principle of ‘see and avoid.’ ”

Instrument flight rules, or IFR, on the other hand, are mandated when an aircraft cannot be safely operated visually. This may occur at night, during adverse weather or in complex topography. The FAA publishes a list of minimum conditions required to permit VFR flying. If those can’t be met, then a flight is required to plan its course under IFR restrictions.

Pilot requirements for IFR flights are considerably more stringent than for pilots permitted to fly VFR flights.

As the helicopter transitioned into Burbank’s airspace, the pilot requested — and was approved — to press on under “special VFR” flight conditions.

That’s a designation that allows pilots to continue operating as a VFR flight even in conditions below VFR limits. A switch to IFR could have been more time-consuming in such busy airspace. There’s also some question as to whether this particular S-76B model had the necessary avionics for a single pilot to operate it under IFR conditions.

“IFR changes the game quite a bit,” said Jerry Kidrick, a 35-year Army helicopter veteran and a professor of advanced helicopter operations at Embry-Riddle Aeronautical University.

“If you’re filing IFR, then you’ll take off from your departure airport and you’ll climb up and [Air Traffic Control] will give you an altitude where you’ll mix in with all the other IFR traffic. Typically, they’re going to take you up on a routing that is not convenient. It’s way more difficult to do that and make it to your destination [quickly]," Kidrick said.

Between 9:40 and 9:44 a.m., the chopper flew between 1,000 and 1,200 feet high. The helicopter passed in the vicinity of Van Nuys Airport before reportedly turning southwest toward the Calabasas Hills. The 9:51 a.m. weather observation at the airport, taken around the time of the crash, recorded visibility of 2.5 miles with 80 percent relative humidity and a ceiling of 1,300 feet. Winds were calm. Haze was also reported.

During this period, it would appear as though the helicopter was flying below the clouds. Much of Simi Valley tends between 600 and 700 feet ambient elevation, and the aircraft was probably about 300 to 500 feet above the surface. Under special VFR conditions, aircraft are generally required to maintain an altitude of at least 500 feet above the ground and 1,000 feet above the nearest obstacle. Helicopters, according to the FAA, “may be operated at less than the minimum altitudes prescribed” for other aircraft operating in special VFR.

Flowers and mementos lie outside Bryant Gymnasium at Kobe Bryant's alma mater Lower Merion High School outside Philadelphia on Monday. (Michael A. McCoy/Reuters)

Helicopters are also permitted to fly in special-VFR mode even if the visibility is below the 1,000-foot threshold required by pilots of other aircraft.

Meteorological analysis suggest the pilot’s altitude at the time allowed the helicopter to continue just below the layer of dense overcast/low cloud cover. It was flying parallel to U.S. 101, based on FlightAware tracking data. (Following major highways is common during special-VFR conditions.)

But it appears the pilot encountered trouble, possibly fog, substantially reduced visibility or a mechanical problem.

At 9:44 a.m., the helicopter began to ascend rapidly, climbing 875 feet in the following 36 seconds. That’s a rapid ascent — one that Kidrick describes as a “spatial disorientation profile.”

“That pace at which he climbed indicates he’s trying to get out of there, or he got spatially disoriented,” Kidrick said. “That’s a very common thing [investigators] find post-crash; the pilot makes rapid corrections and rapid maneuvering. … He does something radical to try to change that. It appears something went wrong.”

In situations like that, Kidrick said, pilots “do things with the controls they otherwise wouldn’t normally do.” He described it as a “panic situation.”

That sudden climb probably carried the helicopter into a layer of fully saturated air and extremely low visibility. Cheeseboro, a weather station northwest of the crash site at 1,700 feet elevation, reported 100 percent relative humidity at the time of the crash, indicating the helicopter was probably in the clouds at the time.

It’s likely that the nearby hilly high terrain was largely or completely obscured by fog. Photos from the scene depict a misty veil of fog, while satellite imagery from above captured low clouds and fog dominating early in the day. The fog did not begin to burn off until around lunchtime.

Los Angeles County firefighters were on the scene of the helicopter crash in Calabasas on Sunday. (Mark Ralson/AFP/Getty Images) (Mark Ralston/Afp Via Getty Images)

At 9:45 a.m., the helicopter abruptly turned from a south-southwesterly direction to southeasterly and eventually easterly. During the same time frame, the helicopter’s ground speed surged, and its altitude — if the FlightAware data is correct — dropped 350 feet in 6 seconds. No additional data is available after 9:45 a.m. and 29 seconds.

Based on this and the resulting debris patterns, many pilots (see here, here, here and here, as well as the video below) have conjectured that this may have been an instance of “controlled flight into terrain,” during which a mechanically stable aircraft is flown under controlled conditions into an object or the ground. These incidents typically occur when visibility is an issue and historically have been one of the biggest causes of aviation accidents.

The National Transportation Safety Board dispatched a “go team” that arrived in California on Sunday night. They will make the ultimate determination as to primary causes of the accident.

What pilots do when ‘spatially disoriented’

According to Kidrick, when a helicopter pilot suddenly finds himself or herself “inside the soup,” or shrouded in fog, the best option is not to turn around. That can exacerbate ongoing disorientation. “That’s the key. Your brain is telling you one thing, and your instruments are telling you another. That’s the critical time.”

Instead, the best course of action is to slow forward speed, stabilize the aircraft and level out, and gradually climb. Only once pilots feel safe should they contact air traffic control, which will route other aircraft around and away from the distressed or disoriented helicopter.

Fog’s role in aviation disasters

People mourn former NBA star Kobe Bryant near Staples Center. (David McNew/AFP/Getty Images)

Low clouds and fog have long been catastrophic to aviators, with controlled flight into terrain (CFIT) a common occurrence. These accidents usually happen when pilots are unaware of their proximity to the ground. Loss of situational awareness is a common factor.

According to Boeing, CFIT crashes have caused over 9,000 deaths since the beginning of commercial jet operations. The deaths resulted from aircraft “inadvertently flying into terrain, water or an obstacle.”

The addition of adverse weather makes the hazard all the more prominent. These disasters have affected jumbo jets and helicopters alike, but helicopters present a particular challenge because they operate at low altitudes.

On Nov. 27, 1979, an Air New Zealand DC-10 flew into the side of Mount Erebus in Antarctica during a sightseeing tour. The flight plan in the computer had programmed the wrong coordinates, guiding the aircraft directly into the mountain. A layer of clouds blended with the snow-covered mountainside — probably obscuring the terrain to the pilots.

CFIT was also to blame on April 3, 1996, in Croatia. A modified U.S. Air Force Boeing 737 crashed into a mountainside while attempting an instrument landing, killing all 34 aboard. Among the victims was Ron Brown, secretary of commerce during the Clinton administration.

A number of other foul-weather CFIT accidents have occurred, in addition to numerous CFITs at night.

In 2016, a helicopter crashed into a mountainside in Alaska amid thick fog, wind and snow. The NTSB deemed it a pilot-error CFIT accident. In 1975, an airplane flew into a mountainside on Alaska’s St. Lawrence Island, killing 10.

Rex Alexander, a 30-year veteran of the helicopter industry who specializes in helicopter safety, points to weather as a major player in crashes.

“Weather’s always been a challenge, but there’s not a lot of money from the government … to provide that same level of security and reporting [of weather] at low altitude as there is at high altitude,” said Alexander, who now serves as a consultant at his company, Five-Alpha.

Alexander said helicopters often land between airports or spend time at low altitudes where little monitoring takes place.

“When you look at the billions spent on weather at airports, the amount spent at heliports is infinitesimal,” he said.

Alexander has worked throughout his career to bring “real-time weather into the cockpit.” He said that weather reports fed to pilots upon takeoff “have an extremely short shelf life and can change rapidly.”

Alexander was one of three lead authors on a report that looked into 14 years of helicopter crashes from eight countries. Among the data set were 672 fatal accidents that resulted in 1,308 fatalities.

Sixteen percent of accidents, the team found, occurred when helicopters flew in VFR mode into conditions that were below VFR limits. That, the report concluded, “involves both a lack of pre-flight planning and risk.”

Overall, “by far the most common and the most lethal fatal accidents are loss of control and controlled flight into terrain.” During the time frame of the study, 118 fatal accidents occurred, the leading cause being VFR flying at night or into poor weather.

“Most of the loss-of-control accidents turned into controlled flight into terrain,” Alexander said.

According to Kidrick, situations that involve weather in the cockpit are very demanding.

“I’ve experienced similar conditions. … They call for good decision-making, an assessment of how important it is to fly in these conditions. Sometimes, it’s very hard to say no … turning to the people in the back. [It’s] easy to say ‘I think I can make it through, I think I saw an opening,’ but you have to ask yourself what the downside of those decisions are.”

Andrew Freedman and Jason Samenow contributed to this report.