You're not alone if you’re at all confused about the terminology experts are using to talk about the crash of the helicopter carrying Kobe Bryant and seven other passengers. In this case, the experts are not being needlessly complex. The accident aircraft, a Sikorsky S76-B helicopter, crashed only at the end of a complex flight through the Los Angeles basin before it ultimately crashed in the foothills of the San Fernando Valley, just to the north. The flight involved interactions with numerous air traffic controllers and requests by the pilot for certain permissions and communications from ATC that reference specific terms describing how pilots fly in such conditions and how controllers route those flights.
1. MSL versus AGL: The acronyms MSL (mean sea level) and AGL (above ground level) are used in aviation because altitude, for obvious reasons, is critical, and just one measure won’t do. AGL is used when you need to know how high you are above terrain. This is needed for such things as coming in to land on a runway, when you obviously need to know how high you are above the ground. MSL is used more as a general reference for altitude, which is handy for air traffic control, which needs to keep airplanes from running into each other over a wide area that might have vastly different terrain elevations. Here's an easy visual to get the gist of MSL versus AGL: If an airplane is sitting on a runway that’s 3,000 feet in elevation, the plane’s MSL altitude is 3,000 (give or take) and its AGL altitude is zero.
2. VFR, IFR and Special VFR: “VFR” stands for visual flight rules, which are the rules pilots use when they can see where they're going. The rules are super specific about required cloud height and visibility, but what it comes down to is this: Can you navigate with your eyeballs or not? If you can’t, you need to fly IFR (instrument flight rules), under which you do almost everything but land and take off by looking at your aircraft’s instruments. Special VFR isn't really special. It’s just another form of VFR (visual, again) rules that controllers have to give you permission (clearance) to do. The weather can be lower and you can still be legal by following roads, for instance, to stay in the clear. It isn't widely used and many commercial outfits don’t allow their pilots to fly Special VFR.
3. Flight Following: Flight following is a popular way for pilots not flying under instrument rules to get air traffic controllers to follow them on radar. Pilots are still not allowed to fly in clouds (IFR is required for that), but they can stay in touch with controllers and get a heads up if there are other planes nearby that pose a collision risk.
4. HTAWS: This term stands for Helicopter Terrain Awareness and Warning System. It’s a hardware system that uses a color-coded and audible warning system to tell pilots if they’re at risk of running into the ground, a tower or some other obstacle. HTAWS is for some reason not mandated for commercial helicopter ops, though a similar system is required for commercial airliners and has been for decades.
5. CFIT: CFIT, which is pronounced “cee-fit,” stands for “controlled flight into terrain.” This is a type of accident in which a plane flies under the control of the pilot into the ground or the side of a mountain or some other obstacle. This almost always happens in dark of night or in cloudy weather. For CFIT to happen, the pilot has to be unaware (at least until the final moment) that the terrain is close enough to run into. CFIT used to be the most common cause of airliner accidents until Terrain Awareness and Warning equipment was mandated for them. The Bryant crash profile is largely consistent with that of a CFIT accident.
6. Charter Vs. Part 91: The FAA has rules for how flights need to be conducted based on whether they're money-making flights (Part 135 charter) or not (Part 91 private flying). The subject is vastly complicated, so much so that reams of paper are generated when the FAA suspects a violation. In this crash, the FAA will be looking at whether Island Express, the company that was responsible for the flight, was legally operating as a charter flight or a private flight and how that might have affected the pilot's actions.
7. Holding: Holding is simply when a pilot is given instructions by ATC to wait until proceeding farther along on the planned flight. It’s a very common technique by ATC to keep aircraft from getting bunched up. To hold, pilots typically fly a circle or oval-shaped pattern to bide time.