In the United States, the National Transportation Safety Board began its investigation on Monday 2 November into the crash of the Learjet 60 registered N722BD in Palm Beach County, Florida7. The business jet came down on Sunday on its way from Austin, Texas, to North Eleuthera in the Bahamas, and all nine people on board died3. The flight profile, flight data and the impact on the ground are covered in our first report on the crash. This follow-up looks at the radio calls, the investigation and what an ice report at FL350 and minus 47 degrees means.

Listen to the ATC audio of N722BD with Miami Center, with a timeline and quotes.

The radio calls on a timeline

Exact times for the individual radio calls have not been published. The recording of the exchange with Miami Air Route Traffic Control Center is on LiveATC.net. The sequence below comes from the descriptions by CNN, AP, CBS, AIN and Flightradar24; the times come from the ADS-B data published by Flightradar2412345.

• At about 17:41 UTC the aircraft levels at FL350 after descending from FL4101.
• At FL350 the crew reports "light, clear ice that is manageable", moderate turbulence moving the aircraft by as much as 700 feet and an outside air temperature of minus 47 degrees Celsius. The crew asks whether there is a better ride ahead123.
• The crew then reports precipitation and asks to climb to FL37015.
• At 17:48 UTC the aircraft climbs briefly to 35,250 feet. Shortly after the climb clearance the crew reports a rapid descent1.
• A pilot reports descending at 8,000 and then 10,000 feet per minute and declares a mayday36.
• Next comes: "We are in an overspeed condition descending 20,000 feet per minute"4. Aerospace Global News describes a further call reporting 22,000 feet per minute6.
• About 30 seconds later the transmissions end with "we're out of time". Controllers then try without success to re-establish contact5.
• At 17:49:22 UTC Flightradar24 receives the last altitude value: 8,750 feet1.

How much time passed between the ice report and the start of the descent cannot be established from the public descriptions. Up to Sunday evening the NTSB had not addressed the radio calls in its public statements8.

What the NTSB is doing now

According to The Palm Beach Post, the NTSB said on Monday morning that a team of specialists was on its way to the crash site. A spokesperson said the NTSB would hold a news conference in West Palm Beach; the time and place had not been released on Monday morning7. According to the NTSB, as cited by WPTV, NTSB Member Tom Chapman gives that briefing on Monday at 1 p.m. local time (18:00 UTC) at the Palm Beach County Emergency Operations Center. He is expected to discuss the investigative process and the work investigators plan to do at the site in the coming days1415. What the NTSB said there is covered in our report on the briefing16.

In this phase the go-team gathers the information needed for the analysis. That happens at the site itself, but according to the NTSB it also covers flight logs, maintenance records and interviews9. Through its party system the NTSB can bring other organisations or companies into the investigation9.

Whether the Learjet carried a cockpit voice recorder or a flight data recorder, and whether either has been recovered, has not been released. CNN reports that US rules require a voice recorder in multi-engine jets flown for hire; whether this was a charter flight is unconfirmed138. CNN analyst Mary Schiavo said the flight recorder could have been destroyed in the explosion2.

US media expect a preliminary report within about 30 days11. In another recent accident, a Cessna 441 in Alaska on 20 August, the NTSB published its preliminary report on 16 September12. The NTSB aims to complete an investigation within 12 to 24 months; only then does it determine a probable cause9.

Ice at minus 47 degrees: what the FAA guidance says

For pilots, the combination of a clear ice report and an outside air temperature of minus 47 degrees stands out. According to the FAA pilot guide to flight in icing conditions, Advisory Circular 91-74B, nearly all aircraft icing occurs in supercooled clouds with liquid drops. Below about minus 20 degrees most clouds are made up entirely of ice particles and liquid water content is usually negligible, the circular says. Above FL250 clouds contain little liquid water10.

Near convective weather, however, large quantities of ice crystals can be found at high altitude. According to the FAA they typically do not accrete on external airframe surfaces, but they can build up inside turbine engines and cause power loss. The circular cites engine events from ice crystals at up to 42,000 feet and at temperatures colder than minus 45 degrees10. Conventional ice detectors and airborne weather radar typically do not detect these crystals; clouds without liquid water have about 20 times less radar reflectivity than raindrops10. CNN reports that a convective SIGMET for thunderstorms covered the area2.

Ice crystals can also clog the heated temperature probe. The indicated temperature then rises falsely to 0 degrees10. A blocked pitot tube gives a wrong airspeed indication. If the pitot tube and its drain hole are both blocked while the static port stays clear, the airspeed indicator reads higher than actual in a climb and lower than actual in a descent10.

Which of these mechanisms, if any, played a part on N722BD is not known. The crew's report is an observation from the cockpit, and the NTSB has not commented on icing. Aviation safety expert Steve Arroyo told AP that icing and turbulence could have been factors and may have caused a stall in cruise. He said the NTSB will not rule anything out at this early stage3.

What is still unknown

• The cause of the crash9.
• Who else was on board: the authorities have not released any names. According to her family, the co-pilot was Nathalia Gadelha Filgueiras, 2071718. According to their family, Adam Robinson and Helen Sharp were also on board with their two young children1920.
• Whether the cockpit voice recorder, which maintenance records show was installed, was on board for this flight16.

Everything established so far about this accident is collected in the dossier on Learjet 60 N722BD.