Learjet crash probe seeks to understand high-altitude upset in convective weather

Executive jet bound for Bahamas came down near West Palm Beach after steep descent from cruise.
As the inquiry into the Bombardier Learjet 60 crash in Florida opens, investigators will inevitably be examining whether the aircraft strayed outside of its high-altitude flight envelope in turbulent conditions.

Eastern Florida was subject to convective weather, with thunderstorms and heavy rain, at the time of the accident, near West Palm Beach, on 1 November.

The aircraft, bound for North Eleuthera airport in the Bahamas, had been heading towards Freeport at 35,000ft.

One of the pilots had referred to “moderate” turbulence at this altitude – indicating fluctuations of 600-700ft – but that the crew was “managing the ice”, in an exchange with Miami’s en route air traffic control centre.

The pilot advised the centre that the outside air temperature was minus 47°C and queried whether there was an option to fly in smoother air, to which the controller replied that they could “expect better rides ahead”.

But the pilot subsequently stated that the aircraft was “picking up precipitation” and that they needed to climb to 37,000ft. This clearance was granted by air traffic control.

High-altitude flight presents particular aerodynamic challenges owing to the effect of increasing Mach number, including greater sensitivity to angle-of-attack – especially in icing conditions.

Turbulence from convective cell conditions can increase aerodynamic demand on the wing, and reduce the operating margin between Mach buffet and stall buffet already narrowed at high altitude.

Supercritical wings are designed to improve efficiency but exceeding critical Mach number thresholds can lead to a pitch-down tendency – known as ‘Mach tuck’ – as the wing centre of pressure moves aft, and overspeed which can be difficult to counter.

The Learjet 60 features a Mach trim system intended to stabilise pitch at critical speeds.

Although the aircraft involved in the Florida accident entered a high-speed dive, and a banking right turn, the inquiry will have to understand whether an excursion from the flight envelope contributed to the upset, or whether it was triggered by other factors, such as a mechanical or structural failure.

Investigators have yet to determine whether the aircraft – which had nine occupants – was carrying a cockpit-voice or flight-data recorder. The status of the flight has not been confirmed but Part 135 regulations state that aircraft with six or more passenger seats need a cockpit recorder.

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