Tackling Turbulence: NRC Tests May Help Improve Flight Safety

NRC CT-133 research aircraft used for studying commercial aircraft wakes

New instrumentation installed and tested on NRC's CT-133 aircraft will enable researchers to gather the most detailed data yet on turbulence behind en route commercial aircraft. Once analyzed, this information is expected to result in a better understanding of aircraft turbulence and could improve flight safety.

NRC's CT-133 aircraft equipped with its new air data acquisition system performed successfully in six trial instrumentation development and calibration flights in May. Using the aircraft staff at the NRC Institute for Aerospace Research carried out a subsequent wake vortex flight behind three commercial aircraft (an A310, A319, and Boeing 767-300) en route over the St. Lawrence Seaway. Wake vortex is the turbulence formed behind an aircraft.

Turbulence is an ongoing concern for people involved in air travel and air transportation safety. Although most wake vortex encounters occur during takeoff and landing, a few have occurred with the aircraft in the en route configuration. Every aircraft generates a pair of trailing wake vortices which undergo instability for a substantial length of time and distance. If another aircraft travels through these wake instabilities it could experience substantial aerodynamic loading and flight path upsets — turbulence severe enough to knock passengers or flight attendants off their feet and possibly cause injuries as well as aircraft structural overload.

To gain a better understanding of the flow mechanisms that can cause such upsets, the NRC Institute for Aerospace Research initiated an earlier pilot study in 2004. During that study, four flights were conducted using the NRC Falcon 20 research aircraft to fly behind another aircraft cruising at altitudes of 7.3 – 11.9 kilometres (24,000 – 39,000 feet) with 1.6 – 48.3 horizontal kilometres (1 – 30 miles) separating the two aircraft.

During those flights, the Falcon experienced aerodynamic g-loading, flight path upsets and an engine flameout, when it became entrained by the trailing vortices. These problems occurred at wake lengths of 12.9 – 20.9 kilometres (8 – 13 miles), well within permissible distance separations for aircraft, and were large enough to knock anyone standing within the aircraft off their feet. The results led to a decision to continue research with the NRC CT-133, a more rugged aircraft.

Since then, NRC installed inertial and air data acquisition systems on the CT-133, carried out software development on the air data system, and flew several airworthiness and air data gathering test flights. These were followed up in late May 2006 with a single flight in the wake vortex environment behind commercial aircraft flying between Quebec City and Toronto.

Condensation outlining B767-300 trailing vortex cores

Anthony Brown, an NRC aerospace research officer and test pilot, said, "We gathered data at 600 Hz, which is 600 samples per second. To our knowledge, this is the first atmospheric research aircraft to do so at that high a rate. It gave us an immediate impression of the wake vortex flow field that we just didn't observe when we gathered data at 32 Hz using the Falcon".

He noted, "We showed events happening over time periods of a couple hundredths of a second, with rise or fall times of one hundredth of a second. As a result, we feel that this is a good platform to obtain detailed information on wake vortex flow fields."

"We compared the CT-133 and the Falcon's data on a Boeing 767 wake vortex encounter, and we were able to compare CT-133 data between an A310 and a Boeing 767. The data shows very high-speed events which may be associated with flying through characteristic funnel vortices within the vortex cores," Brown added.

The next step is to refine the air data gathering system and obtain additional wake vortex flow field data through further flights. "Our goal is to obtain data on the flow phenomena to clarify the risk and highlight the desirability, or the need, to develop wake vortex turbulence warning instrumentation that can be installed on aircraft," Brown stated.

The National Research Council Institute for Aerospace Research is Canada's national aerospace laboratory, undertaking and promoting research and development in support of the Canadian aerospace community in matters affecting the design, manufacture, performance, use and safety of aerospace vehicles.  

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