Vortex flow field data

Citation

(2007), "Vortex flow field data", Aircraft Engineering and Aerospace Technology, Vol. 79 No. 1. https://doi.org/10.1108/aeat.2007.12779aaf.004

Vortex flow field data

Vortex flow field data

The National Research Council Institute for Aerospace Research (NRC Aerospace) has recently installed and tested new instrumentation on its CT-133 aircraft that will enable it to gather the most detailed data yet on turbulence behind enroute commercial aircraft.

This information will result in a better understanding of turbulence and could improve flight safety. The aircraft's new air data acquisition system, which collects data at 600Hz (600 samples of air data and inertia! data per second), performed successfully in six trial instrumentation development and calibration flights in May, and one subsequent wake vortex flight behind three commercial aircraft (an A310, A319, and Boeing 767-300) enroute over the St Lawrence Seaway. These flights demonstrated that the CT-133, possibly the first atmospheric research aircraft to gather air data at such a high rate, is an effective platform for collecting detailed information on wake vortex flow fields.

Turbulence is an ongoing concern to air transportation safety and although most wake vortex encounters occur during takeoff and landing, a few have occurred with the aircraft in the enroute configuration. Every aircraft generates a pair of trailing wake vortices which undergo instability for a substantial length of time and distance, and an aircraft traveling through these wake instabilities could experience substantial aerodynamic loading and flight path upsets – enough to severely knock passengers or flight attendants off their feet and possibly cause injuries as well as structural overload.

To gain a better understanding of the flow mechanisms that can cause such upsets, NRC Aerospace initiated a pilot study in 2004. In that study, four flights were conducted out of Ottawa, Canada, using the NRC Falcon 20 research aircraft to fly behind a cruising aircraft at altitudes of 24,000-39,000feet with separation distances of 1-30miles. 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 perturbations, which occurred at wake lengths of 8-13mile, well within permissible separations, 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 Aerospace has installed inertia! and air data acquisition systems on the CT-133, carried out software development on the air data system, and flown 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.

Anthony Brown, NRC Aerospace research officer and test pilot, said, “We gathered data at 600Hz, 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 32Hz using the Falcon; it 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.”

He added, “We were able to compare the CT-133, and the Falcon, 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 highspeed events which may be associated with flying through characteristic funnel vortices within the vortex cores.”

The next step is to refine the air data gathering system and obtain additional wake vortex flow field data through further flights later this summer. “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.

A study was also initiated in June 2003 with the objective of conducting investigations and gathering data to support recommendations, which would be made to the International Civil Aviation Organization (ICAO), regarding safe wake vortex separation criteria for aircraft following an A380 for various conditions of flight.

The study was led by a Steering Group, comprising Joint Aviation Authorities (JAA), Eurocontrol, US Federal Aviation Administration (FAA), and Airbus. The detailed scientific work was conducted by a subgroup consisting of the leading international experts in this complex field. It was supported by an unprecedented program of flight tests with innovative aspects such as back to back comparative testing of different aircraft, cruise wake encounter tests, and ground and airborne LIDAR (light detection and ranging) wake measurements, totaling over 180h flight time.

As an interim measure, pending completion of the scientific work, temporary guidance material was recommended to ICAO in 2005, to enable ICAO Member States to safely accommodate, from an air traffic management aspect, the A380 during its worldwide developmental flights, prior to entry into operational service. This interim guidance was necessarily conservative because data collection, processing, and analysis were still ongoing at that time.

The Steering Group is now in a position to recommend more specific guidance, based on the completed flight test program. A significant aspect of this new guidance is that it provides for a future review/revision based on further study or modifying current aircraft categories and operational experience.

The following summarizes the key elements of the guidance for ICAO heavy, medium, and light aircraft categories.

Approach/landing

A380 followed by heavy=+2nm extra to existing ICAO separation f 6nm absolute distance).

A380 followed by medium=+3nm^* extra to existing ICAO criteria f 8nm absolute distance).

A380 followed by light=+4nm^* extra to existing ICAO separation criteria f 10nm absolute distance).

No wake constraint for the A380 as a following aircraft.

^*These values are subject to review and possible reduction based on further study or changes in aircraft categories and operational experience.

Departure following A380

Heavy=2min medium, light=3min.

Or same radar spacing as for approach/landing no wake constraint for the A380 as a following aircraft.

Vertical spacing

Vertical spacing in all cases to be the same as for other aircraft.

Evidence and data from encounter flight tests at cruise altitude, supported by airborne LIDAR measurements, have demonstrated that the A380 wake characteristics are equivalent to those of the B744 (chosen as the benchmark aircraft) for this phase of flight. Therefore, the current ICAO vertical separations are confirmed to be appropriate for A380 operations.

Horizontal spacing en-route

En-route horizontal spacing to be the same as for other aircraft.

Holding

Vertical spacing to be the same as for other aircraft.

Though not specifically addressed, flight tests provided no indication of impact on parallel runway operations for runways separated by more than 760m (2,500ft). This should be monitored in operational service for verification.