Home' RTCA Documents for Review : DO-220A Change 1 Contents Appendix A
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DO-220A Change 1
16. Justification for Drift Angle Approach Flight Scenario:
It has been determined that microbursts are driven along the ground by upper winds. Their downflows
penetrate the lower air mass, which contains the airplane, and can therefore have relative motion within the
local air mass. Relative drift obtained from data during TDWR testing shows that windshear events either
side of a fixed narrow beam (± 5 degrees) sensor, that just looks along the airplane’s projected longitudinal
axis or ground track, could be missed. However, with the alert boundary extended to 0.25 nautical miles
either side of the airplane projected longitudinal axis, adequate warning will be given. An event approaching
along the outer edge of the display, due to a 25-degree drift angle, should generate a warning alert 10
seconds prior to the encounter with the airplane flying at 200 KTAS.
The 25-degree drift angle specification was established by determining the demonstrated crosswind values
for a number of current large transport airplanes from their airplane flight manuals. Thirty knots at 50 feet
AGL represented a reasonable consensus, with none higher than 31 knots. This value was extrapolated to
1200 feet AGL using the standard correction method of the height ratio to the one-seventh power. The 47.2-
knot crosswind at 1200 feet AGL will produce a 23.2 drift angle for a 120 KTAS approach speed. The 25-
degree requirement will provide margin to allow some variation in actual conditions.
The windshear event has been located at the threshold to give the longest possible time from the system
alerts enabling altitude (1200 feet AGL minimum) to evaluate the detection and displays.
Since this evaluation is only to assess the system’s ability to detect windshear s with the worst-case drift
angle, it is sensible that only a limited number of windfield models need to be evaluated. Data Sets 4 and 5
were selected as they have small diameter outflows, and being on the edge of the system’s scan, they will
stress the system’s ability to detect, display, and issue timely alerts.
Since for a given value of crosswind low airspeed will give a higher drift angle than high airspeed, only the
120 KTAS case has been picked for evaluation.
17. Justification for Go-Around Flight Scenario:
This scenario is to evaluate the system mode transition from approach to go-around. Alerting ranges,
logic changes, system gains and biases, antenna scan elevation, etc., are possible effects that need to be
evaluated. The leading edge of the hazard is located 1.8 nm from the go-around point as this provides
a reasonably low altitude encounter which could be hazardous.
Data Sets 2 and 5 were selected because of their benign appearance (5 especially being dry) and the
pilot may mistakenly assume they are safe to penetrate.
18. General Meteorological Description for DFW Microburst, Data Set 1:
The 2 August 1985, Dallas – Fort Worth (DFW) microburst was a high-reflectivity microburst that
resulted in the crash of a commercial jetliner. This event is simulated with the 2-D axisymmetric
Terminal Area Simulation System (TASS) model by assuming an environmental sounding interpolated
from observed data. The simulated microburst is associated with high reflectivity due to rain and hail,
moderate rainfall rates, pronounced temperature drop, and hazardous wind shear with strong outflow
winds. The data set is taken near the time of peak intensity, at 11 minutes simulation time . Although
the numerical simulation is 2-D, there is reasonable comparison with observed data taken from aircraft
flight data recorders.
19. General Meteorological Description for Orlando Microburst, Data Set 2:
The 20 June 1991, Orlando microburst, was encountered by a NASA aircraft instrumented with in-situ
and forward-look windshear sensors. It was also measured within the Terminal Doppler Weather Radar
(TDWR) test bed. The parent storm and ensuing microbursts are simulated with 3-D TASS.
Comparisons of the simulation with observed data indicate a reasonable agreement. The simulation, as
verified from measurements, indicates a high-reflectivity microburst with hazardous shear and heavy
rainfall rates. Although the area covered by the outflow is roughly symmetrical, complex regions of
windshear hazard are embedded within the outflow. The data set is taken at a simulation time of 37
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