In conducting a survey about the RDR-4000 weather radar, Honeywell safety specialist Dr. Ratan Khatwa asked more than 50 ATP-rated pilots about their experience with weather radar. The average age of the respondents was 52 years; the average flight time was 12,500 hours. The answers these experienced pilots provided were illuminating.
• 62 percent of the pilots surveyed answered correctly that a straight radar beam is not aligned with an aircraft’s current flight level (because of Earth curvature)
• 15 percent mistakenly thought that antenna down-tilt was required to offset a nose-up pitch angle. (That is offset by antenna stabilization.)
• 63 percent did not appreciate the need for weather-radar antennas to be set to compensate for earth curvature, which blocks weather targets beyond, say, 150 nm ahead for nominal cruise altitudes. “Curvature [effects] become noticeable at ranges above 40 nm, and if ignored can lead to weather-image interpretation errors,” said Khatwa.
• 55 percent of pilots did not realize that a weather target falling inside the radar beam will not necessarily be shown in its true color on the display. “The color selected for display is a direct function of the power returned to the receiver. Where the beam is partially filled, the total power returned may not represent the calibrated value associated with the target cell,” he said.
• Five in every eight pilots incorrectly thought green (short-range) radar targets shown near to cruise levels above FL310 need not be avoided. “Typically, at these altitudes, targets are less reflective. At high altitudes, there is a possibility of unstable air and hail above the storm cell. It is therefore not advisable to penetrate the less-reflective part of the storm top,” Khatwa explained.
• 73 percent of flight crew understood that antenna tilt angle does not need to match a climb (or descent) angle to detect weather on their flight path. “The antenna should be pointed at the base of convective weather during climb. Generally, the lower 18,000 feet is the most reflective part of the storm.” Radar can be used to analyze weather characteristics (such as vertical extent of cells) and to avoid strong convective activity. “Returns along the flight-path angle may not provide full indication of storm intensity and turbulence levels [to be encountered within the cell].”
• Almost 90 percent of pilots did not know the range at which their current weather radar was no longer calibrated and did not show returns at their true levels. Radar beams broaden with distance, so a smaller proportion is filled with moisture. “At shorter ranges, returned power is more representative of the target cell, and it is more likely to be displayed at its true calibrated value. Typically, returns are calibrated within a range of 60 to 80 nm.”