Radar Corner – 8 Things You Might Not Understand About Airborne Weather Radar – Part 1

In this month’s Radar Corner, we start a 3-part series looking at why Honeywell developed the RDR-4000 and RDR-7000 weather radar systems and the problems we were trying to solve. The “8 things” came from, and were a result of, the RDR-4000 human factors study. Try the short quiz in this this article, see how you do, and we’ll explain the answers and how we solved these issues with the RDR-4000.

Background

In 1999, AlliedSignal purchased Honeywell, taking their name. On the legacy Honeywell side of the house, the radar lineage went back to the RCA and Sperry radars while on the AlliedSignal side of the house it went back to the Bendix radar series. Industry firsts included:

• The first airborne weather radar in 1953, the RDR-1,
• The first transistorized radar in 1967, the RDR-1E,
• The first long range weather radar in 1968, the RDR-1F, and
• The first windshear detection radar in 1994, the RDR-4B.

Ok, the point is we have a lot of radar knowledge and experience. Based on this, we thought we knew what pilots didn’t understand about operating radar. Creating training is the obvious solution, right? While somewhat true, that’s the wrong answer (and that wasn’t the quiz). Since radar training time in the classroom is limited, pilots learn a lot of myths and misconceptions about radar on the job.

Human Factors Study

When development of the RDR-4000 began, we wanted to confirm what we believed pilots didn’t understand about radar operation. Every new design that has flight deck effects requires a human factors evaluation. It was also an excellent opportunity to test what we thought we knew and gauge the acceptability of the new automatic weather detection and analysis modes. The end goals of the study were: high end-user acceptance, enhanced weather awareness, and improved crew decision-making while maintaining or reducing workload and training.

We asked each participant a series of questions and provided a brief 15-minute training session on the new RDR-4000 modes. For each scenario pilots were given a pre-flight briefing including ATIS, METAR, PIREPs, and necessary charts. A radar task simulator was used to evaluate different scenarios, but they were only allowed to use one mode (AUTO or altitude slices). The same scenarios were also evaluated using a conventional tilt control radar they had used their entire career. In the group of “line pilots” for the study, the mean pilot age was 52 and mean total time was 12,500 hours. So, it was a very experienced group including pilots from international airlines, business jet operators, Airbus, Boeing and the DGAC (France).

So how did our test group do? You’ll have to take our short 8 question quiz to find out! We’ll explain the answers to the first 4 questions this month. Next month we’ll look at the answers to the last 4 questions, and in the third article we’ll look at how the test group did in the radar task simulator and talk about the new RDR-7000 radar.

The Quiz

We started out with a simple warm-up question: “Is your current weather radar training sufficient?” Over 68% responded NO with comments like, “what training” and “we learn on the job”. Try the simple 8-question quiz below and we’ll see how your results compare with our test group.

1. In level flight with the antenna tilt set to zero degrees, the radar will show me everything at my flight level.    TRUE or FALSE?
2. At cruise level with the aircraft nose up 2 degrees, I should set tilt two-degrees down to compensate for the nose-up attitude.   TRUE or FALSE?
3. Objects that pose a threat can be positively identified by adjusting tilt so the bottom of the beam is parallel to the ground? For example, with a 12-inch antenna and an 8-degree beam width, setting tilt to +4 degrees.    TRUE or FALSE?
4. The antenna tilt should be set to compensate for the earth’s curvature effect. TRUE or FALSE?
5. For radars with an 18-inch antenna the beam is 5.6 degrees wide, and the radar will show targets at their true color level if part of the target is inside the beam. TRUE or FALSE?
6. At cruise altitudes (> FL310), radar targets, displayed as green at short range should be avoided. TRUE or FALSE?
7. If I am climbing (or descending) at a three-degree flight path angle, I should set my tilt angle to 3 degrees to see weather along my flight path. TRUE or FALSE?
8. With the weather radar you are currently using, do you know the range at which it is no longer calibrated, and returns are not displayed at their true levels? If yes, what is the range?

The correct answers are shown below.

1. In level flight with the antenna tilt set to zero degrees, the radar will show me everything at my flight level.    FALSE
2. At cruise level with the aircraft nose up 2 degrees, I should set tilt two-degrees down to compensate for the nose-up attitude.   FALSE
3. Objects that pose a threat can be positively identified by adjusting tilt so the bottom of the beam is parallel to the ground? For example, with a 12-inch antenna and an 8-degree beamwidth, setting tilt to +4 degrees.    FALSE
4. The antenna tilt should be set to compensate for the Earth’s curvature effects. TRUE

Looking at Figure 2, over 1/3rd of our test group got question #1 wrong. They did much better on question #2 with only 15% wrong. On question #3, our test group was back to over 1/3rd incorrect. Interestingly, questions #1 and #3 are false because of the earth’s curvature effect but in question #4 over 63% got the answer wrong even though it is the reason questions #1 & #3 are false. These questions were mainly related to understanding the antenna, beamwidth, the earth’s curvature and antenna stabililization. 

We’ll start with question #2 which looks at the pilot’s understanding of antenna stabilization. The purpose of antenna stabilization is to maintain the antenna scanning relative to the horizon regardless of the attitude of the aircraft. If 0-degrees tilt is set it doesn’t matter if the aircraft pitches up 10-degrees and rolls 30-degrees right, the antenna will continue to scan 0-degrees relative to the horizon (Figure 3). If stabilization fails, or is turned off, the antenna will scan relative to the longitudinal axis of the aircraft. Of course, there are limits, usually expressed in terms of pitch, roll and combined pitch and roll. However, if you exceed those pitch and roll limits, weather detection is the least of your problems at that point.

Questions 1, 3, and 4 expose some misconceptions about tilt management. Even though Christopher Columbus proved the earth isn’t flat, pilot’s often set tilt like it is. Figure 4 shows the magnitude of the earth’s curvature. At 100nm the earth falls off about 6,600 ft but at 200 nm it is over 26,000 ft and at 300 nm it is almost 60,000 ft. Therefore, you cannot set the tilt at zero, or near zero and expect to see all the hazards in your flight path unless you’re on a rocket and heading into space. At longer ranges you would also be looking at less reflective frozen storm tops or over-scanning the weather completely.

On conventional weather radar systems, we recommend the cruise ground park technique where you lower the beam until ground returns appear at the outer edge of the display (Figure 6). There are several benefits to this method:

1. This method will detect all storm cells and they can easily be seen as they walk out of the ground returns.
2. It is independent of antenna size and beam width.
3. You won’t over scan cells.
4. Attenuation (radar shadows) can easily be seen.